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Thunderstorm activity over India and Indian southwest monsoon

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This paper describes the results of monthly latitudinal (8°-30°N) and latitude belts (8°-10°, 10°-15°, 15°-20°, 20°-25°, and 25°-30°N) averaged seasonal thunderstorm activity over India by using monthly data from a large number of Indian stations from 1970 to 1980. The latitudinal variation in the premonsoon (March-April-May) and monsoon season (June-September) months is described and the results are discussed. An examination of the seasonal thunderstorm day activity in the first four belts indicated systematic changes in their signals of semiannual oscillation. These changes are noted to be a function of latitude and season and appear to be consistent with the seasonal migration of the Intertropical Convergence Zone and solar heating of the Indian landmass. We compare the thunderstorm day activity with the monthly mean maximum values of the surface wet-bulb (Tw) temperatures in the five latitude belts over the Indian region. By using rainfall data for the same period of study, the relationship between seasonal rainfall and number of thunderstorm days over the 11 year period is examined. The results of variation of the ratio of monthly rainfall to thunderstorm days (RTR) during different phases of the southwest monsoon are also presented. Results of the monthly mean electrical conditions of mesoscale and isolated deep convective storms at Pune are summarized. It is noted that the electrification of the premonsoon season thunderstorms dominated by a factor of 3-4 over the monsoon ones. We have examined at length the possible influence of the El Nino on the occurrence and electrification of thunderstorms over the Indian region.
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D4, PAGES 4169-4188, FEBRUARY 27, 1999
Thunderstorm activity over India and the Indian southwest
monsoon
G. K. Manohar, S. S. Kandalgaonkar, and M. I. R. Tinmaker
Indian Institute of Tropical Meteorology, Pune, India
Abstract. This paper describes the results of monthly latitudinal (8ø-30øN) and latitude belts
(8o_10 o, 10 o -15 o, 15 o -20 o, 20 o -25 o, and 25 o -30 o N) averaged seasonal thunderstorm activity
over India by using monthly data from a large number of Indian stations from 1970 to 1980. The
latitudinal variation in the premonsoon (March-April-May) and monsoon season (June-
September) months is described and the results are discussed. An examination of the seasonal
thunderstorm day activity in the first four belts indicated systematic changes in their signals of
semiannual oscillation. These changes are noted to be a function of latitude and season and
appear to be consistent with the seasonal migration of the Intertropical Convergence Zone and
solar heating of the Indian landmass. We compare the thunderstorm day activity with the monthly
mean maximum values of the surface wet-bulb (Tw) temperatures in the five latitude belts over
the Indian region. By using rainfall data for the same period of study, the relationship between
seasonal rainfall and number of thunderstorm days over the 11 year period is examined. The
results of variation of the ratio of monthly rainfall to thunderstorm days (RTR) during different
phases of the southwest monsoon are also presented. Results of the monthly mean electrical
conditions of mesoscale and isolated deep convective storms at Pune are summarized. It is noted
that the electrification of the premonsoon season thunderstorms dominated by a factor of 3-4 over
the monsoon ones. We have examined at length the possible influence of the El Nino on the
occurrence and electrification of thunderstorms over the Indian region.
1. Introduction
The tropical region of the Earth is now well understood as a
central player in the convective overturn of the atmosphere [Riehl
and Malkus, 1958]. In recent years, atmospheric scientists have
shown much concern about the pronounced differences in the
precipitation yield and dynamical and electrical properties of the
tropical mesoscale cumulonimbus regimes embedded in the
monsoonal convection during the monsoon season and the more
vigorous but sparsely distributed thunderstorms of the
premonsoon season [Rutledge et al., 1992; Williams et al., 1992;
dayaratne, 1993; Petersen and Rutledge, 1996]. Williams et al.
[1992] have pointed out that although the above mentioned
differences are usually common in the tropical monsoonal storms,
there is a need for an assessment of similar information from the
Indian region where land and warm waters are juxtaposed for
monsoon development. Therefore a study of the rain/2all and
thunderstorm activity for India is needed.
Very. few studies are reported in the literature [e.g., Rao et al.,
1971] of convective thunderstorms in space and time over the
Indian region. In this paper we present the results of the analysis
of thunderstorm activity and associated southwest monsoon
season rainfall over the Indian region. This infom•ation will be
useful in understanding the role of these thunderstorms in
tropical convection and the global electrical circuit.
Copyright 1999 by the American Geophysical Union.
Paper number 98JD02592
0148-0227/99/98 JD-02592 $09.00
In this stud)' we exanfine (1) monthly latitudinal (8ø-30øN) and
seasonal variation (in 5 ø interval latitude belts across the Indian
region) of mean number of thunderstorm days and (2) the
difference between monthly and seasonal ratios of mean rainfall
to number of thunderstorm days for the monsoonal and
continental thunderstorms for the country and at 10 Indian
stations representing dift•:rent climate regimes. We also compare
the monthly mean number of thunderstorm days in each latitude
belt with the monthly mean maximum surface wet-bulb
temperature. This facilitates understanding the sensitive relation
between the occurrence of thunderstorms and modest changes in
the wet-bulb temperature.
In the hilly regions of the United States the climatological
studies of variation of number of thunderstorms associated with
forest covered mountains have suggested their increased
occurrence compared to thunderstorn• occurrence in the low
country sites. This intbnnation is presently not available tbr the
Indian subcontinent. This presents an outstanding topic for future
study.
2. Data
The data used in this study consist of the monthly (January-
December) rottuber of thunderstorn• days (THN) and mean
maximum wet-bulb temperatures (Tw) for 78 Indian observatory
stations for a period of 11 years from 1970 to 1980. These data
have been extracted from the Monthly Weather Reports of India
Weather Review published by the India Meteorological
Department [IMD, 1970-1980]. The names of 78 stations along
with their abbreviations and other details are given in Table 1.
The locations of these stations are shown on the map of India
4169
41'70 MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
Table 1. Station Network Used in This Study aria its Particulars Table 1. (continued)
Serial Stations Station Name Latitude (N), Longitude (E), Serial Stations
No. Abbreviation deg. and min. deg. and min. No.
Station Name Latitude (N), Longitude (E),
Abbreviation deg. and min. deg. and min.
1 Minicoy MNC 08 18 73 00
2 Trivandmm* TRV 08 29 76 57
3 Tuticorin TTC 08 48 78 09
4 Kodaikanal KDK 10 14 77 28
5 Nagapattinam NPT 10 46 79 51
6 Coimbatore* CMB 11 00 76 58
7 Salem* SLM 11 39 78 10
8 Port Blair PBL 11 40 92 43
9 Veilore* VLR 12 55 79 09
10 Bangalore BNG 12 58 77 35
11 Madras* MDS 13 04 80 15
12 Chitradurg* CHT 14 14 76 28
13 Nellore* NLR 14 27 79 59
14 Anantapur* ANT 14 41 77 37
15 Bellary* BLY 15 09 76 51
16 Marmugao* MRG 15 25 73 47
17 Gadag GDG 15 25 75 38
18 Belgaum* BLG 15 51 74 32
19 Masulipatnam* MPT 16 11 81 08
20 Raichur* RCH 16 12 77 21
21 Kakinada KND 16 57 g2 14
22 Gulburga* GLB 17 21 76 51
23 Hyderabad* HYD 17 27 78 28
24 Sholapur* SLP 17 40 75 54
25 Vishakhapatnam* VSK 17 41 83 18
26 Calingpattnam CLN 18 20 84 08
27 Pune* PNE 18 32 73 51
28 Bombay* BMB 18 54 72 49
29 Jagdalpur JGD 19 05 82 02
30 Gopalpur* GPL 19 16 84 53
31 Chandrapur* CHN 19 58 79 18
32 Cuttack* CTK 20 28 85 56
33 Sandheads SDH 20 51 88 15
34 Veraval VVL 20 54 70 22
35 Nagput* NGP 21 09 79 07
36 Surat* SRT 21 12 72 50
37 Gondia GND 21 28 g0 12
38 Balasore* BLS 21 30 g6 56
39 Sagar Island* SGRI 21 39 88 03
40 Bhavanagar* BHV 21 45 72 11
41 Seoni* SNI 22 03 79 33
42 Midnapur* MDP 22 25 87 19
43 Calcutta* CAL 22 32 88 20
44 Indore* IND 22 43 75 48
45 Pendra PND 22 46 81 54
46 Jamshedpur JSD 22 49 g6 11
47 Ahmedabad* AHM 23 04 72 38
48 Pumlia* PRL 23 06 86 30
49 Bhuj* BHJ 23 15 69 40
50 Bhopal* BHP 23 17 77 21
51 Ranchi* RNC 23 23 85 20
52 Asansol ASL 23 41 86 59
53 Dhanbad DNB 23 47 86 26
54 Agartala* AGT 23 53 91 15
55 Neemuch* NMC 24 28 74 54
56 Satna* STN 24 34 80 50
57 Udaipur* UDP 24 35 73 42
58 Guna* GNA 24 39 77 19
59 Gaya* GYA 24 45 84 57
60 Silchar* SLC 24 49 92 48
61 Imphal* IMP 24 51 93 58
62 Banaras* BNS 25 18 83 01
63 Allahabad* ALB 25 27 81 44
64 Patna* PTN 25 37 85 10
65 Fatehapur* FTP 25 56 80 50
66 Gauhati* GHT 26 11 91 45
67 Gwalior* GWL 26 14 78 15
68 Jodhpur* JDP 26 18 73 01
69 Lucknow* LKN 26 52 80 56
70 Jaisalmer* JSM 26 54 70 55
71 Phalodi PLD 27 08 72 22
72 Agra* AGR 27 10 78 02
73 Dibmgarh* DBH 27 28 94 95
74 Baharich* BRC 27 34 81 36
75 Chum CRU 27 50 75 00
76 Bikaner* BKR 28 00 73 18
77 New Delhi* DLH 28 35 77 12
78 Roorkee RRK 29 51 77 53
* These are the stations from which rainfall data was used
in this study.
(Figure 1). The other data used in the present study consist of
monthly (March-September) rainfall (in millimeters) for 58 out
of the 78 stations mentioned above for the same period. The
rainlhll stations are marked by an asterisk in Table 1. The rainfall
data were obtained from the Climatology and Hydrology
Division of Indian Institute of Tropical Meteorology, Pune.
It may be noted in Figtire 1 that each 5 ø interval latitude
belt (8ø-10 ø, 10ø-15 ø, 15ø-20 ø, 200-25 ø, and 25ø-30øN) has nearly
the same nmnber of stations. The aerial distances between some
of the stations in the northem latitude belts 4 and 5 are higher
colnpared to the stations in the near-equator belts. However, the
minimum distance between the stations in the lowest three belts
is nearly 80 kin, and maximum distance is about 130 kan. The
stations are thus fairly xvell separated from each other. This would
effectively reduce the chance of a single storm being reported
simultaneously by t•vo stations. For examination of the monthly
latitudinal variation of mean number of thunderstorm days
across the latitude range 8ø-30øN, monthly data of the stations in
each belt over a period of 11 years have been considered, and
monthly means across the five belts are computed. Figures 2a-2d
show monthly latitudinal variation across the latitude range 8 ø -
30øN, and Figure 3 shows seasonal variation of mean number
of thunderstorm days in 5 ø interval latitude belts over the Indian
region.
An index was introduced by Zipset [1994] to study the relation
between rainfall and number of thunderstorm days for the west
African region. This index is defined as the ratio of monthly
rainthll to number of thunderstorms days (abbreviated as RTR).
Earlier studies in which rainfall has been compared to lightning
activity were by Battan [1965], Maier et al. [1978], and
Piepgrass and Krider [1982]. These authors have calculated rain
yields over cloud scales or mesoscales based on single or multiple
thunderstorm case studies. The recent study by Petersen and
Rutledge [1996] has examined this relationship by using a ratio
of total rain mass to cloud-to-ground flash density over large
spatial and temporal scales for several different regions of the
globe. In the present study we have followed the RTR index
method of Zipset' [1994]. Monthly rainfall and number of
tlmnderstonn days data at 58 stations are used to work out the
premonsoon (March, April, and May (M-A-M)) and monsoon
(June, July, August, and September (J-J-A-S)) season means for
11 years. By using the above means, the seasonal RTR indices are
prepared for a period of 11 years t?om 1970 to 1980.
3. Results and Discussion
3.1. Monthly Latitudinal Variation of Thunderstorm Days
The monthly latitudinal variation of thunderstorm days across
the latitude range 8ø-30øN over the Indian region during different
MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON 4171
N
25
15
RRK
ß
DLH
BKR
ß Cl •U
PLD AGR BRC
JSM ß ' _KN'
ß JDP OWL GHT
LB BNS P'I'N
uOFNM(' GN^ )IN GYi ,SLCNJP
''
ß ' ' ß ß C• ACT
AHM BHP t; N ß
•' * ND ß PND *' JS•OMDi•AL
ß SNI ß ••_
•v,• N•FG •D B,LS
C.HN JGD
SL, PGLB HYD VS..I•"
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.5 ,
6BE 75 85E 95
17
30
17
Figure 1. Map of India showing the locations of 78 stations used in this study. The number of stations contained in 5 ø interval
latitude belts from 80N to 30øN are also indicated.
seasons is shown in Figures 2a-2d. The monthly mean number of
thunderstorm day's for the five latitude belts is plotted against the
mean latitudes (9 ø, 12.5 ø, 17.5 ø, 22.5 ø, and 27.5øN). The
months in the diagrams have been grouped by season that is,
M-A-M (premonsoon), J-J-A-S (monsoon), October and
November (O-N) (postmonsoon), and December, January, and
February (D-J-F) (winter), which constitutes the total mmual
period. In Figures 2a-2d the curves show a systematic latitudinal
behavior. Significant results of their seasonal variation are
described below.
Figure 2a describes the latitudinal variation during
premonsoon season. For clarity' and better understanding, Figure
2a is partitioned into three identifiable latitude sections (I, II, and
111). Section I is from 8 ø to 14 ø, section II is from 14 ø to 22øN, and
section 111 is from 22 ø to 30øN. It is observed in section I that
during the months of March and April the thunderstom• activity
decreases with increasing latitude, but in the month of May this
decrease is not visible in the low latitudes (i.e., 8ø-12.5øN). This
observation indicates that thunderstorm activity' is widespread in
the low latitudes during the month of May'. The number of
thunderstorms during the months of March and April decreases at
the rate of one thunderstorm per month per latitude northward. In
the month of May' the decrease begins at 12.5øN, and the rate
is 0.3 thunderstorms. This comparison suggests a nearly three
fold increase in the thunderstorm activity' in the month of May.
Intermonth comparison of thunderstorm activity' shows that in
the month of April the thunderstorm activity' increases nearly 3
times faster than that in the month of March. In the month of May
the activity' is about 10-15 times higher than that in the month of
March and is about 2 times higher than that in April.
In section 111 (i.e., 22ø-30øN) the thunderstorm activity is
nearly steady' at an average of 2, 3.5, and 5.0 in the months of
March, April, and May', respectively. Intermonth comparison
indicates a clear increase in thunderstorm activity by a factor of 2
in April with respect to that in March and by a factor of 1.5 in
May' with respect to that in April. Thus the differences in
thunderstorm activity in sections I and III show the latitudinal
dependence of thunderstorm activity.
In section II the thunderstorm activity' is lower than that in
section I and is higher than that observed in section 111 except in
the month of March. This result appears to be consistent with the
general view that thunderstorms are most frequent over the moist
humid regions of the equatorial low- pressure belt and that their
frequency decreases with increase in latitude. A survey of the
literature shows that similar studies for the tropical regions are
not available elsewhere.
Figure 2b shows the curves for the 4 months of the monsoon
season. An inspection of the trends of these curves shows that the
thunderstorm activity', by and large, increases with the increase in
latitude in each month. The maxima in the latitudinal
4172 MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
x
.....
8 9 10 11 12 13 14 15 16 17 18 16 20 21 22 23 24 25 26 27 28
10'0 -•- June
.... ,.
S.O -- •/: .............
4.0 .•'-
3.0
•.oT• .......................
0.0
10.0
6.0
•e 7.0
E
•.0
ß
.• 5.0
4.0
3.0
ß
2.0
8 9 10 11 12 13 14 15 16 i: 1S :• 20 21 22 23 24 25 26 27 28
Latitude 'N Lat•ud½ 'N
(a) (b)
October
Novembe•
-'- December
,i'1,1'1'1,1'1'1'1'1'1'1 '1'1'1'1'1'1'
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
January
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Latitude 'N Latitude 'N
(c) (d)
Figure 2. Monthly latitudinal variation of mean number of thunderstom days across the latitude range 8ø-30øN over the Indian
region: (a) for the premonsoon season months (March, April, and May), (b) for the monsoon season months (June, July, August, and
September), (c) for the postmonsoon season months (October and November), and (d) for the winter season months (December,
January, and February). Two circles in Figure 2b show the latitudinal mean locations of the Intertropical Convergence Zone at 85øE
and 79øE longitudes, respectively, during June-September [Rajkumar and Narasimha, 1996].
thunderstoms in each month are located in the latitude range correlation at ditlErent locations in the tropics is worth noting.
25ø-30øN, and the minima are located in the latitudes 8ø-10øN. Our result of a low number of thunderstorm days and high rainfall
The slope of the best fit curve through these data points is also in the peak period of the monsoon season is in partial agreement
positive. This latitudinal dependence of thunderstorm activity in with the result of Zipser.
the ]nonsDon season is in contrast to the pre]nonsoon season. The dip region in Figure 2b was matched with the data of the
These observations therefore suggest the preponderance of the India AJIeteorological Department [IMD, 1981]. It was found that
thunderstorm activity in the monsoon season [Rao et al., 1971]. this is also the same arid zone/less rainthll region of
Inspection of the intercurve spacing and their trends brings out Marathawada, Telangana, and its adjoining parts in south
the following interesting results. It is observed that the curves for central hldia where seasonal thunderstorm days as well as rainfall
July and August (midmonsoon season) run quite parallel to each are less. Further, we also note that in the months of June and
other, and they maintain a seasonal low profile of thunderstorm September (onset and withdraxval phase months of the monsoon
activity. We also note that these curves show a pronounced dip season) the thunderstorm day activity exceeds that in the two
centering around latitudes 15.5ø-18.5øN. These results suggest months of July and August. This fact suggests that the onset and
that during the prime period of the monsoon season, the tYequency withdrawal phases of the monsoon season are associated with
of occurrence of thunderstorms is relatively diminished. Zipset widespread occurrence of thunderstorms. This inference is well
[1994] studied the correlation between rainfall and number of corroborated by the most common and consistent observation that
thunderstorm days for tropical stations, mostly north-south, along the onset and withdrawal, and even the revival of the ]nonsDon
the west coast of Africa. His analysis showed that the correlation activity after the break monsoon situation, in most cases, ]s
between rainfall and thunderstorm days for the northenm•ost associated with the merging of thunderstorms [Rutledge et al.,
stations was high. Going southward, the stations showed a 1992].
striking minima in the seasonal number of thunderstorm days Figure 2c shows a systematic diminution in the latitudinal
with high rainfall. He concluded that this contrast in the thunderstorm activity during October and November months. The
MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND INDIAN MON•;OON
4173
Jin •1• •' r A ' r M' --'------ ' ' ' ' p Oot Nov
Z =:= ........... I •ø- ....
4.•
•n M• A• M•y Ju• J p 0 t Nov
=:• .................. :: _• ......
Mon•
Figure 3. Seaso•l v•ation ofme• nm•r of •dersto• •ys for •e latitude •lts (8'-10 *, 10'-15 *, 15'-20 *, 20'-25 *, •d 25 *-
30 * N) over •e •dian region.
November frequencies appear to be far less than those in 3.2.1. Interbelt comparison. In the first three belts (8ø-10 ø,
October. The higher frequency of ~ six thunderstorms in the 10ø-15ø, and 15ø-20øN) the semiannual oscillation of the seasonal
lower latitudes in the month of October is associated with the variation is clearly seen. In the lowest belt, which is nearest to the
presence of the convective nature of the disturbed weather during equator, the first maximum is seen to occur in the month of
the northeast monsoon season over most parts of the east coast of April. In the second belt this maximum has appeared in the month
south India. Results of the analysis of seasonal frequencies of the of May. In the third belt it is spread over the two months of May
cyclonic storms which develop over the Bay of Bengal region and and June. In the fourth belt (20ø-25øN) the first maximum is
strike theeastcoast of southIndiaalsoshowedtheirpeakactivity faintly seen. In the fifth belt (25ø-30øN) the semiannual
in these two months [India Meteorological Department, 1979]. cmnponent has completely vanished. We notice a single peak in
In Figure 2d it is seen that the latitude belt between 13 ø and the month of July. The amplitudes of the first maximum in the
20øN experiences the least seasonal activity. The regions on
either side of this latitude belt are flanked by low activity in the
range of 0.25-2.5 thunderstorms per month. We also note that the
month of January is almost free from thunderstorm activity over
the Indian subcontinent during the annual period.
3.2. Seasonal Variation of Number of Thunderstorm Days
in 5 ø Interval Latitude- Belts From 8 ø to 30øN
Over the Indian Region
Seasonal variation of the thunderstorm activity in five latitude
belts across the Indian region is shown in Figure 3. Some
observations of the phase shift and amplitude variation in these
belts are summarized below:
first and second belts are seen to be nearly the same, but in the
third belt this amplitude is diminished by- 38% as compared to
that in the first and second belts. Moreover, it is noticed that the
amplitude of the annual signal in this belt is more pronounced. In
the fourth and fifth belts these amplitudes have again increased.
We thus notice the pronounced semiannual signal variations of
the thunderstorm day activity across the latitude belts over the
Indian region as we move northward away from the equator.
Variations in the incoming shortwave radiation from the Sun
are attributed [Williams, 1994] to be ultimately responsible for
variations in global meteorologicalparameters on the seasonal
timescale. The two principal causes for seasonal variationsare (1)
the slightly elliptic orbit of Earth around the Sun and (2) the
tilting of Earth's rotational axis with respect to the ecliptic plane.
4174 MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
2,(
2O
Feb 14 ar Apr llt ay Jun Jul Aug 8 ep O c! Nov Dec
Feb 14 ar Apr 14 ay Jun Jul Aug $ ep Oct Nov Dec
2 !
2s
20
t g
J i n
Feb M ar Apr M ly Jun Jul Aug $ ß p O c t Nov D e c
2 S
2 2
2 0
J I n
F e I.
ß , ,
,d., A •, M .y Ju'. J•. A .'. ..p o •, . o. O .c
26.5
26 0
25 5
25 0
24 5
24 0
I
2 3 5
Jan Feb M ;it Apr M a y Jun Jul Aug S ß p O c I Nov Dec
Months
Figure 4. Same as Figure 3 but for the monthly mean maximum surface wet-bulb (Tw) temperature.
MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND INDIAN MONSOON 4175
The first cause produces an annual variation in insolation (i.e.,
one maximum and one minimum each year). The second cause
produces a semiannual variation (i.e., two maxima and two
minima each year) as the Sun crosses the equator txvice a year.
The semiannual variation of the thunderstoma day frequency is
basically linked with the north-south oscillation of the Sun and
the associated migration of the Intertropical Convergence Zone
(ITCZ) across the country. Riehl and Simpson [1979] have shown
that the ITCZ has its largest annual amplitude of oscillation over
India and the oceanic areas to the south. The ITCZ is a zone of
intense moist convective activity. It may therefore be concluded
that the observed variations in the signals of semiammal
oscillation of the thunderstoma day activity in these belts may be
attributed to the movement of the center of high - cloudiness
•vithin the tropics [Markson, 1986].
3.2.2. The Influence of ITCZ. It is noteworthy in Figure 3
that the trough in the month of August in the first belt gets filled
very. systematically as the latitude belts advance thrther
northward. This observation suggests that the lower thunderstorm
day activity in the July and August months in the lower latitudes
is replaced by more activity in the latitudes north of-• 20øN
where ITCZ is normally located nearly east-west across the Indian
region in these months [Rajkumar and Narasimha et al., 1996].
Our results of seasonal variation of the thunderstorm day activity
are in good agreement xvith the studies of Williams [1994,
1997] tbr the global tropics (_ 25ø).
3.3. Monthly Mean Maximum Surface Wet-Bulb
Temperature (Tw) in the 5 ø Interval
Latitude Belts Over the Indian Region
Previous studies by Williams et al. [1992], Williams I1992,
1994], and dayaratne [1993] have used the monthly mean
maxinmm wet-bulb temperature to compare its association with
lightning and deep convection in the tropics [Williams, 1997]. The
use of surface wet-bulb temperature was more coimnon and
consistent in these studies because the wet-bulb temperature
records simultaneously the effect of temperature and moisture,
both of which are important for the thermodynamics of moist
convection. The results of their studies have shown a reasonably
well defined seasonal variation of lightning flash rate with wet-
bulb temperature at various locations in the tropics. The results
discussed above have motivated us to examine the seasonal
association of wet-bulb temperature with the number of
thunderstorm days over the Indian region because thunderstorms
are the primary seat of the cloud electrification. In the lbllowing
paragraph we present the seasonal variation of monthly mean
maximum sur/;ace wet-bulb temperature in the five latitude belts
.just as in section 3.2.
The monthly mean maximum variation of Tw in the five
latitude belts across the Indian region is presented in Figure 4.
Similar variation in the number of thunderstorm days (THN) is
presented in Figure 3. Both the data sets are similar and are
represented by monthly mean values drawn over the same period.
The focus of the comparison of both the data sets is to investigate
the relation between THN and Tw for the Indian region. The data
presented in Figure 4 show that the seasonal nfinimum and
maxinmm values of the Tw in the five belts are 24.0-26.5øC, 19.5-
24.5øC, 19.5ø-25.0øC, 16.5ø-26.0øC, and 14.5ø-26.0øC. Similar
data tbr the THN (Figure 3) show that the occurrence of the
thunderstorm days varies between 0.5 and 8.5, 0.8 and 8.0, 0.0
and 5.5, 0.5-7.5, and 1.0 and 8.5, respectively. The above ranges
of variation of Tw and THN are used to compute the number of
occurrence of THN days per 1 øC change in the Tw in these belts.
It is noted that per IøC rise in Tw, the occurrences of THN are
3, 2, 1, 0.7, and 0.7 in these respective belts. This analysis shows
that the frequency of occurrence of THN per IøC rise in the Tw is
about 3-4 times larger in the near-equator latitude belt than in
the belts lhrther l¾om the equator. This sensitive association
between the Tw and THN appears to be consistent since the
thermodynamics of moist convection is dependent on Tw.
The other significant result is the association between the
monthly values of Tw and THN in the five latitude belts. It may
be noted from Figures 3 and 4 that the seasonal variation of Tw
shows excellent one-to-one correspondence with that of the THN
in each latitude belt. We present some qualitative details of their
association in the following paragraph.
Figures 5a-5e show the scatter diagram of X-¾ plots of the
monthly values of Tw- THN for the five latitude belts. An
exponentially growing best fit curve is also drawn through the
scatter of these points. The best fit statistics of the curves are
thrnished in the diagrams. The exponentially growing best fit
curves in all the sets suggest that the THN tend to increase
nonlinearly with increase in Tw. From the exponential nature of
these curves it is noted that in the lower range of the Tw (---22øC
or less), the THN could be linearly related to the Tw. The
nonlinearity in the THN is usually seen to arise mostly in the
higher range of Tw (> 22øC). This result suggests that in the
lower latitudes, where magnitude of Tw is larger and its range of
variation is smaller (see the previous paragraphs), the number of
THN per 1 øC rise in Tw is larger than that in the higher latitudes.
This result is in good agreement with the result deduced in the
beginning of this section. This suggests that a modest increase in
the higher value of Tw may lead to still higher frequency of
occurrence of thunderstorms. This result seems to be consistent
with the fact that the convectively available potential energy
(CAPE), lightning flash count, and updraft velocities usually
increase nonlinearly with increase in Tw [Williams, 1992,
Williams et al., 1992, Rutledge et al., 1992, dayaratne, 1993,
Williams, 1994]. The nearly linear relationship between THN and
lower Tw values suggests reduced occurrence of THN. This is
usually attributed to colder dry air aloft and to low ground
temperature that gives rise to lower updraft velocities and
shallower cloud depth [dayaratne, 1993].
3.4. Premonsoon and Monsoon Season Rainfall
and Thunderstorm day Relation
Figures 6a and 6b show the plots of premonsoon and
monsoon season mean rainfall (in millilneters) and number of
thunderstonn days averaged over the Indian region for 11 years
(1970-1980). Table 2 gives yearly seasonal values of RTR. It can
be seen from Figure 6a that the premonsoon season rainfall and
thunderstorm day curves run almost parallel to each other
throughout the 11 year period. The correlation coefficient between
their values is 0.74, which is significant at 99.99 confidence level.
It is remarkable to note that the two deficit monsoon rainfall years
of this decade, that is, 1972 (-23%) and 1979 (-17%)
[Parthasarthy et al., 1990], show the minimum thunderstorm day
activity during the corresponding premonsoon seasons. The
center column of Table 2 shows the variation of RTR during the
premonsoon seasons of the years 1970-1980. It is noted that at
any point of time during the 11 year period, the RTR during the
premonsoon season has remained between 8 and 13 and has not
4176 MANOHAR ET AL.' THUNDERSTORMS OVER RqDIA AND INDIAN MONSOON
I ß i ß i ß i ß ! ß 1 ß ! ß ! ,-
c) .- o
f.(a• 0 t,LUOi• Jo ,•q•n N ume•l /•Im41uo•
II •)
i - ! - ! - ! - ! - o
ß m,
ii
f i ! ß 1
sAI 0 uJ. JOlIJ,14;]J'lm4. Jo ji.q• N •
m
e',q ,
sJ•C] uuo•uY• jo J)quJnN u•eN J4q•J•N
MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND IND• MONSOON 4177
' ' ' +IRa•nfall ',• No•Thn. ays
i I I
I I i
- -F---'• -' --'------•---•---*'---• ....
- -k---•l J _ _'__ _ ' _ _'_'_ -•_-'---' ....
45 di_ _L___J , , , ß L__•_L___J ....
g t--•---+ --•- .... -•-•---•---4---
4 -•--•---• ,-•-•-•---•r-•-, .... •v--•---
I [I f / ,xx, ,/ / , ,xx ,
•--•--- ½-/--• .... •¾-4---
i I
i___ L ....... 4--
d --l•h/---I/ ......
I
' ,
, ¾ ..... •//
' 3 ' L _Z___ 25 --4 T ' : , ••- .... •-- *---
I I I I I I
I I I I I t I
15-
_
i
6,0 250
-55 240
5,0 230
n• 220
.... 4.5[
4.0 •210
200
3.5 õ
3,0 _•
5 180
-2,5õ
..... 2.0 170
160
- 1.5
19701971 197219731974197519761977197819791980
150
YEARS
Figure 6a. Plot of mean rainfall (in millimeters) and number of
thunderstom days for the pre-monsoon seasons of 11 years
(1970-1980);
....... :;-Rainfall *- No.Thn. Day• 7.0
6,5 e
6.0 •o
5.5 _•
o
5,0 <
- , 1 T , .... r , , 4.5
19701971 197219731974197519761977197819791980
YEARS
Figure 6b. Same as Figure 6a but for the monsoon seasons.
that the RTR in the monsoonal regime of the cumulonimbus
clouds is steady and higher by a factor of 3-4 than those values in
the premonsoon season. We note that the correlation coefficient in
Premonsoon Monsoon
1970 10 40
1971 9 39
1972 10 31
1973 9 37
1974 9 33
1975 8 38
1976 10 36
1977 13 36
19.78 10 43
1979 9 43
1980 9 38
Mean 9.6 36.5
Standard 1.2 3.7
Deviation
Median 9 37
RTR is ratio of monthly rainfall to number of thunder-
storm days.
of l 1 years. A remarkable feature of the curve showing the
number of thunderstorm days is the pronounced diminution of
the thunderstorm days in the year 1972 which matches with the
deficit rainfall of this year. Further, the correlation coefficient
between rainthll and number of thunderstorm days in this set is
0.54; that is significant at 99.9 confidence level. The number of
thunderstorm days curves in Figures 6a and 6b show that in the
deficit monsoon year 1972, both the premonsoon and monsoon
seasons exhibited the lowest thunderstorm activity (2.90 and
5.04, respectively) of the decadal period. Earlier studies
correlating the seasonal rainfall with frequencies of
thunderstorms were by Landsberg [1971] and Freier [1978].
Their results showed that, in general, the low-precipitation
regimes matched with lower frequencies of the thunderstorms or
were due to lack of thunderstorm-producing weather conditions.
The last colunto of Table 2 gives values of RTR for the
monsoon season. It is significant to note that the range of
variations of RTR values is between 31 and 43. This range of
variation of RTR in both the seasons is in close agreement with
the criteria deduced in the studies of Zipset [1994], (RTR << 25
and RTR >> 25 in the two respective cumulonimbus regimes).
The mean, median, and standard deviation of this set of RTR
values are 36.5, 37, and 3.7, respectively. This result indicates
Years RTR
Table 2. Yearly Variation of RTR for the
Premonsoon and Monsoon Seasons
During 1970-1980.
shown any noticeable variation. The mean, median, and standard the monsoon season in Figure 6b is inferior to the one in the
deviation of this series of RTR are 9.6, 9, and 1.2, respectively. It premonsoon season in Figure 6a. One can also see that the mean
appears that this consistency in the seasonal values of RTR in the number of thunderstorm days in the monsoon season is actually
premonsoon season is a remarkable feature and a characteristic of higher than that in the premonsoon season. One possible reason
the cumulonimbus regime of the continental type. for the lower correlation may be the mixed modes of the monsoon
Figure 6b shows the plot of monsoon season mean rainfall (in and continental regimes of the convective rainfall. There may be a
millimeters) and number of thunderstorm days for the 11 year mixing of these two modes because even in the monsoon months
period. We note that the rainfall and thunderstorm day curves the premonsoon kind of convection can dominate for the days on
tbllow each other quite well in their trends throughout the course
4178 MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
both ends of the monsoon season. In this context we refer to the
results of Figure 2b. This consideration may give a possible
explanation for the factor of 10 differences with Williams et al.
[1992] and a factor of 3-4 difference xvhen RTR is considered.
The high value of RTR in the monsoon season indicates that the
contribution of thunderstom rains to monsoon season rainfall is
significant.
3.5. Southern Oscillation - El Nino Phenomena and Their
Possible Linkage With Large-Scale Occurrence of the
Number of Thunderstorm Days Over the Indian Region
Figtires 6a and 6b show two important results: (1) the number
of thunderstorm days in the monsoon season is higher than that
in the premonsoon season and (2) the number of thunderstorm
days in the premonsoon seasons of the years 1972 and 1979 and
the monsoon season of the year 1972 are the minima of the
decadal period. Coincidentally, these 2 years are also the bad
monsoon years of this decade. It is felt that in the light of this
result, some discussion is desirable regarding (1) the E1 Nino-
Southern Oscillation (ENSO) phenomenon and its possible
linkage with thunderstorm activity and (2) whether the warin
ENSO years are also the warm years in the sense of wet-bulb
potential temperature.
3.5.1. Earlier work. Sikka [1980] has previously examined
the major fluctuations observed in the historical (1875-1975)
records of the monsoon rainfall series over India and linked the
anomalous epochs of the Indian rain/hll with the Southern
Oscillation index as deterinined by the E1 Nino phenomenon.
Table 3 shows the years of monsoon failure that were also E1
Nino years, the years of monsoon failure but not the E1 Nino
years, and the years of E1 Nino but not of monsoon failure for the
period 1875-1980. Table 3 shows the relationship of the E1 Nino
years with the occurrence of the monsoon failtire. It can be seen
that there have been txvo major epochs, 1911-1920 and 1963-1976
(total period of 24 years), of E1 Nino xvhen monsoon failed in 8
years (1911, 1918, 1920, 1965, 1966, 1968, 1972, and 1974). The
epoch 1929-1962 (34 years) is marked by rarity of draughts over
India as the rainfall failed only in 2 years (1941 and 1951) in this
epoch.
From Table 3 and Figures 6a and 6b it can be noted that
during the E1 Nino year 1972 the number of thunderstorm
days as •vell as the rainfall in both the seasons was
significantly reduced. During the E1 Nino year 1974 the
rain/hll was much reduced (-12%) only during the monsoon
season. The year 1979 was not an E1 Nino year, but the
number of thunderstorm days and rainfall was less during the
premonsoon season, xvhile the rainfall during the monsoon
season only was much less. This analysis perhaps indicates
that the occasions of monsoon rainthll failure are well
associated with the warm phase of the E1 Nino, and the
consequence of E1 Nino may be witnessed through reduced
number of thunderstorm days during the premonsoon and
monsoon seasons.
Kent et al. [1995] studied the longitudinal and latitudinal
tropical cirrus cloud distribution within the tropics to differentiate
between the "xvarm" and "cold" ENSO years from 1984 to 1991.
To the extent that the cirrus clouds are a product of the tropical
thunderstorins, the results of the studies of Kent et al. [1995] are
worth mentioning in the context of the above result of the
influence of the ENSO on thunderstorm frequency over the Indian
region. Kent et al. [1995] observed enhanced upper level cirnls
throughout the tropics with particular enhancements over the
eastern Pacific and several land zones, including India in warm
ENSO years. This is attributed to more vigorous moist convection
and/or more active "ice factories." This enhancement is greatest
over the central and eastern Pacific Ocean and is minima
(etlEctively zero) over the maritime continent. Gray and Sheaffer
[1991] noted that anomalously large ocean surface temperatures
in the eastern equatorial Pacific Ocean produced upper level (200
mbars) outflow imo other equatorial regions. Rasmussen [1991]
also noted that upper tropospheric divergence occurring over
•vann equatorial waters is coupled not only with the equatorial
Walker circulation but also with the extratropical Hadley
circulation. In particular, during an ENSO warm period the
outflow region increases in size, resulting in a weakening in the
Walker circulation and intensification in the Hadley outflow. The
descending branch of the Hadley circulation in the subtropics is
enhanced, causing a decrease in high-cloud activity at these
latitudes.
Table 3. El Nino Events and Monsoon Rain Failures for the period 1875-1980.
Both Monsoon Failure and El Nino Years
Monsoon El Nino
Failure Years
Years Only
Only
1877 1877
1899 1899
1905 1905
1911-12 1911
1913-14 1913'
1914-15 1915'
1918-19 1918
1927-28 1928
1939-40 1939'
1940-41 1941
1951-53 1951
1965-66 1965
1968-69 1968
1972-73 1972
1974-75 1974
1901
1920
1966
1979
1921-22
1923-24
1925-26
1930-31
1956-58
1963-64
1969-70
1976
Total for both monsoon failure and El Nino years is 15 years. Total for monsoon failure
Years only is 4 years. Total for El Nino years only is 8 years.
* These are years of minor failure of monsoon rainfall
MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND INDIAN MONSOON 4179
Table 4. Lightning Flash Count Details Based on the Analysis of Electric Field Records for
the Premonsoon Season Thunderstoms at Ptme.
Date of
Thunderstorm
Total Duration Flashes Flash Rate Seasonal Total Total
Seasonal of per Minute Average Seasonal Seasonal
Thunder- Lightning per Storm Rate Duration Flashes
storms Flashes*, (Standard
Deviation)
C rain A B
B/A B/C
April 24, 1972 7 165 178 1.1
April 25, 1972 21 17 0.8
May 7, 1972 252 146 0.6
May 13, 1972 399 390 1.0
May 17, 1972 221 206 0.9
June 11, 1972 78 86 1.1
June 12, 1972 32 61 1.9
April 1, 1973 5 105 274 2.6
April 2, 1973 132 337 2.5
April 3, 1973 110 302 2.7
April 8, 1973 152 393 2.6
June 27, 1973 96 160 1.7
1.1
(0.4)
n=7
2.4
(0.4)
n=5
April 3, 1974 4 50 92 1.8 4.3
April 4, 1974 204 1038 5.1 (3.1)
April 23, 1974 150 190 1.3 n = 4
April 26, 1974 10 90 9.0
May 7, 1975 4 231 299 1.3 2.1
May 28, 1975 100 248 2.5 (0.9)
May 30, 1975 170 593 3.5 n = 4
May 31, 1975 113 132 1.2
March 4, 1977 5 37 175 4.7
May 13, 1977 164 155 0.9
June 7, 1977 143 551 3.9
June 8, 1977 64 167 2.6
June 26, 1977 105 195 1.9
2.8
(1.4)
n=5
1168 1084 0.9 155
595 1466 2.5 293
414 1410 3.4 353
614 1272 2.1 318
513 1243 2.4 248
The normal date of the onset of the Indian southwest monsoon at Pune is June 10.
* The choice of the period of electric field considered for lightning flash counting is the corresponding one when point discharge current existed.
This procedure thus ensures unambiguously the period of electrically active state of the thunderstorm.
In Table 4 we furnish the results of lightning flash counts for
the premonsoon season thunderstorms during 1972-1977 (except
1976) at Pune. Lightning flashes were determined by the electric
field change measurements using the potential equalization
method. The details of the electronics and accuracy of the
measurement of this system are described in section (3.7). The
durations of the electric field records chosen for the study were
the corresponding ones when point discharge current (PDC)
existed. All our records are comprehensive and continuous and
were personally attended during the events of these thunderstorms
by one of the authors (G.K.Manohar).
This analysis is based on the manual counting of the electric
field changes. Allowance has to be given to the errors and
approximations arising during the flash counting. Ho•vever, since
these results are based on a fairly large number of available
records of thunderstorms, it is felt that the generality of the
results may not be overlooked. Our results of flash counts suggest
that during the warm ENSO year 1972 the seasonal flash rate as
well as flash production per storm was much less than that during
the other years. These results are indicative of reduced overall
electrification during the premonsoon and monsoon seasons over
the Indian region in the •vann ENSO years. Amarasekara et al.
[1997] attributed the reduction in thunderstorm activity to the
global scale subsidence associated with major upwelling in the
eastern Pacific Ocean. In a recent status report of global circuit
response to temperature on distinctive time scale, Williams
[1997] made further studies of the ENSO phenomenon. It was
pointed out that when the eastern Pacific Ocean wam•s in the El
Nino (warm) phase of the cycle, the surface air temperature
tlxroughout the tropical continental regions also increases [Sanga-
Ngoie and Fukuyama, 1996]. These continental perturbations
were dominant contributors to the tropical temperature anomaly
in Schumann resonance study by Williams [1992] and Satori and
Zieger [1996]. On the basis of the evidence for a systematic
temperature change in the continental regions of the ENSO
timescale, evidence was sought for systematic changes in deep
convection in the same regions. We note that our present result
runs counter to the result of Kent et al. [1995]. It is not possible
to give an explanation for this opposite nature of our result.
Um:esolved differences in the results on important issue like this
may lead to new understanding. It is felt that, possibly, a clear
picture of this situation might emerge through organized studies
in filture. The surmnaries of the above results indicate influence
of ENSO on the occurrence and the electrical state of
thunderstorms. Further studies are needed for clear understanding
of ENSO influence on the occurrence of thunderstorms.
3.5.2. Examination of Surface temperature conditions in
the monsoon environment during El Nino and La Nina
4180 MANOHAR ET AL.: THUNDERSTORMS OVER LNDIA AND INDIAN MONSOON
Table 5. Monthly and Seasonal Mean Maximum Wet-Bulb Potential Temperature
Anomaly Assessed From the Data of 78 Indian Stations Used in This Study
June July August September Seasonal result
Number of stations showing negative
Temperature anomalyin 1972 as
Compared to 1975, %
Range of temperature anomaly, øC
45 40 76 75 59
-0.1 to-l.7 -0.1 to-2.1 -0.1 to-3.6 -0.1 to-3.5 -0.1 to-3.6
Mean -0.5 -0.7 -0.8 -1.0 -0.8
Standard Deviation 0.4 0.5 0.6 0.8 0.6
Median 0.5 0.6 0.7 0.9 0.7
Rainfall departures, %
1972 -24 -33 -11 -25 -23
1975 +10 +6 +9 +33 +13
T6nnperature is given in degrees Celsius. Anomaly is derived by subtracting 1972 data from 1975 data. Rainfall departure staffsties for the years
1972 and 1975 [Parthasarathy et al., 1994] are also presented.
episodes over the Indian region. Earlier study [Sanga-Ngoie
and Fukuyama, 1996] of tropical land zones has shown positive
temperature anomalies in the warm phase of ENSO and vice
versa. Sanga-Ngoie and Fukuyama sununarized that E1 Nino
years tend to be warmer than the non-E1-Nino years over Zarie
river basin, Africa. A pertinent question in the context of the
present study is, whether the warm ENSO years during the period
of present study were also the wam• years in the sense of wet-
bulb potential temperature over the Indian region? This
consideration forms the hypothesis that the tropical land zones
will be more conditionally unstable in the warm phase of ENSO.
The following results from this study are submitted.
During the period under study the years 1972 and 1975 were
identified for minimum (653 nun) and maximum (963 ram)
seasonal rainfall [Parthasarathy et al., 1994], respectively, and
also for the warm and cool episodes of ENSO [Rasmussen and
Carpenter, 1983], respectively. The choice of these 2 years was
therefore well suited for the examination of this question. The
surface monthly mean maximum potential temperature data of 78
Indian stations for the 4 months (June-September) of the year
1972 were compared with year 1975. This analysis has therefore
offered results of surface temperature conditions in the monsoon
enviromnent during E1 Nino and La-Nina episodes over the Indian
region.
Table 5 presents the monthly and seasonal results of this
analysis. Table 5 also gives the rainthll departure statistics for the
years 1972 and 1975. It is noted that surface temperature over
greater portions of the Indian subcontinent were substantially
colder during the monsoon season of the year 1972. The rainfall
statistics of the year 1972 show below normal values of
precipitation while those of the year 1975 show above normal
values.
The elevated values of temperatures and the associated CAPE
values [Williams et al., 1992] and the positive departures of
rainfall over greater portions of the Indian subcontinent during the
year 1975 support the observations of Hendon [1988] that
conditional instability is fundamental to monsoon energetics.
One more important result of this analysis needs to be
mentioned. Monthly temperatures over the land stations
composing six categories of land zones (south central India,
central India, Bengal, northeast India, monsoon trough region, and
northxvest India) were averaged for the 4 months of the years
1972 and 1975. It xvas noted that the highest temperatures
(25.5ø-27.1øC) xvere consistently associated with the monsoon
trough region while the lowest (23.2ø-24.3øC) were usually
associated with south central India. The climatological atlas of
India shoxved that the seasonal rainfall over these two regions
averages about 100-250 and 20-75 cm, respectively.
From the above analysis and from the studies of Keshavamurty
andAwade [1974], Sikka [1980], and Ve•ta [1982] we are let to
infer that, on average, during good monsoon years the temperature
structure in the loxver to upper troposphere is characterized by
xvanner than normal temperatures leading to instability which
manifests itself in synoptic scales convergent air motions driven
by horizontal pressure gradients that, in turn, are set up by
latitudinal gradients in surface temperatures.
The unanswered question is, why are our results of low-
temperature conditions and overall reduced cloud electrification
obtained during the warm phase of ENSO dill•erent from Sana-
Ngoie and Fuku. vama [1996] and Kent et a/.[1995]. It is hoped
that a clear picture of this situation nfight emerge through an
organized study of Indian region data for the same period as in the
studies of Kent et al. [1995] and Sanga-Ngoie and Fukuyama
[19961.
3.6. RTR Variation at 10 Selected Indian Stations
During Different Phases of the Monsoon Season
In section (3.4) the results of variation of RTR index during
the premonsoon and monsoon seasons of the years 1970-1980 are
presented tbr the entire country (Table 2). hi the present section
we examine the monthly (March-September) and seasonal
(premonsoon and monsoon) variation of RTR index at 10 selected
Indian stations representing oceanic, coastal, island, and deep
inland and arid zone [Rao, 1976] clintate regimes. Table 6
provides the monthly mean values of RTR at these stations and
their other details. It may be noted that the normal dates of
the advance of monsoon around these stations range from the
earliest (-• May 20 at PBL) to the latest (July 1-15 at JSM) and
thus cover the entire period of advance of the monsoon across
the country. We first describe in brief the monsoon scenario over
the Indian region.
MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND INDIAN MONSOON 4181
Table 6. Monthly Mean RTR During March-September at 10 Indian Stations.
Station/(Climate Regime) Onset March April May June July August September
Dates
PBL (oceanic May 20 8 7 25 61 43 61 94
TRV (coastal June I 8 7 13 51 94 76 25
SGR I (island June 1-5 17 10 19 41 66 87 42
VSK (coastal) June 5-10 8 9 6 13 12 18 13
NGP (deep inland June 10-15 4 I 3 14 26 40 15
SNI (deep inlan June 10-15 12 2 7 37 118 120 44
BHV (coastal) June 10-15 I 2 5 52 67 76 35
STN (deep inland) June 15 to 5 3 3 16 35 45 27
July 1
GWL (deep inland) June 15 to I 2 2 10 19 30 17
July 1
JSM (arid zone) July 1-15 I I 7 9 15 20 6
The abbreviated names of the stations, their corresponding onset dates of the southwest monsoon, and climate regimes are also
shown. The onset date refers to the normal date of the monsoon onset for the region around the station. The stations are arranged in the
ascending order of the onset dates of the monsoon.
3.6.1. Monsoon Scenario Over the Indian Region. The stations a similar change in the RTR lies betxveen these extreme
significant feature of the •nonsoon season is the presence of a (1-2 and 10-50) values. This observation suggests that there is a
highly moist air mass in great depth over most part of the country substantial temporal and spatial variation in the RTR index across
[Srinivasan and Sadasivan, 1975; Rao, 1976]. During this season the country. For example, at PBL, TRV, and SGRI where the
the country receives rainfall from deep convective clouds [Rao monsoon onset is during the first week of June or earlier (see
and Rajamani, 1970; Rajamani and Sikdar, 1989]. These clouds Table 6), the June RTR values are significantly higher (much
develop as a consequence of tbrmation of low-pressure systems
and depressions [Mooley and Shukla, 1989] xvhich usually
originate in the head of the Bay of Bengal and travel westward
across the country. Rainfall associated with a low occurs over a
relatively much wider area than that associated with depressions.
Hence the lows and the depressions could be considered equally
important with respect to their contributions to the Indian
monsoon rainfall. The lows produce rainfall over a wide area
through convergence and vertical motion associated with them. As
a result of the transport of heat and moisture upward over the
lows, the periodical passage of these lows across the country
maintains the normal location and activity of the monsoon trough
that are conducive to good rainfall distribution over the country.
The significant climatological feature of the pattern of the
monsoon rainfall is the difference between its ascending and
descending phases [Ananthakrishnan and Pathan, 1991]. The
onset of the monsoon in early June is followed by progressive
increase in rainfall over the entire country. This phase attains its
larger than 25, an index derived by Zipser [1994]); and at NGP,
SNI, and BHV where the onset is between June 10-15, the June
values are in range from 14 to 52. By July 1-15 the country as
a xvhole is under the regime of the monsoon season. We note that
the RTR values in July at 80% of the stations are higher in range
from 30 to 220% of the values in June. It is noteworthy that in
August the RTR values are higher than that in June and July each
at 90% of the stations. This systematic change in the RTR values
appears to be consistent with the climatological onset phase of the
monsoon described earlier. In the month of September the RTR
values at 90% of the stations are lower than in the month of
August. The systematic variation in the RTR values during June-
August and August-September months thus appears to be quite
consistent with the ascending and descending phases of the
lnonsoon rainfall. One important observation of this study is that
for the first seven stations where the onset period lies between
May 20 and June 15, the RTR in most cases in June itself is
significantly enhanced. For the last three stations where onset
maxima in the last week of July and stretches over the months of period lies betxveen June 15 and July 15, the appreciable
June and July and part of August. The descending phase covers
the later part of August and September. The average monthly
rainfall for June-September is approximately 19, 32, 29, and
20%, respectively, of the seasonal rainfall for the country as a
whole. July and August, which together account for 61% of the
rainfall of the season and 50% of the annual rainfall of the
country, are generally regarded as constituting the established
phase of the summer monsoon. The climatological picture of the
•nonsoon rainfall described above now allows us to examine the
variation of monthly mean RTR at the stations representing
typical climate regimes (Table 6).
3.6.2. Examination of Monthly Mean RTR During March-
September at 10 Indian Stations Representing Typical
Climate Regimes. It is noted from Table 6 that at each station
the RTR value in the month of June is higher than any of the
months of the premonsoon season. At VSK the June value is 1-2
times higher, and at BHV it is 10-50 times higher. At other
enhancement in RTR is noted not in June but in July. This result
suggests that the RTR index could be useful for the identification
of the onset phase of monsoon over a region. This inference is
drawn on the basis of monthly mean data. However, the
application of this teclmique over shorter timescales and over a
large number of stations may show more details of the
application of RTR in a useful manner.
Comparison between the monthly mean RTR values during
June-September at PBL, TRV, SGRI, SNI, and BHV with
those at NGP, STN, and GWL shows that the values in the
former group of stations are much higher than in the latter. The
stations NGP, STN, and GWL are situated deep inland, and
hence the RTR values are relatively smaller. The stations where
the values are large are either situated along the onshore flux of
the monsoon current or are oceanic/coastal in character or are
located south of the axis of the monsoon trough (SNI) where the
monsoon rainfall is usually high. The characteristic difference
4182 MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
Table 7. Summary of Seasonal Mean Rll• for Four Categories of Stations
Climatological Location Premonsoon Monsoon
Category Mean Mean
(Station (Standard (Standard
Abbreviation) Deviation) Deviation)
arid zone situated in the far 3 (3) 13 (5)
(JSM) NW region of India
continental located deep inland 4 (3) 38 (32)
(NGP, SNI, STN, in the central parts
and GWL) of India
Coastal within the tropic, 9 (3) 61 (26)
(TRV) close to the equator
and on the west
coast of India
(BHV) within the tropic, 3 (2) 57 (15)
along the west
coast but nearer to
the tropic of cancer
with proximity to
arid zone
(VSK) within the tropic, but
on the east coast
oceanic/island situated in the Bay of
(PBI) Bengal and near the
equator
(SGRI) situated at the head
Bay of Bengal
7(1) 14(2)
14 (7) 62 (19)
in the values of the RTR is thus attributed to the continentality or
the climatic regimes to which the stations belong. Table 7
summarizes the seasonal mean RTR in the four categories of
stations and makes this point of viexv more clear.
At station JSM we note that the premonsoon season RTR value
is the lowest, and the value even in the monsoon season is
not very high. In this respect the station JSM stands out from the
rest of the stations considered in this study. An explanation for
this phenomenal behavior of the RTR is given below. The station
JSM is situated in west Rajasthan, that is, in a semiarid zone of
NW India. Over this area the premonsoon season rainfall occurs
mostly from the thunderstorms [Rao, 1976], but their seasonal
frequency is less. During the monsoon season this region comes
under the heat low with subtropical ridge aloft. According to
Bryson and Baerries [1967] this region coincides with the
divergent sinking of the subsiding air at 700 mbars. In this region
the mean monsoon flow brings monsoon air below 1.5 Ion after
travel over hot land across the Indian subcontinent. Only under
the influence of synoptic systems does the thick monsoonal air
mass occasionally reach this region. These conditions do not favor
the occurrence of rainfall. Hence there is not much change in the
RTR in the monsoon season either.
3.7. Monthly Mean Electrical Conditions of the Tropical
Mesoscale Convective Systems and Isolated
Deep Convective Storms
The RTR analysis in section 3.6 has shown significant
variation for the continental and maritime convective rainfall over
different climate zones of the Indian region. These results thus
suggest that the application of the RTR technique is suitable for
analyzing the tropical rainfall regimes. The present results,
however, cam•ot be compared with those of Rutledge et al.
[1992], Williams et al. [1992], and Petersen and Rutledge [1996]
because the cloud electrical conditions of tropical mesoscale
convective systems and isolated deep convective storms over the
Indian region are not lmown. This argument is particularly true
because only the thunderstorm days data are not the best measure
of cloud electrification. An important requirement here is to
obtain actual infom•ation about the degree of cloud electrification
of the premonsoon and monsoon season thunderstorms over the
Indian region. This significant information may throw light on the
contribution of lightning and point discharge current (PDC) to the
AC and DC parts of the global electrical circuit t¾om the monsoon
and premonsoon season thunderstorms.
The monthly (April-September) mean electrical properties of a
large number of mesoscale and isolated deep convective
thunderstorms that occurred over the Pune region during 6 years
from 1972 to 1977 are presented below.
Figures 7a and 7b show storm-associated monthly
mean (1) maxinmm amplitude and (2) duration of PDC
averaged over the monthly number of thunderstom•s during 1972-
1977 at Pune, respectively. Figures 8a-Sf present the samples of
the actual recordings of PDC and electric field for the two dates
of thunderstorms each in the premonsoon and monsoon season at
Pune. These samples are added to complement the results shown
in Figure 7a. The details of the electronics, measurement
technique, accuracy of the measurement of PDC and electric field,
and their sign convention and the location details at Pune are
available elsewhere [Selvarn et al., 1977, 1980; Manohar et al.,
1991; Manohar and Kandalgaonkar, 1995]. The number of
storms used to work out the monthly means are shown on the
PDC curve of Figure 7a. These are 40, 70, 70, 50, 100, and 10%,
respectively, of the total monthly storms during the period
mentioned earlier. The storm-associated maximum amplitude of
the PDC for each storm during a given month was picked up
MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON 4183
7.0 4 .... 1 250
, _
TE 200
6.1 3
E
.Ol-o 2
100 ß
•3.5
50
O , •0
Mar Apr May Jun Jul Aug Sep
Months
•isure ?a. Sto•-ass•iated mo•y (Apdl-Scptcm•) m• ma•• •p]imdc of •int di•h•Sc c•ent (•) •d mo•y
me• •a•a]] (• •]]•ete•s) at •e d•8 1972-1977.
from the daily recordings of PDC, and the monthly mean was
worked out over the number of storms during the month.
Similarly, the daily duration of PDC on a storm day was found by
adding the durations of individual spells of current (see Figure
7b). The monthly mean duration was worked out over the
number of storms during the month.
In Figure 7a the scale to the left of the ordinate refers to the
electric field corresponding to the PDC data. The conversion of
the PDC to electric field was made using the empirical relation of
I4q•ipple and Scrase [1936]. The other curve in Figure 7a presents
the monthly mean rainfall for the period under study. Several
250 .....
:3
..c 200 ....
2
_
.c_
o 150
:3
a lOO-
O '
50
0
Mar Apr May
Figure 7b. Same as Figure 7a but for •nonthly mean duration
(minutes) of PDC.
previous studies have also employed the mesonetwork of PDC
to document the electrical activity of thunderstorms [Asuma et
al., 1988; Engholm et al., 1990; Williams et al., 1992].
It is noted from Figure 7a that the storm-associated monthly
mean maximum electric field and PDC in April and May
thunderstorms are nearly the same ( -- 5.7 kV m'l; 2.7 IXA). The
high values of the surface electric field and PDC are an indication
of the high, electrically active state of a thundercloud. Under such
conditions the cloud region electric field is usually very intense to
cause dielectric breakdown of the atmosphere and thus trigger
lightning [Vonnegut, 1963]. The intensity of electrification of a
thundercloud is reckoned by the frequency of lightning (intracloud
or cloud to ground). The frequency of lightning is normally
determined by the frequency of transient field changes/reversals.
Such field reversals are usually seen even in the recordings of
the vl_xJ (see Figure 8). Such occasions of very strong
electrification of thunderclouds are usually encountered during
the later period of the month of April and run through the month
of May at Pune [Manohar and Kandalgaonkar, 1995]. Their
frequency of occurrence and the electrical activity gradually
reduces by the first week of June over the Pune area. In the June
thunderstorms the monthly mean maximum value of PDC is
seen to be diminished (1.05 IxA). This diminution in PDC is -*
61% of its value in May. This point is further explained below.
At Pane the normal onset date of the monsoon is June 10. The
mean maximum amplitude of the PDC for the June thunderstorms
prior to the onset date of monsoon is shown in Figure 7a (by an
asterisk). This value shows -• 45% diminution of PDC in
comparison with that in May; while the PDC value for the
postonset date thunderstorms (shown by a triangle in Figure 7a)
showed-* 73% diminution in comparison to that in May. Thus it
is inferred that with the advent of the monsoonal regime, the
electrical vigor of the thunderstorms in this area is significantly
diminished. The features of cloud electrification in the
Jun Jul Aug Sep premonsoon and monsoon season thunderstorms described above
are themselves clearly evident in Figure 8. The monthly mean
Months rainfall curve in Figure 7a shows a systematically increasing
trend from April until July, while the PDC curve and the electric
field values show systematic reduction during the same period.
4184 MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
2.80
2.10
0.70
0
-0.7G
-I.4
-2-
--2
1.0
o
-I.0
i i i i i i i I
PUNE ( 1'8' 1974)
I I I I i
i i I i i i i i
I i i I t ; I I 814
630 1700 1730 18 0 1830 I 5
TIME ( IST )
'i 3'50
-1/2 ß 80
•2.10
'Jl.40
I
•0'70
'0
i
-•-0.70
4-•.,•o
i
4-2.80
J-3.50
/I
/
i
71 O•A
----• 0
,,
• -1.0 lU. A
',
,,
190C
i i !
z
u
•' -IO•A o
u
z
_
o
a_ I I 3
1930 2030 21 0 2230
TIME (IST)
Figure 8. Shows sample records of surface atmospheric electric potential gradient and PDC for two thunderstorm days each in the
monsoon season i.e. August 1, 1974 and August 11, 1977 (see diagrams a, b, e and f) and premonsoon season i.e. May 14, 1974 and
May 30, 1975 (see diagrams c and d) at Pune. Letters A, B, C, and D in the monsoon season thunderstorm record on August 11, 1977,
indicate the occasional occurrence of lightning marked by the transient changes in the two parameters. Records for the premonsoon
season thunderstorms show highly enhanced activity of these two parameters associated with long-time severe occurrence of lightning.
Pronounced differences in the electrical activity of the seasonal thunderstorms may be noted (see text for more details).
Thus it is noted from these data that the rainfall yield and the
cloud electrification during the two convective rainfall regimes
are differently associated. This analysis also suggests that
although the •nonsoonal convection is deep and the rainfall is
higher, it is not deep enough to cause lightning.
The PDC duration curve in Figure 7b shows one-to-one
correspondence •vith the monthly mean maximtm• amplitude of
PDC. This systematic behavior of PDC suggests that the
thunderstorms in the premonsoon season are strongly electrified
and are of longer duration than the monsoonal ones. Thus
contribution to the AC and DC parts of the global electrical
circuit is more from the continental deep convection in this part of
the h•dian region. The results of the sununary of cloud
electrification presented here pertain to only one location,
namely, Pune. The authors do not have similar data from other
parts of the country. Further, we have presented our results
the months of premonsoon and monsoon seasons. The
presentation of these results on the basis of break period
MANOHAR ET AL.: THUNDERSTORMS OVER [NDIA AND [NDIAN MONSOON 4185
SURFACE ELECTRIC POTENTIAL GRADIENT, PUNE (:50-5.1975)
/ I i ! l•l• Kv m -I
0 0
Kv• •
0.7 5 6 '"' •5.6
-2.9/ ' i-2'9
C POINT DISCHARGE CURRENT (,,u.,A) / •A
L -1'4 5,,u.A
- 1'45').L A '-•
2.c, I 2.9•A
415 1515 1615
TIME(IST)
-5'6/ . L SURFACE ELECTRIC POTENTIAL GRADIENT (Kvm-').
o• PUNE ( $0.5. 1975)
5. \6 'll\ •'•\' ¾1"" ' '• NO RECORD AHAD DUE TO INSTRUMENT FAILURE
-5'6
Kvm-•
Kvm -•
-2.9
t - I. 45..,u.A I
2.
POINT DISCHRGE CURRENT (/u.A) /-,,•'A 9
J / • -1' 45,u.A "/
0
1620 1720 1820 1920
TIME (IST)
7.63
/6.11
4'58
$'05
1.52
'•- 3.02
t•.•_ 4' 58
i i -7.63
,,o! .....
2'46/- /•l•j•/i ] tl '
64
-,.4• •V
-,.,o• , , , , -4.,o
,•o• ,•, ,•,• ' ,•,• ,•'• ,•
TI ME ( I ST )
Figure 8. (continued)
4186 MANOHAR ET AL.' THUNDERSTORMS OVER INDIA AND INDIAN MONSOON
9.16
- ?? •'? PUNE (11'8'1977)
,.z, x •' x A 5.49
•.0
0
-I.83
z
I I I I I -9.16
1545 I 1615 1650 1645 I?00 1705 I710 1715
6OO
e T,M[ ( ,ST )
,9.16
/ PUNE ( 11.8.1977 ) '
I -'5-40
; .•,.,•
o L •o
4-1-85
-5-49
,
, c D
i I I I I I I I I-9-16
17•'0 11'25 11'30 11'35 1740 1745 I?SO 1755
TIME (IST)
POINT DISCHARGE CURRENT RECORD. PUNE (11.8.1977)
- 2.87.,u. A
B C O
-I-43•A •____ _•_••
I. 43,a.A A -•
I
2'87.,u.A
1553 1623
I I I I
1653 1723 1753 1823
TIME ([ST)
- 2'87,a.A
- 2.87
-I-43
1.43
2.87,•,A
1853
Figure 8. (continued)
and active monsoon conditions would have been better for
their comparison with the results of Williams et al. [1992] and
Rutledge et al. [1992]. As the results are averaged over a
reasonably long period, it is felt that they may not be
largely different from the break period and active monsoon
conditions. Petersen and Rutledge [1996] examined the
characteristic differences in cloud-to-ground lightning flash
densities and rain-yields over large spatial and temporal scales for
different climate regimes of the globe. Our results cannot be
directly compared with theirs. However, in a broader sense, it
appears that some common conclusion in relation to rain yield
versus cloud electrification and RTR exists between our study and
that of Petersen and Rutledge [1996].
4. Conclusions
1. The latitudinal intermonth comparison of the thunderstorm
activity during the premonsoon season (M-A-M) showed a
significant increase in the number of thunderstorm days, and their
activity decreased with increasing latitude. The analysis for 4
months of the monsoon season (June-September) showed that the
latitudinal variation was in contrast to the premonsoon season
such that the thunderstorm day activity increased with increasing
latitude. An interesting result of the intraseasonal comparison
indicated that during the months of July and August, midmonsoon
season, thunderstorm activity maintained a seasonal low profile
throughout the country, whereas during the months of June and
September, onset and withdrawal phase months of the monsoon
season, the activity was more pronounced. The other result of the
study showed that the month of January is the most preferred
period for minimum thunderstorm activity over the Indian region.
2. The latitude belts associated seasonal variation of the
thunderstorm activity within the four belts (8ø-10 ø, 10ø-15 ø, 15 ø-
20 ø, and 20ø-25øN) and indicated clear signals of their
semiannual oscillation. These semiannual signals also showed
systematic changes in their amplitudes and a phase shift
according to the northward shift of these belts from the equator.
These systematic changes in their signals are noted to be a
function of latitude and season and appear to be consistent with
the seasonal migration of the ITCZ and the solar heating of the
Indian landmass.
3. We have compared the above mentioned latitude-belt
averaged seasonal thunderstorm day activity with the similar data
of monthly mean maximum values of surface wet-bulb
MANOHAR ET AL.: THUNDERSTORMS OVER INDIA AND INDIAN MONSOON 4187
temperatures (Tw). The seasonal variation of Tw showed excellent
one-to-one correspondence with that of the number of
thunderstorm days in each latitude belt. This comparison has
shown that the occurrence of the thunderstorms per 1 øC rise in T•,
is nearly 3-4 times larger in the lower latitudes where the average
magnitude of Tw and its range of variation are higher and
smaller, respectively, than those in the higher latitudes. This
sensitive association between Tw and rottuber of thunderstorm
days appears to be consistent with the results where Tw and
lightning flash counts have been studied in the other regions of
the tropics.
4. Using monthly rainfall data of 58 conunon stations, a relation
between seasonal rainfall and number of thunderstorm days was
examined for the premonsoon and monsoon seasons of the 11 year
period from 1970 to 1980. It was noted that the yearly variation of
the two quantities was highly correlated.
5. The variation of the RTR index for the Indian region during
the premonsoon and monsoon seasons of the years 1970-1980 was
studied. Comparison between these seasonal mean values of the
RTR indices showed that the RTR index undergoes a change from
the average 9.6 in the premonsoon season to 36.5 in the monsoon
season. It was observed that these seasonal values of the RTR are
consistent with previous works in the other regions of the tropics.
It was found that the premonsoon season and monsoon season
rainfall are often associated with the loxver and higher values,
respectively, of the RTR. The RTR variation at 10 selected Indian
stations representing different climate regimes were studied in
relation to the ascending and descending phases of the Indian
southxvest monsoon rainfall. It was found that the rainfall
quantity in the RTR is the major factor that decides its variation.
It was also found that the RTR is higher as one enters the
monsoon and is still higher when one proceeds to the mature
stage of the monsoon season. The other result of the RTR analysis
suggested that the large-scale application of the RTR technique
about the validity of the use of just thunderstorm days as a
general measure of their contribution to the global electrical
circuit.
Acknowledgments. The authors are thankful to the two
reviewers of this paper for offering constructive suggestions. This
has helped in considerable improvement of the text and the
manner of its presentation. The help received from E.R. Williams
of M1T on several occasions during the progress of this work is
worth mentioning and is thankfully acknowledged. The authors
express their indebtedness to the Director General of
Meteorology, IMD, New Delhi, for the data used in this study.
Thanks are due to T.S. Pranesha for going through the manuscript
and for providing valuable suggestions. The authors wish to thank
R.H.Kriplani for useful discussions. The authors are also thankful
to A.S.R. Murty, A.M. Selvam, and the Director, ITYM, for their
constant support of our research activity. Help received
from R. Vijayakumar and V.R. Mujumdar in computer graphics is
also acknowledged.
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$. $. Kandalgaonkar, G. K. Manohar, and M. I. R. Tinmaker, Indian
Institute of Tropical Meteorology, Dr. Homi Bhabha Road, Pashan, Pune 411
008, India. (manoh•opmet.emet. in ß sskandal•opmet. emet. in)
(Received August 21, 1997; revised July 9, 1998;
accepted July 13, 1998).
... Therefore, further understanding the effects of ENSO on TS variations over Bangladesh is necessary for disaster prevention and mitigation. Some studies have performed globally to investigate the effects of ENSO on TS activity [5][6][7][8][9][10]. Manohar et al. [5] found the influences of El Niño on thunderstorm occurrences during the Indian monsoon season. ...
... Some studies have performed globally to investigate the effects of ENSO on TS activity [5][6][7][8][9][10]. Manohar et al. [5] found the influences of El Niño on thunderstorm occurrences during the Indian monsoon season. Allen and Karoly [7] showed a significant influence of ENSO on the spatial distribution of thunderstorms in Australia. ...
... El Niño 1976, 1977, 1986, 1987, 1988, 1997, 1998, 2006, 2007, 2014La Niña 1975, 1983, 1984, 1985, 1989, 1996, 1999, 2000, 2001, 2005, 2008, 2010, 2012Normal 1978, 1991, 1992, 1994, 1995, 2002, 2003, 2004, 2005, 2009IOD-positive 1982, 1983, 1994, 1997, 2006, 2012IOD-negative 1975, 1981, 1989, 1992, 1996, 1998, 2010, 2014 2.3. Methods ...
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Thunderstorms (TS) are one of the most devastating atmospheric phenomena, which causes massive damage and adverse losses in various sectors, including agriculture and infrastructure. This study investigates the spatiotemporal variabilities of TS days over Bangladesh and their connection with El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). The TS, ENSO and IOD years' data for 42 years (1975-2016) are used. The trend in TS days at the spatiotemporal scale is calculated using Mann Kendall and Spearman's rho test. Results suggest that the trend in TS days is positive for all months except December and January. The significant trends are found for May and June, particularly in the northern and northeastern regions of Bangladesh. In the decadal scale, most of the regions show a significant upward trend in TS days. Results from the Weibull probability distribution model show the highest TS days in the northeastern region. The connection between TS days and ENSO/IOD indicates a decrease in TS activities in Bangladesh during the El Niño and positive IOD years.
... Consequently, studying of lightning activity within convective systems have great significance over the Indian subcontinent as they are the most frequent cause of fatalities more commonly occurring during the rainy and summer seasons (Singh & Singh, 2015). Seasonality of lightning activity over India was depicted by Manohar et al. (1999) and Ranalkar and Chaudhari. (2009). ...
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X‐band radar observations are integrated with lightning location network observations to investigate the relationship between convective storm properties and lightning over the Western Ghats during a monsoon season (June–September 2014). Convective storms (cells) were identified using an objective‐tracking method and instantaneous lightning strikes within the radar domain were then linked with observed storms. This spatio‐temporal sampling of individual convective cells and lightning has facilitated process‐based study of electrified convection over the Ghats for the first time. Storm and lightning occurrences are typically high during monsoon onset and withdrawal months of June and September, respectively. A spatial correspondence between deep‐intense storms, lightning, and intense convective cores indicated presence of large hydrometeors in the mixed‐phase region of storm supported by strong updrafts and is essential for lightning. The large‐scale instability that peaked during afternoon hours was a key factor in the formation of deep‐intense storms and lightning. Results show that majority of lightning‐producing storms are located on the leeward side as opposed to the windward side. These storms have deeper top‐heights, larger areas and vertically integrated liquid, and an enhanced hail probability than those devoid of lightning. Warm season convection in the study area is accompanied by the preponderance of negative Cloud to Ground (−CG) flashes over positive Cloud to Ground (+CG) lightning. Storms with +CG features exhibited much higher (>2 times) vertical airmass flux in the mid‐troposphere (6–9 km) than storms without +CG features. Furthermore, for majority of +CG storms, intracloud flash occurrences increased significantly above the freezing level.
... The tropical area is well understood as a belt of the strong, convectively active region (Manohar et al. 1999). In tropical India, the sub-Himalayan West Bengal experiences the highest number of TS with a frequency of more than 100 days in a year and shows an increasing trend (Tyagi 2007;Singh et al. 2011). ...
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The Kolkata metropolitan region, located in eastern India, is one of the most densely urbanized areas, with significant thunderstorms reported during the pre-monsoon season. The Weather Research and Forecasting (WRF) model is used to investigate the influence of urban-induced land use and land cover (LULC) change over Kolkata during the pre-monsoon thunderstorms. Multiple thunderstorm events reported during 2014–2017 are simulated using a high (Hurb) and low (Lurb) urban LULC scenario. The presence of higher urban pixels in Hurb case favors the enhancement in precipitation mainly over central and northern parts of the city in the downwind direction. Urban Heat Island (UHI) effect is more evident during the nighttime, with a temperature difference of up to 0.5 °C. However, the UHI impacts the vertical structure of the boundary layer (BL) more during the daytime due to prevailing higher temperatures and dominant surface heating. The analysis reveals positive contributions of the ground and sensible heat fluxes to the near-surface UHI intensity. The surfaces over the urban patch and surrounding areas experience a relatively drier atmosphere than their rural counterparts. Over the identified urban patches, a significant impact on meteorological variables is seen near the surface and within the BL in the case of Hurb compared to Lurb LULC scenario. The urbanization over Kolkata stimulates the BL and the local meteorology encouraging nighttime UHI, afternoon or evening moist convection, and consequent occurrence of thunderstorms to result in enhanced and distinctly distributed rainfall over the city and its neighborhood during pre-monsoon months.
... Many studies indicated the interconnection between lightning and CAPE (Manohar et al. 1999;Kandalgaonkar et al. 2005a, b;Manohar and Kesarkar 2005). Kandalgaonkar et al. (2003) observed that highest lightning activity is recorded in May month in-particular at 0800 UTC. ...
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The current work was attempted to analyze various atmospheric parameters that influence the convective potential energy available (CAPE) development over Puducherry region, India during pre-monsoon season. k-index (KI), total totals index (TTI), improved K-index, improved total totals index, total precipitable water (TPW), deep convective index (DCI), S Index (SI), Dew point depression (DPD) and humidity index (HI) are the atmospheric related parameters that are utilized for this study. Fifth generation ECMWF atmospheric reanalysis (ERA5) daily data for the PRMS of 2021 were used to measure all rainfall-related variables. High CAPE values were seen during May month and low values are seen during March month. CAPE values have increased dramatically in the last 5 years (2016–2020) compared to the previous 15 years, indicating the severity of thunderstorms and convective activity over Puducherry. In all pre-monsoon seasons, even on moderate CAPE days, high daily mean rainfall was observed. The improved KI and TTI values were more helpful in detecting the severity of instability on some high CAPE days than the regular KI and TTI parameters that indicate fewer chances for instability. During the month of April, high TPW values were observed, indicating a high availability of moisture in the atmosphere that contributes to convective instability. With an increase in temperature and potential temperature parameters, the relative humidity parameter rises, contributing to severe atmospheric instability.
... The threshold value of VTI for thunderstorm occurrence is 26; the greater the value, the larger the vertical total index, and the more chance for a rising air parcel to continue its upward movement (Miller 1972). The ratio of mean rainfall to lightning flash count provides RLR as an indicator of distributions of lightning activity over the Earth's surface (Manohar et al. 1999;Zipser 2002). show that the lower RLR value is associated with high lightning activity and heavy convective rainfall over the land. ...
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This paper presents the analysis of the frequency of lightning strikes associated with thunderstorm and precipitation distributions over smooth oceanic surface relative to that over solid earth surface in the tropical region. Long-term (1998–2014) data retrieved from the Lightning Imaging Sensors (LIS) of the Tropical Rainfall Measuring Mission (TRMM) satellite shows lightning flash counts over Indian landmass found to be 9.1 times more than those over the smooth oceanic surface of the Arabian Sea and the Bay of Bengal. On the other hand, the annual variation of rainfall-to-lightning ratio (RLR) is found to be 0.8 over Indian landmass, whereas it is 10 over the oceanic surfaces. We discuss the convective strength of thunderstorm distributions over land and oceanic regions by examining the relationships of RLR to the Bowen ratio, sea surface temperature (SST), and maximum air temperature over land, maximum updraft speed, and Aerosol Optical Depth (AOD) and cloud ice water content. The RLR shows high positive Pearson’s correlations with the Bowen ratio, maximum updraft speed, AOD, and cloud ice water content over land region relative to those measured over the oceanic region. The RLR also shows negative correlations with SST, maximum updraft speed, AOD over the oceanic region. The results are applicable in understanding of the convective characteristics of thunderstorm distributions and lightning flashes over the tropical regions of the world.
... Hansen and Lacis (1990) studying the role of solar radiation and atmospheric aerosol concentrations compared with anthropogenic greenhouse gases in global climate change, found that solar variability will not counteract greenhouse warming. Manohar et al. (1999) investigated the seasonal thunderstorm day activity of five latitude belts over the Indian region and concluded that with each increase of one degree Celsius from wet temperatures, the incidence of thunderstorms increases by 3-4 times in the lower latitudes, and January is the minimum period of thunderstorm activity in the vast region of India. Schlegel et al. (2001) investigating thunderstorms, lightning and solar activity in middle Europe claimed that there is a significant correlation between lightning frequency, sunspot number and other descriptions of solar activities. ...
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In this study, using various statistical methods such as trend analysis, Mann–Kendall (MK) test and Morlet Wavelet Analysis (MWA), the Frequency of Thunderstorm Days (FTD) and its relationship with Sunspots Frequency (SF) and Global Atmospheric Carbon Dioxide Concentration (GACDC) in the 68-year statistical period of 1951–2018 in Tabriz Synoptic Station has been studied. In terms of distribution and temporal concentration, the highest incidence of Thunderstorm Days (TD) is concentrated in the months of May and June and the lowest in the cold months of the year, i.e. January, February and December, with spring having the highest and winter having the lowest FTD. The analysis of the trend of FTD in the study area showed that except for the trends of January, February, June and December, which have no clear and significant trend, other months, especially April, have an upward and significant trend and the seasonal trend of FTD in Tabriz increase significantly in all seasons, especially in the peak of spring; Also, in the annual time scale, a significant upward trend was observed. Decadal changes in the FTD show a sharp increase in the occurrence of TD in Tabriz, especially in the second decade of the twenty-first century. The results of MWA showed that the FTD, like sunspots, has a periodic cycle in its emergence and occurrence. Based on the results of cross-wavelet analysis, there is a strong negative and anti-phased relationship between SF and the FTD in Tabriz. Also, according to the results of MWA, no significant relationship between the GACDC and the FTD in Tabriz has been found.
... According to the past investigations, thunderstorm activity has correlation with the meteorological parameters (Manohar et al., 1999). For revealing any possible connection between meteorological parameters behavior during fair weather and thunderstorms we have investigated atmospheric pressure, wind speed, temperature and humidity during the above discussed cases from 2012 to 2019. ...
Article
Ground-based electric field measurements during fair weather and thunderstorms are key parameters for Global Electric Circuit investigations and climate change assessment. We study the variations of near surface electric field during fair weather and thunderstorms. In this work we present daily, monthly and yearly distribution of thunderstorm activity from 2012 to 2019 at different research stations of Cosmic Ray Division (CRD) of the A. Alikhanyan National Science Laboratory (Yerevan Physics Institute) which has been done the first time for mentioned locations. The distribution of fair-weather days based on electric field variation and meteorological parameters are also discussed. According to results, thunderstorm activity is very high from May to June and the most active part during the day is from 15:00 to 20:00 LT at three different altitudes.
... A total of 5259 people died due to lightning strike from 1979 to 2011, with a fatality rate of about 0.24 per million population per year (Singh and Singh 2015). The climatological features of lightning strikes have been studied based on the Tropical Rainfall Measurement Mission satellite dataset, where the monthly and seasonal variations in lightning strikes across different geographical regions of India were discussed (Manohar et al. 1999;Nath et al. 2009;Lal and Pawar 2009;Tinmaker et al. 2010;Kumar and Kamra 2012;Tinmaker and Chate 2013;Murugavel et al. 2014). The extensive hot and humid land region of the inter-tropical convergent zone in eastern India is more likely to lead to the development of high flash rate density (Tinmaker and Chate 2013). ...
Article
Full-text available
Lightning has emerged as one of the major weather hazards in India. Lightning forecasts are introduced into the operational National Centre for Medium Range Weather Forecasting regional unified model (NCUM-R) to predict the events in advance. A new blended electric scheme following McCaul et al. (2009) is employed to predict the lightning flash count as a useful tool for day-to-day prediction of thunderstorm activity and intensity. A total of four numerical experiments, namely CNTL, EXP1, EXP2, and EXP3, were conducted by using the NCUM-R based on the graupel water path (GWP) amount and the process allowing the snow-rain collisions to form a graupel. The numerical simulation forecasts are compared with the Indian Air Force and Indian Institute of Tropical Meteorology earth network lightning sensor data. A case study of a convective system associated with a severe lightning event that occurred on 7 February 2019 over the northern region of India is diagnosed. The observations indicate stronger lightning cells present over the Haryana–Punjab region, with a leaf-like extension through south-eastwards and continuing up to the Himalaya foothills. Such south-eastward progression of the lightning system is well captured in all the experiments. However, when the GWP threshold is set to 200 g m−3, and allowing for the snow-rain collision process, the counts are improved by approximately 50% compared to the control run, and is closely agree with the observation count. Temporal evolution characteristics of the vertical distribution of the hydrometeors and vertical velocity support the formulation of the revised lightning parameterization scheme. Statistical metrics were computed for the pre-monsoon month indicating the robustness of the model with the revised scheme. Hence, the revised scheme is chosen for the operational implementation of the lightning flash prediction system of the NCUM-R. Further modifications of the electric scheme are warranted based on the cloud microphysics response over different weather regimes.
... A recent study by Liou and Kar (2010) explained that CAPE and lightning are interconnected over Taiwan. Some researchers tried to analyze the lightning activity and its relationship with CAPE, rainfall, air temperature, relative humidity and specific humidity (Manohar et al. 1999;Kandalgaonkar et al. 2005a, b;Manohar and Kesarkar 2005;Tinmaker et al. 2010;Tinmaker et al. 2015). Factors such as surface air temperature, rainfall and CAPE showed good correlation which indicates their influence on lightning activity over Maharashtra, India (Tinmaker et al. 2015). ...
Article
This paper brings out the interconnection of flash rate density (FRD) with convection and stability parameters over Andhra Pradesh (AP), India. The convection parameters include rainfall, relative humidity, specific humidity, surface air temperature (SAT) and air temperature (at 850 mb). The stability parameters include convective available potential energy (CAPE), lifted index, K-index, total totals index (TTI), humidity index and total precipitable water. Both convective and stability parameters indicate good correlation with lightning activity. SAT and AT 850 mb have shown good correlations with lightning, which is a clear indication of interaction between warm air and dry air. CAPE and TTI have shown strong positive correlation with lightning activity. The correlation coefficient between FRD and CAPE is 0.81. We have also studied the influence of convective and stability parameters during lightning and no lightning activity. Later, we also attempted the estimation of lightning activity by using artificial neural network model. By using convection and stability parameters, six learning algorithms were used for training the artificial neural network. Scaled conjugate gradient backpropagation training algorithm has given the better estimation, whereas resilient backpropagation training algorithm has shown the poor estimation of FRD.
Article
This paper presents a relationship of lightning activity with maximum air temperature, Bowen ratio, rainfall, cloud ice contents and Aerosol Optical Depth (AOD) over India during transition period from dry to wet seasons. Lightning flash count data and weather parameters are retrieved from the Tropical Rainfall Measuring Mission (Lightning Imaging Sensor - LIS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) satellites for the period of 17 years (1998–2014). The Pearson correlation between lightning flash count and Bowen ratio is found to be coefficient of R = 0.95 for both dry and wet seasons. For dry season, the Pearson correlation of lightning flash count with surface maximum air temperature is found to be coefficient of R = 0.97 whereas, that for wet season negative coefficient of R = −0.89. The comparative analysis of Pearson correlations of lightning flash counts with AOD, rainfall and cloud ice content are found to be coefficients higher by 20%, 28% and 34% for dry season than those of during wet season, respectively. The results of lightning activity and weather parameters in comparative analyses over the Indian region may be useful for better understanding the differential convection characteristics during transition period of dry to wet season. The results are also important for estimations of impact associated with lightning strikes to ground during dry and wet seasons.
Article
Vertical velocities at 850, 700, 500 and 300-mb surfaces, associated with a monsoon depression have been calculated making use of a 4-level geostrophic baroclinic model. The region of upward vertical velocities agrees well with the region of rainfall. The contributions by the vorticity advection and divergence terms in the vorticity equation have been evaluated, and the importance of the divergence term in formulating a numerical model for the monsoon is brought out.
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In recent years 1965 and 1972 have been years of severe drought over India and neighbourhood. The dynamical and thermal features of these seasons are contrasted with those of normal monsoon, The main local abnormality during drought years is the shift of the monsoon trough northwards and development or anti-cyclones over central India in the lower troposphere. The main abnormality in the thermal field is that temperatures are considerably below normal over southern parts of USSR. Iran. Afghanistan and north India. Quasi-geostrophic 'w' is used to delineate the vertical circulations. It is seen that the north-south circulation and associated energy conversion is weaker and the east-west circulation more marked during breaks. A model of the vertical circulations during drought periods is suggested. The intensity phase movement and vertical and horizontal tilts of the different wave number in the mid-latitudes during drought and in the preceding months is contrasted with the normal, In 1972 wave Nos, 5.9 were abnormally pronounced and showed some dynamical differences.
Article
The differences in the thermodynamic structure of the atmosphere over India and the adjoining areas between active and weak monsoon (southwest) periods in the various meteorological sub-divisions are presented. The mean dry bulb and dew point remperatures and equivalent potentioal temperatures during active and weak monsoon were calculated at all the radiosonde stations in India and neighbourhood and studied. The results show that whatever be the monsoon activity, there is no significant change in the dry bulb temperature at any level and the moisture content in the lower tropospheric levels remains high without any appreciable variation. The main change is in the moisture content in the mid-tropospherica levels. The implication of these results in the vertical circulation associated with active and weak monsoon and in the radiation budget are brought out, since the vigorous ascending air in the areas of strong monsoon ultimately subsides slowly over the regions of weak monsoon. The region of the seasonal monsoon trough has been indentified as the area of maximum total energy both in active and weak monsoon is least. This is also the area where convective instability is present upto greater heights than elsewhere. The estimated heights of cb tops are highest in the region of the seasonal monsoon trough. It is also shown that there is no reversal of virtual temperature gradient over the northwest India in the midtropospheric levels during the SW monsoon season.
Article
Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity. Upper tropospheric monthly mean values of geopotential height; temperature and wind at about sixty Indian and neighbouring stations during the pre-monsoon months of April and May, and monsoon months of June, July and August, have been used to study the inter-annual variation in the upper tropospheric fields for the years 1967 to 1976. Thickness anomalies at selected Indian stations suggest, ( 1) that warming/ cooling of upper troposphere is a large-scale phenomenon over north and northwest Indian region and (2) that the anomalies as observed in the months of April and May (more predominantly in May) , generally persis1 during the subsequent monsoon months. It is found that the years of warmer (cooler) upper troposphere over north and northwest Indian region, beginning from the pre-monsoon month of May (and perhaps from April) and generally persisting during the subsequent monsoon months, are the years when the summer monsoon rainfall over India was good (poor). The upper tropospheric circulation over Indian sub-continent during the month of May in the contrasting monsoons show marked difference in the wave structure of sub-tropical westerlies over northwest of Indian region. The study suggests that the upper tropospheric quasi-permanent wave in the westerlies can significantly effect the thermal field over north and northwest Indian region during the pre-monsoon - months, which in turn, may influence the activity of the subsequent summer monsoon over India. This may have value in long-range prediction of monsoon activity.
Article
The vorticity budget terms are examined for the eastern and western halves of three depressions in a Lagrangian reference frame. The vorticity divergence term and the horizontal advection term (in the upper troposphere) apparently caused the westward movement, whereas the vertical advection term and tilting term had little influence on the movement. The residual term is predominantly negative in the western half of the monsoon depression where convective activity and rainfall are greater, compared to the eastern half. The computational study of energetics shows that the zonal available potential