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Evolution of the impacts of the 2009–10 El Niño and the 2010–11 La Niña on flash rate in wet and dry environments in the Himalayan range
Abstract
Impacts of the 2009–2010 El Niño and the 2010–2011 La Niña events on the lightning activity in the climatologically dry and moist regions of the Himalayan range are studied from the 18-year (1995–2012) data obtained from the combination of Optical Transient Detector and Lighting Imaging Sensors on the TRMM satellite. Average flash rates in both regions are higher than the 18-year normal during both El Niño and La Niña events. Our results suggest that the impacts of El Niño and La Niña need to be examined season-wise separately in moist and dry regions. During El Niño, the flash rate increases from the month of February into the pre-monsoon season but has no significant effect in the monsoon and post-monsoon seasons in the moist region. On the contrary, flash rate does not change during the pre-monsoon but is higher than normal in the monsoon and lower than normal in post-monsoon season in the dry region. During La Niña, it does not change from its normal value in any season of the moist region and even in pre-monsoon season of dry region. However, it is higher than normal in the monsoon and post-monsoon seasons of the dry region. In the dry region, while flash rate is highly correlated with convective available potential energy (CAPE), surface temperature, and convective rain fall, it is highly correlated only with CAPE in the moist region during La Niña events. Moist convection and aerosols appear to be important parameters for production of lightning in moist and dry regions, respectively. Progress of the monsoon current dramatically affects the lightning activity in both moist and dry regions.
... La Niña causes the opposite effect to El Niño, which is responsible for flood monsoons and rainfall above normal values (Gadgil et al., 2019). ENSO is one of the main factors driving lightning and rainfall during the Indian summer monsoon (Kamra and Athira, 2016). The strength of El Niño and La Niña events during the Indian summer monsoon plays an important role in lightning and rainfall (Ahmad and Ghosh, 2017;Guha et al., 2017;Tinmaker et al., 2017). ...
The Indian summer monsoon rainfall (June-September) on a regional scale is critically important for agriculture and water management in India. The current study presents the lightning-rainfall relationship during El Niño (drought) and La Niña (flood) events in the Indian summer monsoon over central India. The results show that the flash count, Bowen ratio, surface maximum temperature, total heat flux, aerosol optical depth (AOD), sea surface temperature (SST), and Niño 3.4 index are increased by 36, 62, 19, 12, 46, 4.7%, and 0.3 ºC (warmer), whereas the rainfall is decreased by 15% during El Niño years with respect to normal years. The flash count, Bowen ratio, surface maximum temperature, and AOD are found to decrease by 15, 11, 3.5, and 11.1% during La Nina years, whereas the rainfall, total heat flux, SST, and Niño 3.4 index are found to increase by 2.4, 1.72, 0.36%, and –0.68 ºC (cooler) during La Niña years with respect to normal years. The increase in the flash count and the reduction in rainfall are associated with the warm phase of El Niño-Southern Oscillation (ENSO) (El Niño), which causes the weakening of the Indian summer monsoon. The decrease in flash count and increase in rainfall is due to the cold phase of ENSO (La Niña) and is associated with the strengthening of the Indian monsoon season. The increase in the number of break days and low-pressure systems also plays an important role in El Niño and La Niña years, respectively, over central India during the Indian summer monsoon.
... The present study is a step in this direction. Our previous studies were addressed to specific problems of how the lightning activity in some regions of South Asia is, (a) associated with the dry and wet convections in two regions of the Himalayas (Penki and Kamra, 2013a), (b) distributed with respect to the position of monsoon trough in the Indian summer monsoon season (Ramesh Kumar and Kamra, 2012b), (c) influenced by the presence of the Himalayas (Penki and Kamra, 2013a), (d) affected by the land-sea contrast over the sea and peninsular regions (Ramesh Kumar and Kamra, 2012b) and (e) influenced by El Nino/La Nina events, (Ramesh Kumar and Kamra, 2012c;Kamra and Athira, 2016), influenced by the topography and vegetation index (Oulkar et al., 2019) and projected change in lightning activity in 21st century (Saha et al, 2017a). ...
The regional variability of lightning activity over 10 different regions, selected with at least one major meteorological/topographical feature, of South Asia (8°N–36°N, 90°E–100°E) have been examined on all time scales from the data obtained from July 1995 to December 2013 from the Tropical Rainfall Measuring Mission satellite. A comparative study of the correlation coefficients calculated between lightning flash rate and convective parameters, aerosol optical depth (AOD) and topography for different regions is carried out. Impact of the progress of the Asian Monsoon on the spatio‐temporal variability of flash rate is also examined. Monthly‐averaged variations are annual at the regions >2,700 m in altitude and are semi‐annual at the regions <2,700 m in altitude. The annually averaged flash rates increase during the study period in most of the regions. However, such long‐term changes are nonlinearly related to the change in convective parameters and AOD and such inter‐relationships differ from one meteorological region to another. Small changes in AOD in high altitude regions are associated with very large changes in the surface temperature and flash rate. The flash rate is strongly correlated with surface temperature in the dry region of the north‐west but with CAPE in moist region of the north‐east. Progress of the Asian monsoon strongly impacts the flash rate in different regions. Ratio of the amplitudes of primary to secondary maxima at land stations increases with latitude in case of the monthly‐averaged variations in flash rate. The primary maxima in these variations are highly correlated with the atmospheric surface temperature. However, the secondary maxima are associated with the low‐level convergence during the withdrawal phase of the monsoon. Incursion of moisture by the Bay of Bengal branch of monsoon current determines the sequence of occurrence of lightning activity in different regions along the Himalayan foothills and north‐west.
Key Points
Lightning flash rate in the South Asian regions is sensitive to the surface temperature and to the altitude in the Himalayas
Progress of the Asian monsoon strongly impacts the regional distribution of the flash rate in the South Asia
Long‐term changes in flash rate during 1995–2013 in the South Asian regions increased with the surface temperature and aerosols
Lightning studies are highly focused on spatial and temporal variability in various scales but very limited studies are focused on dominant spatial modes of variability. This study intends to identify the possible spatial modes of climate variability of lightning over India during different seasons and relate them to regional and large-scale climate modes. Empirical orthogonal function analysis of lightning has been carried out and the first three orthogonally independent modes are considered in order to retrieve the maximum variance explained by each mode. To understand the role of remote and local teleconnections on the lightning flash rate (LFR) variability, we have analyzed two Pacific Ocean modes (El Niño Southern Oscillation; ENSO, Pacific Decadal Oscillation; PDO) and two Indian Ocean modes (Indian Ocean Dipole; IOD and Bay of Bengal (BOB) meridional Sea Surface Temperature (SST) gradient). First mode is positively correlated with the warm phase of ENSO and PDO whereas second and third modes are negatively correlated with the warm phase of ENSO and PDO during pre-monsoon, post-monsoon and winter. Reverse is true for the monsoon season due to the shift in walker cell caused by the changes in the location of the heat sources and sinks. A strong positive correlation of IOD and BOB meridional SST gradient with first mode, suggests the vital role of nearby Indian Ocean in explaining the typical lightning flashes over India due to the enhanced zonal and meridional circulation, thereby moisture supply to the Indian subcontinent. The impact of Nino-3.4, IOD and BOB meridional SST gradient on lightning over India further suggest the role of SST in local and remote influence on lightning variability through the distribution and transport of heat and moisture.
The El Niño–Southern Oscillation (ENSO) modulates the lightning flash density (LFD) variability over India during premonsoon, monsoon and postmonsoon seasons. This study intends to shed light on the impact of ENSO phases on the LFD over the Indian subcontinent using the data obtained from Optical Transient Detector (OTD) and Lightning Imaging Sensors (LIS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. Results suggest the LFD over northeast India (NEI) and southern peninsular India (SPI) strengthened (weakened) during the warm (cold) phase of ENSO in the premonsoon season. During monsoon season, NNWI (north of northwest India) shows above (below) normal LFD in the cold (warm) ENSO phase. It is striking to note that there are three hot spots of LFD over the Indian land region which became more prominent during the monsoon seasons of the last decade. A widespread increase in LFD is observed all over India during the warm phase of ENSO in the postmonsoon season. A robust rise in graupel/snow concentration is found during the postmonsoon season over SPI in the ENSO warm phase, with the lowest fluctuations over the NEI and NNWI regions. The subtropical westerly jet stream is shifted south in association with the warm phase, accompanied by an increase in geopotential height (GPH) all over India for the same period. This exciting remark may explain the indirect influences of ENSO's warm phase on LFD during the postmonsoon season by pushing the mean position of the subtropical westerly towards southern latitudes. However, the marked increase in LFD is confined mostly over the NNWI in the cold ENSO phase.
In this paper, the processes of precipitation, surface flow characteristics and groundwater table status in arid and desert regions have been investigated and compared with wet areas. Local spatial distribution of rainfall on a local scale is significantly affected by the reaction of atmospheric processes. From the qualitative point of view, these processes are similar in arid regions with wet areas, but certain aspects such as the frequency of occurrence and time and spatial variability are often characteristic of most arid regions. Considering that the surface flow in the watersheds consists of two different types of groundwater flow and drainage flow. Investigating the spatial distribution of flood in different directions of the basin is not the priority of hydrologic studies and only provides for flood decomposition maps. From the point of view of the flood discharges, it is studied. The flood water is in the dry and desert regions of the sharp tip, with a steep and sloping branch with a steep slope. The variation coefficient of maximum annual flood discharge in arid areas is higher than that for wet areas. Channels are mostly seasonal or flooded in arid areas, and the flow of zero in a number of months makes flood analysis of monthly data difficult. That is why it can be found in arid and semi-arid regions that make up a large part of Iran. The annual maximum flow rate of flooding is used only for analyzing sustained floods. In low humidity regions, low flow analyzes will result in a minimum flow rate with expected return periods and minimize the problems encountered in hydroelectricity, aquatic ecosystems and water supply. Due to the abundance of zero currents in arid regions, flow analysis is not common. Undue harvesting of groundwater in arid areas causes landslip, land leakage, and, consequently, desertification. In addition, the annual rainfall is low. The rainfall intensity is extremely high and the low soil permeability is mainly due to lack of vegetation and high evapotranspiration, which causes the aquifers to be fed niggling. In the upper reaches of the watershed, due to the slope of the land, the subject becomes acuter and, in addition to erosion of the soil, causes a lot of financial and human damage to the bottom of the basin. For the retrospective climate simulation, consequences were contrasted to a perceived gridded climatology of temperature and precipitation, and gridded hydrologic variables resulting from forcing the hydrologic model with observations. Tree-ring data donate to a better comprehension of the nature of past climatic alternations. For most of these zones, rainfall peculiarities tend to be related with departures in the large-scale atmospheric and oceanic fields that correlate with the template alters in the annual alternation of dry and rainy seasons. The interannual mutability of climate and excursion thus become visible mainly as increment and decrement of the annual cycle. The model assessment leads to some assurance in the trustworthiness of the modeled climate. The wide-scale dispensation of territorial ecosystem complexes is defined in large part by climate and can be varied by climatic alter due to natural causes or due to human processes such as those leading to increasing atmospheric CO2 close attention. systematizations that identify the affiliation of natural vegetation on climate prepare one means of constructing maps to show the effect of climatic change on the geography of major vegetation areas. The drying attitudes are qualitatively stable with other analyses and model prognosis, which propose more severe drying in the coming decades. Strategic-scale evaluations of climate alter efffects are often assumed using the change factor methodology whereby future varies in climate projected by General Circulation Models are used to a baseline climatology. One of the main concerns with a possible vary in climate is that an enhance in utmost events will happen. Model output has been examined that displays alternations in great events for future climates, such as enhances in maximum high temperatures, reduces in extreme low temperatures, and enhances in severe precipitation events. as well as, the societal substructure is becoming more responsive to weather and climate extremes, which would be deteriorated by climate change. In wild plants and animals, climate-induced extinctions, distributional and phenological alters, and species' range shifts are being recorded at an enhancing rate. Several evidently gradual biological alters are linked to answers to extreme weather and climate incidents.
In the present study, we are investigating the role of aerosols-and clouds in modulating the austral summer precipitation (December–February) during ENSO events over southern Africa region for the period from 2002 to2012 by using satellite and complimentary data sets. Aerosol radiative forcing (ARF) and Cloud radiative forcing (CRF) shows distinct patterns for El-Nina and La-Nina years. Further analysis were carried out by selecting the four Southern Africa regions where the precipitation shows remarkable difference during El-Nino and La-Nina years. These regions are R1 (33°S–24°S, 18°E–30°E), R2 (17°S–10°S, 24°E–32°E), R3 (19°S–9°S, 33°E–41°E) and R4 (7°S–0°S, 27°E–36°E). Aerosol Optical depth (AOD) shows considerable differences during these events. In region R1, R2 and R3 AOD shows more abundance in El-Nino years as compared to La-Nina years where as in R4 the AOD shows more abundance in La-Nina years. Cloud Droplet Effective radius (CDER) shows higher values during La-Nina years over R1, R2 and R3 regions but in R4 region CDER shows higher values in El-Nino years. Aerosol indirect effect (AIE) is estimated both for fixed cloud liquid water path (CLWP) and for fixed cloud ice path (CIP) bins, ranging from 1 to 300 gm –2 at 25 gm –2 interval over all the selected regions for El-Nino and La-Nina years. The results indicate more influence of positive indirect effect (Twomey effect) over R1 and R3 region during El-Nino years as compared to La-Nina years. This analysis reveals the important role of aerosol on cloud-precipitation interaction mechanism illustrating the interlinkage between dynamics and microphysics during austral summer season over southern Africa.
In general, convection, refers to the transport of some property by fluid movement, most often with reference to heat transport. As such, it is one of the three main processes by which heat is transported: radiation, conduction, and convection. Meteorologists typically use the term convection to refer to heat transport by the vertical component of the flow associated with buoyancy. Transport of heat (or any other property) by the nonbuoyant part of the atmospheric flow is usually called advection by meterologists; advection can be either horizontal or vertical.
Based on 16 years of Tropical Rainfall Measuring Mission (TRMM) data and NCEP Climate Forecast
System Reanalysis data, the most intense convective systems (ICSs) along the southern Himalayan front
(SHF) are studied using the multivariate techniques of principal component analysis in T mode and k-means cluster analysis. Three clusters, classified according to the near-surface fields of wind, specific humidity, convective available potential energy, and convective inhibition, correspond to the premonsoon (March– May), the establishment of the monsoon (late May–early June), and the Indian summer monsoon itself (June–September), respectively. The location of ICSs along the SHF is closely related to the establishment of the transport passage from the eastern SHF to the northwestern SHF along the Himalayas. During the premonsoon, the southwesterly wind is weak and moist air from the Bay of Bengal is transported to the eastern SHF, where ICSs are densely distributed. The oceanic southwesterly wind is enhanced and the transport passage extends to the central SHF during the monsoon establishment period, when ICSs distribute over the whole SHF homogeneously. The southwesterly wind is the strongest and the transport passage extends to the westernmost SHF after the monsoon is established, when ICSs mainly concentrate over the concave indentation region. Backward trajectory analysis confirms that, besides the local environment, the moisture transport from the Arabian Sea (17%) and the Bay of Bengal (9%) are two important long-range transport pathways for the summer monsoon ICSs at the western end of the SHF.
The contribution of extreme convective storms to rainfall in South America is investigated using 15 years of high-resolution data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR). Precipitation from three specific types of storms with extreme horizontal and vertical dimensions have been calculated and compared to the climatological rain. The tropical and subtropical regions of South America differ markedly in the influence of storms with extreme dimensions. The tropical regions, especially the Amazon basin, have aspects similar to oceanic convection. Convection in the subtropical regions, centered on La Plata basin, exhibits patterns consistent with storm life cycles initiating in the foothills of the Andes and growing into larger mesoscale convective systems that propagate to the east. In La Plata basin, convective storms with a large horizontal dimension contribute similar to 44% of the rain and the accumulated influence of all three types of storms with extreme characteristics produce similar to 95% of the total precipitation in the austral summer.
Diurnal and seasonal variation, intensity, and structure of deep convective systems (DCSs; with 20-dBZ echo tops exceeding 14 km) over the Tibetan Plateau–South Asian monsoon region from the Tibetan Plateau (TP) to the ocean are investigated using 14 yr of Tropical Rainfall Measuring Mission (TRMM) data. Four unique regions characterized by different orography are selected for comparison, including the TP, the southern Himalayan front (SHF), the South Asian subcontinent (SAS), and the ocean. DCSs and intense DCSs (IDCSs; with 40-dBZ echo tops exceeding 10 km) occur more frequently over the continent than over the ocean. About 23% of total DCSs develop into IDCSs in the SHF, followed by the TP (21%) and the SAS (15%), with the least over the ocean (2%). The average 20-dBZ echo-top height of IDCSs exceeds 16km and 9% of them even exceed 18 km. DCSs and IDCSs are the most frequent over the SHF, especially in the westernmost SHF, where the intensity—in terms of strong radar echo-top (viz., 40 dBZ) height, ice-particle content, and lightning flash rate—is the strongest. DCSs over the TP are relatively weak in convective intensity and small in size but occur frequently. Oceanic DCSs possess the tallest cloud top (which mainly reflects small ice particles) and the largest size, but their convective intensity is markedly weaker. DCSs and IDCSs show a similar diurnal variation, mainly occurring in the afternoon with a peak at 1600 local time over land. Although most of both DCSs and IDCSs occur between April and October, DCSs have a peak in August, whereas IDCSs have a peak in May.
Gridded climatologies of total lightning flash rates observed by the spaceborne Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) instruments have been updated. OTD collected data from May 1995 to March 2000. LIS data (equatorward of about 38°) adds the years 1998-2010. Flash counts from each instrument are scaled by the best available estimates of detection efficiency. The long LIS record makes the merged climatology most robust in the tropics and subtropics, while the high latitude data is entirely from OTD. The gridded climatologies include annual mean flash rate on a 0.5° grid, mean diurnal cycle of flash rate on a 2.5° grid with 24 hour resolution, mean annual cycle of flash rate on a 0.5° or 2.5° grid with daily, monthly, or seasonal resolution, mean annual cycle of the diurnal cycle on a 2.5° grid with two hour resolution for each day, and time series of flash rate over the sixteen year record with roughly three-month smoothing. For some of these (e.g., annual cycle of the diurnal cycle), more smoothing is necessary for results to be robust.The mean global flash rate from the merged climatology is 46 flashes s- 1. This varies from around 35 flashes s- 1 in February (austral summer) to 60 flashes s- 1 in August (boreal summer). The peak annual flash rate at 0.5° scale is 160 fl km- 2 yr- 1 in eastern Congo. The peak monthly average flash rate at 2.5° scale is 18 fl km- 2 mo- 1 from early April to early May in the Brahmaputra Valley of far eastern India. Lightning decreases in this region during the monsoon season, but increases further north and west. An August peak in northern Pakistan also exceeds any monthly averages from Africa, despite central Africa having the greatest yearly average.
During July 2010, a series of monsoonal deluges over northern Pakistan resulted in catastrophic flooding, loss of life and property and an agricultural crisis that may last for years. Was the rainfall abnormal compared to previous years? Furthermore, could a high probability of flooding have been predicted? To address these questions, regional precipitation is analyzed using three dataset sets covering the 1981–2010 time period. It is concluded that the 2010 average May to August (MJJA) rainfall for year 2010 is somewhat greater in magnitude than previous years. However, the rainfall rate of the July deluges, especially in North Pakistan was exceptionally rare as deduced from limited data. The location of the deluges over the mountainous northern part of the country lead to the devastating floods. The European Centre for Medium Range Weather Forecasts (ECMWF) 15-day Ensemble Prediction System (EPS) is used to assess whether the rainfall over the flood affected region was predictable. A multi-year analysis shows that Pakistan rainfall is highly predictable out to 6–8 days including rainfall in the summer of 2010. We conclude that if these extended quantitative precipitation forecasts had been available in Pakistan, the high risk of flooding could have been foreseen. If these rainfall forecasts had been coupled to a hydrological model then the high risk of extensive and prolonged flooding could have anticipated and actions taken to mitigate their impact.
Spatio-temporal variability of lightning activity over the Indian land mass region (8°N–33°N, 73°E–86°E) has been studied using monthly satellite based lightning flash grid (5° × 5°) data for 5-year (1998–2002) period. These data have been examined for depicting the annual, seasonal, and spatial distribution of the lightning activity. The study revealed the nonlinear relationship between lightning flash density and latitude on the annual timescale, and it is linked with the convective activity, large-scale circulations, land mass gradient, and orography of the region under study. On the seasonal timescale, the nonlinear positive relationships were observed in the premonsoon and monsoon seasons whereas a bimodal variation is observed in the post-monsoon season. The comparison of flash density with maximum surface air temperature shows the existence of nonlinear relationship between the two parameters. The approximate range of increase of flash density per 1°C rise in temperature is 20 to 44%. The lightning flash density shows a pronounced semiannual oscillation over the latitude belts 8°N–28°N.
A number of experimental studies have shown that sublimating ice acquires negative charge and ice undergoing vapor deposition acquires positive charge. Microphysical calculations are performed to determine the diffusional state (i.e., sublimation versus deposition) of riming graupel particles. Comparisons with earlier laboratory measurements of charge transfer to a rotating rimer in a cloud of supercooled water droplets and ice crystals again suggest that sublimating graupel particles charge negatively and graupel undergoing deposition charge positively. Implications for charge separation in thunderstorms are discussed.
The relationship between cloud height and lightning activity is examined using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Coincident data from the precipitation radar (PR) and Lightning Imaging Sensor aboard the TRMM satellite are used to examine whether lightning flash rate is proportional to the fifth power of cloud top height. This study is unique in that (1) the relationship between instantaneous rather than maximum storm height and flash rate is obtained and (2) relatively unbiased full data sets for different locations and seasons over the globe are used. The relationship between thunderstorm height and flash rate is nonlinear with large variance. The overall trend shows that flash rate increases exponentially with storm height. Some tall thunderstorms do not have large flash rates, but the reverse situation never occurs. The fifth power dependency that is derived from scaling laws is not inconsistent with, but not necessarily required by, the observed data.
We deduce temporal variations of the level of global lightning activity from the long-term Schumann resonance (SR) monitoring performed at the Tottori observatory (35.5°N, 134.33°E). The continuous analog records of the horizontal magnetic field cover the period from August 1967 to November 1970. The data show that temporal changes of the SR amplitudes vary considerably from day to day. Meanwhile, the regular diurnal and longer-scale variations appear after averaging. We found annual, and semiannual components in variations of global thunderstorm activity between 1967 and 1970. Comparison with the long-term SR records made recently in Europe allows us to demonstrate the similarity and stability of the global thunderstorm variations on a seasonal scale. Simultaneously, it revealed a contribution from winter thunderstorms in the Japan Sea to the Tottori SR records.
Major rainstorms can occur in mountainous regions when air is lifted over the terrain. When the environment is especially unstable, storms may form that contain locally intense buoyant updrafts, heavy downpours of rain, and sometimes hail. The stratiform region features gentler rain but covers a much greater area and can last for long periods of time if the environment is favorable. The satellite radar on TRMM provides a unique vision of storms in regions inaccessible to land-based surface observations such as the mountains of Pakistan. The air in the depression is very humid, and the counter-clockwise winds on the northeast side of the depression bring a deep layer of moist air over land in the mountainous region to the northeast of the depression. This flow anomaly extended to lower levels, conveyed moisture into a region of Pakistan orthogonal to the Himalayan barrier, and was thus largely responsible for establishing the environment of the mesoscale rain systems that produced the flooding in the Indus region.
Extreme orogenic convective storms in southeastern South America are divided into three categories: storms with deep convective cores, storms with wide convective cores, and storms containing broad stratiform regions. Data from the Tropical Rainfall Measuring Mission satellite's Precipitation Radar show that storms with wide convective cores are the most frequent, tending to originate near the Sierra de Cordoba range. Downslope flow at upper levels caps a nocturnally enhanced low-level jet, thus preventing convection from breaking out until the jet hits a steep slope of terrain, such as the Sierra de Cordoba Mountains or Andean foothills, so that the moist low-level air is lifted enough to release the instability and overcome the cap. This capping and triggering is similar to the way intense convection is released near the northwestern Himalayas. However, the intense storms with wide convective cores over southeastern South America are unlike their Himalayan counterparts in that they exhibit leading-line/trailing-stratiform organization and are influenced by baroclinic troughs more similar to storms east of the Rocky Mountains in the United States. Comparison of South American storms containing wide convective cores with storms in other parts of the world contributes to a global understanding of how major mountain ranges influence precipitating cloud systems.
Radar and electrical measurements for deep tropical convection are examined for both `break period' and `monsoonal' regimes in the vicinity of Darwin, Australia. Break period convection consists primarily of deep continental convection, whereas oceanic-based convection dominates during monsoonal periods, associated with the monsoon trough over Darwin. Order-of-magnitude enhancements in lightning flash rates for the `break period' regime are associated with 10-20-dB enhancements in radar reflectivity in the mixed-phase region of the convection compared with the monsoonal regime. The latter differences are attributed to the effect of convective available potential energy (CAPE) and its nonlinear influence on the growth and accumulation of ice particles aloft, which are believed to promote charge separation by differential particle motions. CAPE, in turn, is largely determined by the boundary-layer wet-bulb temperature. Modest differences (1°-3°C) in wet-bulb potential temperature between land and sea may account for the order-of-magnitude contrast in recently observed land-ocean lightning activity.
The instruments on the Tropical Rainfall Measuring Mission (TRMM) satellite have been observing storms as well as rainfall since December 1997. This paper shows the results of a systematic search through seven full years of the TRMM database to find indicators of uncommonly intense storms. These include strong (> 40 dBZ) radar echoes extending to great heights, high lightning flash rates, and very low brightness temperatures at 37 and 85 GHz. These are used as proxy variables, indicating powerful convective updrafts. The main physical principles supporting this assertion involve the effects of such updrafts in producing and lofting large ice particles high into the storm, where TRMM's radar easily detects them near storm top. TRMM's passive microwave radiometer detects the large integrated ice water path as very low brightness temperatures, while high lightning flash rates are a physically related but instrumentally independent indicator. The geographical locations of these very intense convective storms demonstrate strong regional preferences for certain land areas while they are extremely rare over tropical oceans. Favored locations include the south-central United States, southeast South America, and equatorial Africa. Other regions have extreme storms mainly in specific seasons, such as the Sahel, the Indian subcontinent, and northern Australia. Because intense storms are distributed quite differently from rainfall, these maps provide some new metrics for global models, if they are to simulate the type of convection as a component of our climate system.
Four distinct meteorological regimes in the Amazon basin have been examined to distinguish the contributions from boundary layer aerosol and convective available potential energy (CAPE) to continental cloud structure and electrification. The lack of distinction in the electrical parameters (peak flash rate, lightning yield per unit rainfall) between aerosol-rich October and aerosol-poor November in the premonsoon regime casts doubt on a primary role for the aerosol in enhancing cloud electrification. Evidence for a substantial role for the aerosol in suppressing warm rain coalescence is identified in the most highly polluted period in early October. The electrical activity in this stage is qualitatively peculiar. During the easterly and westerly wind regimes of the wet season, the lightning yield per unit of rainfall is positively correlated with the aerosol concentration, but the electrical parameters are also correlated with CAPE, with a similar degree of scatter. Here cause and effect are difficult to establish with available observations. This ambiguity extends to the ``green ocean'' westerly regime, a distinctly maritime regime over a major continent with minimum aerosol concentration, minimum CAPE, and little if any lightning.
DUNDEE (Down Under Doppler and Electricity Experiment) is described.
DUNDEE was carded out in the vicinity of Darwin, Northern Territory,
Australia, during the wet seasons of November 1988 through February
1989, and November 1989 through February 1990. The general goal of
DUNDEE was to investigate the dynamical and electrical properties of
tropical mesoscale convective systems and isolated deep convective
storms. Darwin, situated at the southern tip of the "maritime
continent," experiences both monsoon and "break" period conditions
during the wet season. We discuss the observational network deployed for
DUNDEE and present preliminary scientific results. One particularly
interesting observation is a large contrast in the frequency of total
lightning between break period convection (high lightning rates) and
convection in the monsoon trough (low lightning rates). A relationship
between CAPE (convective available potentional energy) and total flash
rate is presented and discussed to explain this observation.
Reflectivity data from Doppler radars are used to construct vertical profiles of radar reflectivity (VPRR) of convective cells in mesoscale convective systems (MCSs) in three different environmental regimes. The National Center for Atmospheric Research CP-3 and CP-4 radars are used to calculate median VPRR for MCSs in the Oklahoma-Kansas Preliminary Regional Experiment for STORM-Central in 1985. The National Oceanic and Atmospheric Administration-Tropical Ocean Global Atmosphere radar in Darwin, Australia, is used to calculate VPRR for MCSs observed both in oceanic, monsoon regimes and in continental, break period regimes during the wet seasons of 1987/88 and 1988/89. The midlatitude and tropical continental VPRRs both exhibit maximum reflectivity somewhat above the surface and have a gradual decrease in reflectivity with height above the freezing level. In sharp contrast, the tropical oceanic profile has a maximum reflectivity at the lowest level and a very rapid decrease in reflectivity with height beginning just above the freezing level. The tropical oceanic profile in the Darwin area is almost the same shape as that for two other tropical oceanic regimes, leading to the conclustion that it is characteristic. The absolute values of reflectivity in the 0 to 20 C range are compared with values in the literature thought to represent a threshold for rapid storm electrification leading to lightning, about 40 dBZ at -10 C. The large negative vertical gradient of reflectivity in this temperature range for oceanic storms is hypothesized to be a direct result of the characteristically weaker vertical velocities observed in MCSs over tropical oceans. It is proposed, as a necessary condition for rapid electrification, that a convective cell must have its updraft speed exceed some threshold value. Based upon field program data, a tentative estimate for the magnitude of this threshold is 6-7 m/s for mean speed and 10-12 m/s for peak speed.
The Optical Transient Detector (OTD) is a space-based instrument specifically designed to detect and locate lightning discharges as it orbits the Earth. This instrument is a scientific payload on the MicroLab-1 satellite that was launched into a 70deg inclination low Earth orbit in April 1995. Given the orbital trajectory of the satellite, most regions of the Earth are observed by the OTD instrument more than 400 times during a 1 year period, and the average duration of each observation is 2 min. The OTD instrument optically detects lightning flashes that occur within its 1300 x 1300 sq km field of view during both day and night conditions. A statistical examination of OTD lightning data reveals that nearly 1.4 billion flashes occur annually over the entire Earth. This annual flash count translates to an average of 44 +/- 5 lightning flashes (intracloud and cloud-to-ground combined) occurring around the globe every second, which is well below the traditional estimate of 100 fl/s that was derived in 1925 from world thunder day records. The range of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument. The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe. An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of approx. 10:1. The Congo basin, which stands out year-round, shows a peak mean annual flash density of 80 fl/sq km/yr in Rwanda, and includes an area of over 3 million km2 exhibiting flash densities greater than 30 fl/sq km/yr (the flash density of central Florida). Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year-round where instability is produced from cold air passing over warm ocean water. Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer. A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle.
The variability of lightning flash and thunderstorm on the ENSO time scales over East/Southeast Asia was investigated by using 17-year (1995-2011) lightning data from the Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS), and 14-year (1998-2011) Tropical Rainfall Measuring Mission satellite (TRMM) precipitation feature data. In addition, ERA-Interim reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF) were used to present related environmental characteristics. It was found that the response of lightning flash to ENSO events shows remarkable seasonal and regional variations. The regions of positive (negative) lightning anomaly are mainly located at both sides of 5°-20°N (5°-15°N) in El Niño (La Niña) spring and winter, and located north of the equator in summer and autumn. There is a significantly positive correlation between lightning anomaly and the Oceanic Niño Index (ONI) over both East China and Indonesia during El Niño episodes, but no obvious correlation during La Niña episodes. The positive thunderstorm anomalies during El Niño periods are dispersed. The distribution of thunderstorm anomalies in La Niña summer and autumn is almost opposite to that in spring and winter. The correlation between thunderstorm anomaly and ONI is better over East China than that over Indonesia. In general, lightning variation follows thunderstorm intensity (number) variation over East China during El Niño (La Niña) episodes, and follows a combination of thunderstorm intensity and number variations over Indonesia on ENSO time scales. During ENSO time scales, variations of surface wind can be considered as one of the key factors to LAs. More lightning flashes present in the regions where warm moist flows intersection, and less in the regions where surface wind changes slightly or diverges. Dramatic lightning increases also occur with higher values of convective available potential energy (CAPE). In addition, higher (lower) 850. hPa relative humidity generally follows with more (less) lightning flashes, which is more obvious during El Niño episodes.
The effect of the Western Ghats on the lightning activity across the west coast of India around the coastal metropolitan city of Mumbai during the 1998–2012 period is investigated using data from the Lightning Imaging Sensor (LIS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. A land–sea contrast of an order of magnitude in the lightning activity is observed even in a small area across the western coast of India. The shape of a zone of high lightning activity formed almost parallel to the Western Ghats during the onset and withdrawal phases of monsoon, strongly suggests the effect of the Western Ghats in its formation. Seasonal variation of the lightning activity in this area and also in each of its four equal sectors (two each over the Arabian Sea and over land) is bi-annual with one peak each in the onset (May/June) and withdrawal months (September/October) of monsoon and a sharp dip to very low values during the monsoon months (July/August) of maximum seasonal rainfall. The lightning activity in each sector is found to increase over the 1998–2012 period. However, the increase in lightning activity over the sector containing Mumbai is found to be greater during the pre- and post-monsoon periods and smaller during the monsoon period as compared to an identical sector immediately south of it.
Satellite radar and radiometer data show that subtropical South America has the world's deepest convective storms, robust mesoscale convective systems, and very frequent large hail. We determine severe weather characteristics for the most intense precipitation features seen by satellite in this region. In summer, hail and lightning concentrate over the foothills of western Argentina. Lightning has a nocturnal maximum associated with storms having deep and mesoscale convective echoes. In spring, lightning is maximum to the east in association with storms having mesoscale structure. A tornado alley is over the Pampas, in central Argentina, distant from the maximum hail occurrence, in association with extreme storms. In summer, flash floods occur over the Andes foothills associated with storms having deep convective cores. In spring, slow-rise floods occur over the plains with storms of mesoscale dimension. This characterization of high-impact weather in South America provides crucial information for socioeconomic implications and public safety.
The location and shift in the position of the maximum lightning activity with respect to the position the of monsoon trough has been investigated in the region of 20°N–32°N and 75°E–95°E during the Indian summer monsoon seasons of 2002 and 2003 from the lightning imaging sensor (LIS) data from the Tropical Rainfall Measuring Mission (TRMM) satellite. The polynomial curve fitting the maximum lightning activity followed the position of the monsoon trough in both the normal monsoon and drought years. Observations of the maximal lightning activity in the TRMM‐LIS data collected during a period of 13 years, from 1998 to 2010, confirm these findings in the monsoon season. When the seasonal position of the monsoon trough moved from the plains to the submontane region of the Himalayas during a monsoon break period, the polynomial fitting of the maximum lightning activity also moved from the central Indian plains to that region. An empirical orthogonal function (EOF) analysis is applied to two different data sets of precipitation and observed high lightning flash rates over the monsoon trough region for the period of 1995–2005. The first and second components, EOF1 and EOF2, of precipitation values are out of phase with those of the flash rates. Our results imply that deep electrified convective systems during monsoon periods are responsible for the lightning activity in the monsoon trough region and that the lightning activity in this region can be used as a proxy for the location and shift of the seasonal monsoon trough.
In this paper, preliminary results are presented showing that the two record-setting extreme events during 2010 summer (i.e., the Russian heat wave-wildfires and Pakistan flood) were physically connected. It is found that the Russian heat wave was associated with the development of an extraordinarily strong and prolonged extratropical atmospheric blocking event in association with the excitation of a large-scale atmospheric Rossby wave train spanning western Russia, Kazakhstan, and the northwestern China-Tibetan Plateau region. The southward penetration of upper-level vorticity perturbations in the leading trough of the Rossby wave was instrumental in triggering anomalously heavy rain events over northern Pakistan and vicinity in mid- to late July. Also shown are evidences that the Russian heat wave was amplified by a positive feedback through changes in surface energy fluxes between the atmospheric blocking pattern and an underlying extensive land region with below-normal soil moisture. The Pakistan heavy rain events were amplified and sustained by strong anomalous southeasterly flow along the Himalayan foothills and abundant moisture transport from the Bay of Bengal in connection with the northward propagation of the monsoonal intraseasonal oscillation.
The spatiotemporal variability of lightning activity in the region of
the Himalayan foothills and perpendicular to it has been examined from
about 16-year data from the TRMM satellite. The monthly mean flash rate
for the period from 1995 to 2010 is maximum in an arc-shaped area along
the Himalayan foothills and decreases on both the north and south sides
of it. Seasonal variation of mean flash rate changes from annual to
semiannual as the area shifts from north to south of the Himalayas and
the average elevation of area decreases from 5043 m to 219 m. Further,
the mean flash rate is higher in the afternoon and lower from midnight
to midday. In the Himalayan foothills, the mean flash rate is highly
correlated with surface temperature but poorly correlated with the
convective available potential energy (CAPE). Lightning flashes
occurring in the high-altitude regions, as compared to the low-altitude
regions, are less energetic but more frequent. The empirical orthogonal
function (EOF) analysis has been applied to three different data sets of
flash rates, surface temperature, and CAPE in the 18.75°N-36°N
and 68.75°E-93.75°E area for the period of 1995-2010 to examine
the relationship between these parameters in terms of seasonal
variations in this region. Spatial distributions of the first mode of
flash rate and CAPE and of the first and second modes of surface
temperature indicate that the Himalayan range exerts a strong effect on
the flash rate distributions in this region. However, while the first
mode of spatial distribution of flash rate is in phase with the surface
temperature, the time series of CAPE and flash rate are out of phase for
the first mode but in phase for their second modes. This result amounts
to that 65% of variability in CAPE cannot be associated with the changes
in flash rate but 22% of CAPE variability can be associated with changes
in flash rate. Higher values of flash rate in the Himalayan foothills
are suggested to be associated with the diurnal cycle of a mountain
breeze front. Relative roles of convective activities due to solar
heating of land, orography of the region, and the synoptic convective
systems embedded in monsoon current are discussed to explain the
results.
Impacts of the 1997–98 and 2002–03 ENSOs on total number of flashes and number of flash days from the LIS (Lightning Imaging Sensor) on board the Tropical Rainfall Measuring Mission satellite and on the average flash rate from the combined OTD (Optical Transient Detector) / LIS data are studied in South/Southeast Asia (8° –30°N, 60°–120°E) for the period 1998–2005. Also examined is the difference between the geographical distributions of these parameters in the El Nino period in 1998 and in the corresponding normal period in 1999. During the 1997–98 ENSO event, the number of flashes and average flash rate increase by 12% and 36%, respectively, during the El Nino period and during the La Nina period decrease by 22% and 5%, respectively, as compared to the corresponding normal periods. Similarly, during the 2002–2003 ENSO event, the number of flashes and average flash rate increase by 28% and 5% during the El Nino period in 2002 as compared to the corresponding normal period. The contrast in lightning activity between the El Nino and La Nina periods is more pronounced in Southeast Asia than in South Asia because of the southeastward shift in the convection activity during the ENSO events. In both regions, however, monthly variations of both total number of flashes and average flash rate are almost parallel to that of surface temperature. Although, the inverse relationship between the Indian Summer Monsoon Rainfall (ISMR) and El Nino broke down in 1997–98, the relationship between the El Nino and lightning maintains its normal increasing trend, indicating that the lightning activity is more sensitive to the convective activity than to rainfall on seasonal time scale. The total number of flashes and aerosol loading increase in the break‐period regime as compared to the monsoonal regime. The number of flash days in both ENSO events is higher in the EL Nino phase and lower in the La Nina phase as compared to their normal values in the subsequent years. Enhancement of lightning activity is mostly concentrated over land areas. Our results show that although diurnal variations in lightning activity can be related to the variations in surface temperature, convective adjustment induced by the corresponding ENSO phase needs to be considered for explaining the lightning activity variations on these longer time scales.
Temporal and spatial variations of convection in South Asia are analyzed using eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data and NCEP reanalysis fields. To identify the most extreme convective features, three types of radar echo structures are defined: deep convective cores (contiguous 3D convective echo ≥40 dBZ extending ≥10 km in height) represent the most vertically penetrative convection, wide convective cores (contiguous convective ≥40 dBZ echo over a horizontal area ≥1000 km2) indicate wide regions of intense multicellular convection, and broad stratiform regions (stratiform echo contiguous over an area ≥50 000 km2) mark the mesoscale convective systems that have developed the most robust stratiform regions.
The preferred locations of deep convective cores change markedly from India’s east coast in the premonsoon to the western Himalayan foothills in the monsoon. They form preferentially in the evening and over land as near-surface moist flow is capped by dry air aloft. Continental wide convective cores show a similar behavior with an additional nocturnal peak during the monsoon along the Himalayan foothills that is associated with convergence of downslope flow from the Himalayas with moist monsoonal winds at the foothills. The oceanic wide convective cores have a relatively weak diurnal cycle with a midday maximum. Convective systems exhibiting broad stratiform regions occur primarily in the rainiest season and regions—during the monsoon, over the ocean upstream of coastal mountains. Their diurnal patterns are similar to those of the wide convective cores.
During its first three years, the Tropical Rainfall Measuring Mission (TRMM) satellite observed nearly six million precipitation features. The population of precipitation features is sorted by lightning flash rate, minimum brightness temperature, maximum radar reflectivity, areal extent, and volumetric rainfall. For each of these characteristics, essentially describing the convective intensity or the size of the features, the population is broken into categories consisting of the top 0.001%, top 0.01%, top 0.1%, top 1%, top 2.4%, and remaining 97.6%. The set of 'weakest / smallest' features comprises 97.6% of the population because that fraction does not have detected lightning, with a minimum detectable flash rate 0.7 fl/min. The greatest observed flash rate is 1351 fl/min; the lowest brightness temperatures are 42 K (85-GHz) and 69 K (37- GHz). The largest precipitation feature covers 335,000 sq km and the greatest rainfall from an individual precipitation feature exceeds 2 x 10(exp 12) kg of water. There is considerable overlap between the greatest storms according to different measures of convective intensity. The largest storms are mostly independent of the most intense storms. The set of storms producing the most rainfall is a convolution of the largest and the most intense storms. This analysis is a composite of the global tropics and subtropics. Significant variability is known to exist between locations, seasons, and meteorological regimes. Such variability will be examined in Part II. In Part I, only a crude land / Ocean separation is made. The known differences in bulk lightning flash rates over land and Ocean result from at least two differences in the precipitation feature population: the frequency of occurrence of intense storms, and the magnitude of those intense storms that do occur. Even when restricted to storms with the same brightness temperature, same size, or same radar reflectivity aloft, the storms over water are considerably less likely to produce lightning than are comparable storms over land.
In the frame of this paper, 1-year of lightning data from the experimental network ZEUS operated by National Observatory of Athens is analyzed. The area of interest is the Mediterranean and surrounding countries. At a first stage, the ZEUS data are compared with the data provided by the UK Met Office long-range lightning detection network for the warm period of the year. It was found that although ZEUS system underestimates the nighttime and early morning activity, during the rest of the day ZEUS system detects increased number of lightning compared to ATD that suffers from saturation when the number of flashes exceeds a certain threshold. Furthermore, the possible relationship between lightning and elevation, terrain slope and vegetation is investigated. The analysis showed that during spring and summer there is a positive relationship of lightning activity with elevation, while this feature is not evident during the rest of the year. The lightning activity was found to be positively correlated with the elevation slope throughout the year except winter. As it concerns the vegetation cover, it was found that over bareground the lightning “yield” is low during the whole year, while the inverse is true for woodland areas. During the warm period of the year, due to drying of the Mediterranean surfaces, the forested and wooded areas that keep soil moisture present increased lightning “yield” in contrast with the shrubland areas that, for the same period, present a decreased lightning yield.
The 1997-98 El Niño was one of the strongest El Niño Southern Oscillation (ENSO) events of this century. The major impact of the sea surface temperature (SST) change during this El Niño event was the shift in convection activity from the western to the central and eastern Pacific ocean affecting the response of rain-producing cumulonimbus. As a result, convective rainfalls were suppressed near the Western Pacific regions and the Maritime Continent including Indonesia. On the other hand, the lightning activity during the El Niño period increased in contrast (on the average by 57%). As observed by the Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor and Precipitation Radar, the convective storms during the El Niño were more intense. This was supported by the evidence that the El Niño storms had greater vertical developments and thicker zones containing ice phase precipitation.
Currently available evidence for the mechanisms thought to be responsible for large-scale charge separation in thunderclouds is reviewed. The evidence suggests that electrically charged precipitation particles are basic to the electrification process but that convective motions are essential in accounting for the electrical energy of active thunderstorms. The relative contributions of air and particle motions remain largely unquantified, and the physical origins of thundercloud electrification have yet to be established.
Ratios of area mean rainfall and cloud-to-ground lightning flash count (termed "rain yields") were computed for several different locations around the globe, over temporal and spatial scales of 1 month and 104-105 km 2, respectively. Values of the rain yield clustered near l0 s kg/fl for a large portion of the midcontinental United States. Rain yields were slightly lower over the arid southwestern United States, averaging --6 x 107 kg/fl. In tropical locations the rain yields increased systematically from a tropical 8 10 continental value of 4 x 10 kg/fl to a value of 10 kg/fl for the tropical western Pacific Ocean. The observed stability of the rain yield, coupled with demonstrated positive correlations between cloud-to-ground flash density and rainfall amount, suggests that cloud-to-ground lightning data may be useful for inferring monthly convective rainfall statistics in certain rainfall regimes.
Laboratory calibration and observed background radiance data are used to determine the effective sensitivities of the Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS), as functions of local hour and pixel location within the instrument arrays. The effective LIS thresholds, expressed as radiances emitted normal to cloud top, are 4.0 ± 0.7 and 7.6 ± 3.3 μJ sr-1 m-2 for night and local noon: the OTD thresholds are 11.7 ± 2.2 and 16.8 ± 4.6 μJ sr-1 m-2, LIS and OTD minimum signal-to-noise ratios occur from 0800 to 1600 local time, and attain values of 10 ± 2 and 20 ± 3, respectively. False alarm rate due to instrument noise yields ∼5 false triggers per month for LIS, and is neglible for OTD. Flash detection efficiency, based on prior optical pulse sensor measurements, is predicted to be 93 ± 4% and 73 ± 11% for LIS night and noon: 56 ± 7% and 44 ± 9% for OTD night and noon, corresponding to a 12%-20% diurnal variability and LIS:OTD ratio of 1.7. Use of the weighted daily mean detection efficiency (i.e., not controlling for local hour) corresponds to σ = 8%-9% uncertainty. These are likely overstimates of actual flash detection efficiency due to differences in pixel ground field of view across the instrument arrays that are not accounted for in the validation optical pulse sensor data.
The remarkable contrasts between El Niño and La Niña in the East/Southeast Asia region and the western Pacific region are reported. One is that the lightning flash rate in the East/Southeast Asia region shows a clear inverse correlation with the Southern Oscillation Index (SOI). The lightning activity increases during the El Niño period and decrease during the La Niña period in this area. The other is that the contrast of the areal distribution of the variation of the flash rate in the western Pacific region. During the El Niño period the moist air from the ocean is warmed over the land, and that makes strong updrafts in high-pressure systems over the oceanic area. As a result, well-developed cumulonimbus with frequent lightning activity is observed in the coastal area. During the La Niña period, low-pressure systems on the land cause the increase of rainfall frequency with less lightning activity.
Global lightning activity has been studied on the ENSO (El Niño Southern Oscillation) time scale based on recordings of the Earth's Schumann resonances at Nagycenk (NCK), Hungary as well as observations from the OTD (Optical Transient Detector) and the LIS (Lightning Imaging Sensor) satellites in space. Both the intensity and position of lightning activity vary on the ENSO time scale. The magnitude of the global variation in lightning flash rate is ~10% from La Niña to El Niño. In general, more lightning is observed in the tropical–extratropical land regions during warm, El Niño episodes, especially in Southeast Asia. Although oceanic lightning activity is a minor contributor to global lightning, an opposite behavior is observed in the Pacific and other oceanic regions. More lightning is present during cold, La Niña conditions than during the warm, El Niño episodes. The annual distribution of global lightning is slightly offset from the equator into the Northern Hemisphere due to the north–south asymmetry of the land/ocean area ratio. Schumann resonance intensity variations suggest a southward (equator-ward) shift and satellite observations support this and show in addition an eastward shift in the global position during warm, El Niño episodes. The greatest lightning contrast between warm El Niño and cold La Niña episodes has been identified at the latitudes of descending dry air in the Hadley circulation.
Cloud-to-ground lightning flash data have been analyzed for the twelve-year period 1989-2000, for a geographical area centered on Houston, Texas. Of the 1.6 million cloud-to-ground flashes in this area of study, approximately 752,000 flashes occurred in the summer months of June, July, and August, and 119,000 flashes in the months of December, January, and February. The highest flash densities, greater than 4 flashes km -2 in the summer and 0.7 flashes/km -2 in the winter, are near the urban areas of Houston. We suggest that the elevated flash densities could result from several factors, including, 1) the convergence due to the urban heat island effect, and 2) the increasing levels of air pollution from anthropogenic sources producing numerous small droplets and thereby suppressing mean droplet size. The latter effect would enable more cloud water to reach the mixed phase region where it is involved in the formation of precipitation and the separation of electric charge, leading to an enhancement of lightning.
During the Asian summer monsoon, convection occurs frequently near the Himalayan foothills. However, the nature of the convective systems varies dramatically from the western to eastern foothills. The analysis of high-resolution numerical simulations and available observations from two case-studies and of the monsoon climatology indicates that this variation is a result of region-specific orographically modified flows and land surface flux feedbacks.
Convective systems containing intense convective echo occur in the western region as moist Arabian Sea low-level air traverses desert land, where surface flux of sensible heat enhances buoyancy. As the flow approaches the Himalayan foothills, the soil may provide an additional source of moisture if it was moistened by a previous precipitation event. Low-level and elevated layers of dry, warm, continental flow apparently cap the low-level moist flow, inhibiting the release of instability upstream of the foothills. The convection is released over the small foothills as the potentially unstable flow is orographically lifted to saturation.
Convective systems containing broad stratiform echo occur in the eastern Himalayas in association with Bay of Bengal depressions, as strong low-level flow transports maritime moisture into the region. As the flow progresses over the Bangladesh wetlands, additional moisture is extracted from the diurnally heated surface. Convection is triggered as conditionally unstable flow is lifted upstream of and over the foothills. The convective cells evolve into mesoscale convective systems (MCSs) with convective and stratiform areas. The MCSs are advected farther into the Himalayan eastern indentation, where orographic lifting enhances the stratiform precipitation. Copyright
Three-dimensional structure of summer monsoon convection in the Himalayan region and its overall variability are examined by analyzing data from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar over the June–September seasons of 2002 and 2003. Statistics are compiled for both convective and stratiform components of the observed radar echoes.
Deep intense convective echoes (40 dBZ echo reaching heights > 10 km) occur primarily just upstream (south) of and over the lower elevations of the Himalayan barrier, especially in the northwestern concave indentation of the barrier. The deep intense convective echoes are vertically erect, consistent with the relatively weak environmental shear. They sometimes extend above 17 km, indicating that exceptionally strong updraughts loft graupel to high altitudes. Occasionally, scattered isolated deep intense convective echoes occur over the Tibetan Plateau.
Wide intense convective echoes (40 dBZ echo > 1000 km2 in horizontal dimension) also occur preferentially just upstream of and over the lower elevations of the Himalayas, most frequently in the northwestern indentation of the barrier. The wide intense echoes have an additional tendency to occur along the central portion of the Himalayas, and they seldom if ever occur over the Tibetan Plateau. The wide intense echoes exhibit three mesoscale structures: amorphous areas, lines parallel to the mountain barrier, and arc-shaped squall lines perpendicular to and propagating parallel to the steep Himalayan barrier. The latter are rare, generally weaker than those seen in other parts of the world, and occur when a midlevel jet is aligned with the Himalayan escarpment.
Deep and wide intense convective echoes over the northwestern subcontinent tend to occur where the low-level moist layer of monsoon air from the Arabian Sea meets dry downslope flow, in a manner reminiscent of severe convection leeward of the Rocky Mountains in the central USA. As the low-level layer of moist air from the sea moves over the hot arid northwestern subcontinent, it is capped by an elevated layer of dry air advected off the Afghan or Tibetan Plateau. The capped low-level monsoonal airflow accumulates instability via surface heating until this instability is released by orographically induced lifting immediately adjacent to or directly over the foothills of the Himalayas.
Broad (>50 000 km2 in area) stratiform echoes occur in the eastern and central portions of the Himalayan region in connection with Bay of Bengal depressions. Their centroids are most frequent just upstream of the Himalayas, in the region of the concave indentation of the barrier at the eastern end of the range. The steep topography apparently enhances the formation and longevity of the broad stratiform echoes. Monsoonal depressions provide a moist maritime environment for the convection, evidently allowing mesoscale systems to develop larger stratiform echoes than in the western Himalayan region. Copyright
A significant correlation between the seasonal rainfall over India and the southern oscillation index (SOI) illustrated by Walker is examined on the basis of a long series of 100 year data. The relationship is discussed in the light of recent applications of global interactions in the regional climatic change models through feedback processes. The years of large scale deficient/excess rainfall over India during June–August season are examined in relation to the concurrent SOI regimes. This aspect may be of importance in examining the large scale circulation features in association with severe droughts/floods over India.Eine signifikante Korrelation zwischen dem jahreszeitlichen Regen in Indien und dem Index der sdlichen Oszillation (SOI) nach Walker wird aufgrund einer hundertjhrigen Reihe von Beobachtungsdaten untersucht. Die Beziehung wird im Hinblick auf neue Anwendungen globaler Wechselwirkungen durch Rckkoppelungsprozesse in regionalen Klimaschwankungsmodellen besprochen. Die Jahre mit grorumigem Mangel oder berschu an Regen in Indien in der Zeit von Juni bis August wird in Beziehung zum gleichzeitigen SOI-Verlauf untersucht. Diese Gesichtspunkte knnen bei der Untersuchung grorumiger Zirkulationsmerkmale in Verbindung mit starken Trockenheiten oder berschwemmungen in Indien von Bedeutung sein.
The seasonal variation of lightning flash activity over the Indian subcontinent (0°N–35°N and 60°E–100°E) is studied using
the quality checked monthly lightning flash data obtained from lightning imaging sensor on board the Tropical Rainfall Measuring
Mission satellite. This paper presents results of spatio-temporal variability of lightning activity over the Indian subcontinent.
The study of seasonal total lightning flashes indicates that the lightning flash density values are in qualitative agreement
with the convective activity observed over this region. Maximum seasonal total flash counts are observed during the monsoon
season. The propagation of the inter-tropical convergence zone over this region is also confirmed. Synoptic conditions responsible
for variation of lightning activity are also investigated with the help of an observed dataset. The mean monthly flash counts
show a peak in the month of May, which is the month of maximum temperatures over this region. Maximum flash density (40.2km−2 season−1) is observed during the pre-monsoon season at 25.2°N/91.6°E and the annual maximum flash density of 28.2km−2year−1 is observed at 33.2°N/74.6°E. The study of the inter-annual variability of flash counts exhibits bimodal nature with the
first maximum in April/May and second maximum in August/September.
Satellite observations of lightning flash rate have been merged with proximal surface station thermodynamic observations toward improving the understanding of the response of the updraft and lightning activity in the tropical atmosphere to temperature. The tropical results have led in turn to an examination of thermodynamic climatology over the continental United States in summertime and its comparison with exceptional electrical conditions documented in earlier studies. The tropical and mid-latitude results taken together support an important role for cloud base height in regulating the transfer of convective available potential energy (CAPE) to updraft kinetic energy in thunderstorms. In the tropics, cloud base height is dominated by the dry bulb temperature over the wet bulb temperature as the lightning-regulating temperature in regions characterized by moist convection. In the extratropics, an elevated cloud base height may enable larger cloud water concentrations in the mixed phase region, a favorable condition for the positive charging of large ice particles that may result in thunderclouds with a reversed polarity of the main cloud dipole. The combined requirements of instability and cloud base height serve to confine the region of superlative electrification to the vicinity of the ridge in moist entropy in the western Great Plains.
The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5–20 min. A distinguishing feature of severe storms is the presence of lightning `jumps' — abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds — wind, hail, tornadoes — is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity.
The present work is aimed to understand direct radiation effects due to aerosols over Delhi in the Indo-Gangetic Basin (IGB) region, using detailed chemical analysis of surface measured aerosols during the year 2007.
An optically equivalent aerosol model was formulated on the basis of measured aerosol chemical compositions along with the ambient meteorological parameters to derive radiatively important aerosol optical parameters. The derived aerosol parameters were then used to estimate the aerosol direct radiative forcing at the top of the atmosphere, surface, and in the atmosphere.
The anthropogenic components measured at Delhi were found to be contributing ∼ 72% to the composite aerosol optical depth (AOD(0.5) ∼ 0.84). The estimated mean surface and atmospheric forcing for composite aerosols over Delhi were found to be about -69, -85, and -78 W m(-2) and about +78, +98, and +79 W m(-2) during the winter, summer, and post-monsoon periods, respectively. The anthropogenic aerosols contribute ∼ 90%, 53%, and 84% to the total aerosol surface forcing and ∼ 93%, 54%, and 88% to the total aerosol atmospheric forcing during the above respective periods. The mean (± SD) surface and atmospheric forcing for composite aerosols was about -79 (± 15) and +87 (± 26) W m(-2) over Delhi with respective anthropogenic contributions of ∼ 71% and 75% during the overall period of observation.
Aerosol induced large surface cooling, which was relatively higher during summer as compared to the winter suggesting an increase in dust loading over the station. The total atmospheric heating rate at Delhi averaged during the observation was found to be 2.42 ± 0.72 K day(-1), of which the anthropogenic fraction contributed as much as ∼ 73%.
To monitor future global temperature trends, it would be extremely useful if parameters nonlinearly related to surface temperature could be found, thereby amplifying any warming signal that may exist. Evidence that global thunderstorm activity is nonlinearly related to diurnal, seasonal and interannual temperature variations is presented. Since global thunderstorm activity is also well correlated with the earth's ionospheric potential, it appears that variations of ionospheric potential, that can be measured at a single location, may be able to supply valuable information regarding global surface temperature fluctuations. The observations presented enable a prediction that a 1 percent increase in global surface temperatures may result in a 20 percent increase in ionospheric potential.
Analysis of the 140-year historical record suggests that the inverse relationship between the El Niño–Southern Oscillation
(ENSO) and the Indian summer monsoon (weak monsoon arising from warm ENSO event) has broken down in recent decades. Two possible
reasons emerge from the analyses. A southeastward shift in the Walker circulation anomalies associated with ENSO events may
lead to a reduced subsidence over the Indian region, thus favoring normal monsoon conditions. Additionally, increased surface
temperatures over Eurasia in winter and spring, which are a part of the midlatitude continental warming trend, may favor the
enhanced land-ocean thermal gradient conducive to a strong monsoon. These observations raise the possibility that the Eurasian
warming in recent decades helps to sustain the monsoon rainfall at a normal level despite strong ENSO events.
The Schumann resonance, a global electromagnetic phenomenon, is shown to be a sensitive measure of temperature fluctuations
in the tropical atmosphere. The link between Schumann resonance and temperature is lightning flash rate, which increases nonlinearly
with temperature in the interaction between deep convection and ice microphysics.