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Atmos. Chem. Phys., 14, 11287–11295, 2014
www.atmos-chem-phys.net/14/11287/2014/
doi:10.5194/acp-14-11287-2014
© Author(s) 2014. CC Attribution 3.0 License.
An important mechanism sustaining the atmospheric “water tower”
over the Tibetan Plateau
X. Xu1, T. Zhao2, C. Lu3, Y. Guo1, B. Chen1, R. Liu4, Y. Li5, and X. Shi1
1State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, 100081, China
2Key Lab for Aerosol-Cloud-Precipitation of CMA, School of Atmospheric Physics, Nanjing University of Information
Science & Technology, Jiangsu, 210044, China
3National Science Foundation, Arlington, VA 22230, USA
4National Satellite Meteorological Center, China Meteorological Administration, Beijing, 100081, China
5Institute of Plateau Meteorology, China Meteorological Administration, Chengdu, 610072, China
Correspondence to: T. Zhao (josef_zhao@126.com)
Received: 27 April 2014 – Published in Atmos. Chem. Phys. Discuss.: 10 July 2014
Revised: 17 September 2014 – Accepted: 18 September 2014 – Published: 27 October 2014
Abstract. The Tibetan Plateau (TP), referred to as the “roof
of the world”, is also known as the “world water tower”
because it contains a large amount of water resources and
ceaselessly transports these waters to its surrounding areas.
However, it is not clear how these waters are being supplied
and replenished. In particular, how plausible hydrological cy-
cles can be realized between tropical oceans and the TP. In
order to explore the mechanism sustaining the atmospheric
“water tower” over the TP, the relationship of a “heat source
column” over the plateau and moist flows in the Asian sum-
mer monsoon circulation is investigated. Here we show that
the plateau’s thermal structure leads to dynamic processes
with an integration of two couplings of lower convergence
zones and upper divergences, respectively, over the plateau’s
southern slopes and main platform, which relay moist air in
two ladders up to the plateau. Similarly to the CISK (condi-
tional instability of the second kind) mechanism of tropical
cyclones, the elevated warm–moist air, in turn, forces con-
vective weather systems, hence building a water cycle over
the plateau. An integration of mechanical and thermal TP
forcing is revealed in relation to the Asian summer monsoon
circulation knitting a close tie of vapor transport from tropi-
cal oceans to the atmospheric “water tower” over the TP.
1 Introduction
It has long been known that the Tibetan Plateau (TP) as the
third pole and “the world water tower” (Xu et al., 2008; Qiu,
2008) plays an important and special role in global climate
and energy–water cycle. In particular, due to its elevated land
surface and thus enhanced sensible heating, the TP becomes
a unique heat source, nonexistent in any other part of the
world (Flohn, 1957; Yeh et al., 1957; Yanai et al., 1992;
Webster et al., 1998; Wu and Zhang, 1998; An et al., 2001;
Sugimoto and Ueno, 2010). From its topographic structure,
we know that the TP possesses steep slopes with dramatic
rising of land surfaces on its south and east rims. Over the
plateau, however, the TP extends into the north and west ex-
tensively in a relatively flat fashion; thus being presented as
an oversized “mesa”, although there are large mountains over
the TP triggering convective cloud formations. In the boreal
summer, this massive “mesa” is strongly heated by solar ra-
diation. One of the consequences of this thermal structure is
its virtual functionality serving as an “air pump”, which at-
tracts warm and moist air from low-latitude oceans up to the
north into the Asian continent (Wu et al., 1997, 2012). During
boreal winter, this flow pattern reverses with the TP’s cool-
ing source (Ding, 1994). Hence, the TP’s role in the world’s
largest monsoon system is explained.
Furthermore, classic studies (Flohn, 1957; Yeh et al.,
1957; Luo and Yanai, 1984; Wu and Zhang, 1998; Yanai et
al., 1992; Hahn and Manabe, 1975; Webster et al., 1998; Xu
et al., 2010; Ye and Gao, 1979) also indicate that the rising
Published by Copernicus Publications on behalf of the European Geosciences Union.
11288 X. Xu et al.: An important mechanism sustaining the atmospheric “water tower”
warm and moist air from the tropical oceans tends to be de-
flected predominantly to the right (carried along the mid-
latitude westerlies), once they encountered the sharply ele-
vated plateau. The deflected warm and moist air forms the
well-known “southwesterly monsoonal flows”, transporting
water vapor down to southeastern China, plausibly explain-
ing the abundant water resources in these areas (Xu et al.,
2010, 2012; Zhao and Chen, 2001) (see the small rectangle
in the low reach of the Yangtze River basin in the upper panel
of Fig. 1). The lower southwesterly driving warm and wet air
transport from tropical oceans to these areas of southeastern
China in the summer season could also be induced by the
conjunction of the TP and Eurasia continental thermal forc-
ing (Duan and Wu, 2005).
However, many environment resource surveys (Lu et al.,
2005; Yao et al., 2012; Qiu, 2008) confirm that the TP it-
self contains a large amount of water resources in the form
of snowpack, glaciers, lakes, rivers and aquifers (the large
rectangle over the TP in the upper panel of Fig. 1). The TP
region contains one of the richest water resources and con-
stitutes one of the densest hydrological systems in the world.
Xu et al. (2008) identified the role of TP as the world water
tower, and elucidated how a hydrological cycle is completed
over the plateau and its surrounding areas, and how atmo-
sphere is able to supplement and reinforce the water that has
been continuously transported away from the TP. These stud-
ies certainly indicate that despite the fact that a large amount
of water vapor is deflected to southeast China, there must be
an appreciable amount of moist flows that are able to climb
over the TP, supplying and depositing necessary amounts of
water onto the TP, to make up the depleting surface flows.
In this study focusing on the climate mean in boreal sum-
mer, we investigate the mechanism in which a portion of
moist air reaches over the TP to maintain the atmospheric
“water tower”, as shown with high vapor contents over the
TP in the lower panel of Fig. 1. The mechanism depicts
an understanding of dynamic and thermodynamic processes
forcing the moist air up to the plateau. In particular, a cou-
pling of two “dynamic pumps” with the CISK (conditional
instability of the second kind) mechanism similar to the ty-
phoon’s thermal forcing, contiguous horizontally but stag-
gered vertically, are revealed. The two “water connected
pumps” will mutually support each other in such a way that
they ladder and relay the moist air over the elevated plateau.
2 Data and method
In this study, we used the reanalysis meteorology data
of years 2000–2009 from the Research Data Archive
at the US NCEP (National Center for Atmospheric Re-
search), Computational and Information Systems Laboratory
(doi:10.5065/D6M043C6) for all atmospheric variable anal-
yses, and the cloud cover fraction data derived from the
Chinese meteorological satellite FY-2F for convective cloud
analyses. Following the studies of Yanai (1961), Yanai and
Johnson (1993), Yanai and Tomita (1998), the apparent heat
source (Q1)and apparent moisture sink (Q2)are calculated.
Atmospheric heat sources and moisture sinks are respectively
gauged with the Q1and Q2. As Q1includes Q2and radia-
tive heating, here we concentrate only on the collective ef-
fect of apparent heating (Q1)over the TP. The heat source
column (in units of Wm−2)over the TP is obtained with
both horizontal and vertical integration of Q1over the TP
area of 78–103◦E and 28–38◦N covering the most region
with the altitude higher than 3000m (see the large TP rect-
angle in the upper panel of Fig. 1) to form a one-dimensional
variable representing the TP thermal forcing. The correlation
coefficients between the TP heat source column and the me-
teorological variables (divergence, U,Vand Wcomponents
of wind and vapor transport flux) are calculated to build their
horizontal and vertical distributions of correlations. Zonal,
meridional and vertical components of the correlation vector
are respectively derived through the correlation coefficients
of the TP heat source column to U,Vand Wcomponents of
vector of wind and vapor transport flux, indicating the vari-
ations in wind and vapor transport flux induced by the TP
thermal forcing.
3 Results
3.1 Elevated heat and wet islands over the TP
The upper panel in Fig. 2 depicts the vertical distribution in
zonal differences of air temperature and specific humidity av-
eraged along 93–94◦E around and over the TP, and these dif-
ferences are calculated respectively by subtracting air tem-
perature and specific humidity in summer (June, July and
August) averaged over 2000–2009 from their zonal means
in the Northern Hemisphere. A “warm–wet island” elevated
in the middle troposphere over the TP is identified from the
positive differences of air temperature and humidity over the
TP (upper panel of Fig. 2). On average, the urban tempera-
ture is 1–3◦C warmer than surrounding rural environments
(Voogt and Oke, 2003; Zhao et al., 2014), while air temper-
atures over the TP is 4–6◦C and even up to 6◦C higher than
its surrounding atmosphere at the same altitude in summer
(upper panel of Fig. 2). This heat island over the massive TP
exceeds that of any urban agglomerations in the world in both
intensity and area.
A high total solar irradiance of 1688W m−2, 23% higher
than the solar constant was observed over the TP (Lu et al.,
1995), as the plateau absorbs a large proportion of solar ra-
diation. Because the TP is the region with strong solar ra-
diation exceeding the solar constant in the world, air tem-
peratures over the TP could be 4–6◦C and even up to 10◦C
higher than its surrounding atmosphere at the same altitude
in summer (Yeh and Chen, 1992). The high solar radiation
on the TP could result in a strong sensible heat exchange in
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X. Xu et al.: An important mechanism sustaining the atmospheric “water tower” 11289
Figure 1. Geographical distribution of water sources in glaciers (snowpack), rivers and lakes over China with white, green and light blue
colors, respectively. Two major lake groups are marked by two red rectangles in the TP and eastern China (upper panel). Column vapor
content (10−2gcm−2)over 500 hPa in summer averaged over 2000–2009 (lower panel).
the surface layer. Air temperature is a measure of the sensible
heat content of the air. A good positive correlation between
surface air temperature and vertical velocity at 500hPa over
the TP (lower panel of Fig. 2) reflects an important role of
the surface sensible heating and its vertical transfer in build-
ing the heat and wet islands over the TP. The surface heating
from the plateau could trigger the air ascent driving the ver-
tical water vapor transport up to the free troposphere. Even
if the surface heat fluxes from the plateau have a negligi-
ble impact on the South Asian summer monsoon circulation
strength (Boos and Kuang, 2010), they could greatly impact
the convective precipitation over the TP. As shown in the up-
per panel of Fig. 2 for the vertical structures of the elevated
heat and wet islands, a heat source column reaching the up-
per troposphere over the TP could be visualized from the dis-
tribution of positive temperature differences with two high
cores, respectively, within near-surface layers and between
200 and 400hPa (upper panel of Fig. 2). Due to a mono-
tonic decrease in surface sensible heating with increasing el-
evation, the “hollow heat island” with a warm core at 200–
400hPa could be dominated by the latent heating released
from the convective cloud and precipitation processes over
the TP in association with the vertical structure of air vapor
in the wet island over the TP (upper panel of Fig. 2).
The elevated land surface with a strong radiative heating
could make the massive TP “mesa” more favorable for initi-
ating a large number of convective cells. These convective
cells over the plateau often give rise to precipitation over
the TP and its surroundings in the boreal summer (Xu et
al., 2012; Sugimoto and Ueno, 2010). In fact, the annual
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11290 X. Xu et al.: An important mechanism sustaining the atmospheric “water tower”
y = 1.011x - 292.89
R² = 0.2836
-20
-15
-10
-5
0
5
280 282 284 286 288 290
Vertical velocity（hPa/s)
Temperature（k）
Figure 2. Vertical sections of air temperature (◦C; filled contours)
and specific humidity (g kg−1; contour lines) differences relative to
the zonal means along 93–94◦E in summer averaged over 2000–
2009. The plateau section is marked with soil color (upper panel).
A scatter plot of surface air temperature and vertical velocity at
500 hPa in the TP region in July 2000–2009 (lower panel).
occurrences of convective clouds (cumulonimbus) over the
TP are observed with 2.5 times of the regional mean over
the other areas of China (Xu et al., 2002), and the TP region
is regarded as a high frequency center of cumulonimbus or
mesoscale convective systems (MCSs) in China (Sugimoto
and Ueno, 2012), which is also confirmed by the mean dis-
tribution of convective clouds over the TP (see Sect. 3.3) in
the plateau low vortex region (upper panel of Fig. 4).
3.2 Processes of water vapor transport upward the TP
Based on the differences of temperature and humidity at a
given pressure level of the atmosphere over the TP and over
adjacent non-elevated areas in boreal summer, the vertical
structures of heat source column and wet island on the TP
are characterized in Fig. 2 (upper panel) with the particularly
surprising “hollow heat island” between 200 and 400hPa in
Figure 3. Vertical sections of wind vectors and divergences (filled
contours) for summer averaged over 2000–2009 along 93–94◦E
(left upper panel); Distribution of summertime 500hPa divergence
averaged over 2000–2009 (right upper panel). Vertical sections of
the correlations of the daily TP heat source column Q1to the di-
vergences (filled contours) and the correction vectors of daily Q1
to V- and W-wind components in July 2000–2009 along 93–94◦E
with the meridional circulations and the uplifting vapor transport
denoted by blue dash lines and black arrows, respectively (middle
panel). Vertical sections of the lag-correlations of TP heat source
column Q1at 10 prior days to divergences and the lag-correlation
vectors in the meridional circulations in July 2000–2009 along 93–
94◦E (lower panel). In all panels, two couplings of lower conver-
gence zones (LC) and upper divergences (UD) are denoted with ∇·
V<0 and ∇ · V> 0 in two dotted rectangles and the interaction of
LC in the TP and UD over the southern slopes in the black ovals.
The plateau section is marked with soil color.
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X. Xu et al.: An important mechanism sustaining the atmospheric “water tower” 11291
y = 0.1055x + 100.06
R² = 0.2876
100
105
110
115
120
125
45 60 75 90 105 120 135 150 165
q(g/kg)
Q1(w/m2)
y = 0.2074x + 20.79
R² = 0.3206
15
25
35
45
55
65
45 60 75 90 105 120 135 150 165
div(quqv)×105
Q1(w/m2)
Figure 4. Correlation vectors of the TP heat source column strength
Q1to the horizontal moisture transport flux components over sum-
mer of 1957–2009. A shear line between southward and northward
air flows (light blue arrows) and the plateau low vortex over the TP
are marked with blue and red dash lines, respectively. The TP region
with the altitude of higher than 3000m is shaded in yellow contour
(upper panel). Correlations of the heat source strength Q1, total wa-
ter vapor q and net vapor transport flux divergence (div) in the TP
air column in summer of 2000–2009 in scatter plots of Q1to q(left
lower panel) and div (right lower panel).
the shape of “warm core” and “mushroom cloud” (high zonal
air temperature deviation) over the TP. The vertical structure
of the elevated wet island over the TP can also confirm that
the large TP topography prevents dry and cool extratropical
air from “ventilating” the moist and warm tropics and sub-
tropics (upper panel of Fig. 2). It is particularly interesting
that the TP “hollow heat island” structure is similar to the
warm core of Typhoon-CISK process (Charney and Eliassen,
1964; Smith, 1997) in the company of the elevated wet island
(upper panel of Fig. 2) and the meridional circulation with a
strong convection (left upper panel of Fig. 3). The “CISK-
like process” relaying warm–moist air up to the TP in two
ladders is identified between two couplings of tropospheric
lower convergence zones (LC) and upper divergences (UD)
corresponding to (1) the LC in the South Asian monsoon re-
gions and the UD over the southern TP slopes as well as (2)
the LC on the TP main platform and the UD in the middle and
upper troposphere over the TP (left upper panel of Fig. 3).
The strength of “heat source column Q1” could be repre-
sented by the atmosphere column integration of apparent heat
source Q1over the TP region. The middle panel of Fig. 3
presents the correlation vectors of the TP heat source column
strength Q1over the TP to the W- and V-wind components
at the vertical sections around the TP averaged in July 2000–
2009. In this study, zonal, meridional and a vertical compo-
nents of the correlation vector are derived through the corre-
lation coefficients of the Q1to U- ,V- and W-wind (or trans-
port flux) components, respectively, where the arrow length
denotes the correlation combination with a longer arrow im-
plying a better correlation, and the arrow direction means the
direction of anomalous wind (or transport flux) induced by
the TP thermal effect. Therefore, the middle panel of Fig. 3
indicates that the air ascent motions induced by the TP heat-
ing are profound over the TP during the summer monsoon
period. The large topography of TP with the “hollow heat is-
land” can force a water vapor pump with the strong upward
air flows. A meridional circulation produced by the thermal
effect of “hollow heat island” and the mechanical impact of
the TP topography can not only result in the Asian summer
monsoon circulations but also enhance the water vapor trans-
port from the oceans crossing the Asian monsoon areas up to
the TP (middle panel of Fig. 3). The strong divergences of
the South Asian high in the upper troposphere are collocated
with the near-surface convergence zones associated with the
plateau low vortex, which is a favorable pattern for vertical
circulation enforcing a strong water vapor uplift over the TP
(left upper and middle panels of Fig. 3; upper panel of Fig. 4).
The TP surface sensible heat and the latent heat release from
the convective cloud and precipitation may maintain the ver-
tical circulation driving the vapor transport up into the at-
mospheric “water tower” over the TP (lower panel of Fig. 2;
Figs. 3–5). A water vapor pump with cloud convective activ-
ities is motivated in the near-surface air convergence zones
over the TP, driven by the plateau heating (upper panel of
Fig. 4; Fig. 6). The atmospheric “water tower” is set up by
the air pump forced with the TP heating (Xu et al., 2008).
A coupling of two “dynamic pumps” with the CISK-like
mechanism, contiguous horizontally but staggered vertically,
are revealed with the cooperative interaction of the “heat
source column” and the elevated wet islands over the roof
of the world (see two dotted rectangles in the middle panel
of Fig. 3). This interaction could be achieved with a positive
feedback, when the forcing effect of the “heat source col-
umn” drives the water vapor flows climbing up the TP in the
vertical motion, in turn, and the phase changes of water vapor
to clouds and precipitation in the moist convection release
latent heating intensifying the “heat source column” and es-
pecially the “warm core” in the upper troposphere associ-
ated with the South Asian high (Sugimoto and Ueno, 2012).
The “heat source column” could enhance convergence zones
at lower levels and divergences at upper levels in the tropo-
sphere for pushing the moist air up the TP (middle panel of
Fig. 3; Fig. 4). There could be a mutual feedback between
the UD on the southern plateau slopes and the LC on the TP
platform through the dynamical interaction of the horizon-
tally contiguous UD and LC (right upper and middle panels
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11292 X. Xu et al.: An important mechanism sustaining the atmospheric “water tower”
Figure 5. The vertical distributions of apparent heat source Q1
(filled contours) and apparent moisture sink Q2(Wm−2)averaged
between 85 and 100◦E in summer over 2000–2009. The Q1and
Q2in two dash rectangles are produced with two ladders of CISK-
like process respectively over the TP’s southern slopes and main
platform. The plateau section is marked with soil color.
longtitude
latitude
0.9
0.8
0.7
0.6
0.5
0.45
0.35
0.25
0.2
0.1
76°E 78°E 80°E 82°E 84°E 86°E 88°E 90°E 92°E 94°E 96°E 98°E 100°E
42°N
40°N
38°N
36°N
34°N
32°N
30°N
28°N
26°N
24°N
Figure 6. Mean distribution of cloud cover fraction in July 2008
(06:00UT) derived from the Chinese meteorological satellite FY-
2F. The black dash line separates the high and low amounts of cloud
cover over the TP.
of Fig. 3). The UD over the southern TP slopes and the LC on
the TP platform could be contributed by the water vapor flow
acceleration at the inflection point between the steep south-
ern slopes and the southern edge of TP platform with the me-
chanical TP impact on the air pump on the platform (upper
and middle panels of Fig. 3).
The two ladders of “CISK-like process” over the South
Asian summer monsoon region and the TP knit a close tie
of vapor transport from tropical oceans to the atmospheric
“water tower” over the TP (Fig. 3). The South Asian summer
monsoon precipitation is produced in the first ladder of air
vapor transport toward the TP atmosphere, which could be
attributed to the TP topographical block at the steep south-
ern slopes with less thermal impact (Boos and Kuang, 2010).
The second ladder resulting in convective cloud precipitation
over the large TP platform with less terrain obstacles for wa-
ter vapor flows is dominantly controlled by thermal forcing
of the “hollow heat island” in a large scale (Wu et al., 2012).
The pump of the “hollow heat island” over the TP could not
only attract air vapor transport from tropical oceans to the TP
but also intensify the dynamic lift of air vapor on the south-
ern slope of the TP for the Asian summer monsoon (middle
and lower panels of Fig. 3). The dynamic structures of two
couplings of tropospheric LC and UD with their interaction
build up a meridional circulation in a two-ladder pump of
moist air along the plateau (left upper and middle panels of
Fig. 3), which could also be explained with the vertical dis-
tribution of apparent heat source Q1and apparent moisture
sink Q2around the TP (Fig. 5). In Fig. 5 two couplings of
high Q1and Q2areas are found between two couplings of
tropospheric LC and UD, respectively, on two ladders in the
process of water vapor transport up to the TP atmosphere
(Fig. 3).
The convective clouds and precipitation of the plateau low
vortex or cyclone are triggered by the plateau heating. The
CISK-like process is found to play an important role in the
local low vortex development for the TP precipitation (Qiao
and Zhang, 1994).
The good correlations of the strength of “heat source col-
umn” Q1to the total water vapor and to the net transport
flux divergence over the TP (two lower panels of Fig. 4) fur-
ther interpret a large-scale effect of “CISK-like mechanism”
with a positive feedback among the heat source column, the
vertical convection and the water vapor supply for the atmo-
spheric “water tower” over the TP. The two ladder “CISK-
like mechanism” is a key process attracting water vapor to-
ward the TP for building the TP’s “water tower” in Asian
water cycle. To further discover the process initiating the up-
ward transport of water vapor flows over the TP, the lag corre-
lations of the TP’s heat source column Q1at 10 prior days to
the divergences and the meridional circulation are analyzed
in the lower panel of Fig. 3, which reflect that the plateau
heating could initiate and trigger the vertical circulations for
the “hollow heat island” process with a leading effect of the
heat source column on water vapor transport toward the TP.
3.3 Cloud distribution over the TP
The TP region is identified as a frequent occurrence center of
MCSs in China (Sugimoto and Ueno, 2012). In association
with Asian summer monsoons, the summertime convective
clouds bring the precipitation over the TP and its surround-
ings (Xu et al., 2012; Sugimoto and Ueno, 2010). To further
clarify the atmospheric “water tower” over the TP in Asian
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X. Xu et al.: An important mechanism sustaining the atmospheric “water tower” 11293
water cycle, Fig. 6 presents the spatial distribution of total
cloud cover over the TP and its surrounding area averaged in
July 2008.
During the Asian summer monsoon period, the dense
cloud covers exist over the regions from the Bay of Bengal,
South Asian monsoon region, to the southern TP (Fig. 6).
As characterized with the correction vectors of the column
heat source over the TP to the moisture transport over and
around the TP (middle panel of Fig. 3), two convergence
zones of moisture transport fluxes (∇· q V < 0) are found on
two ladders over the plateau’s southern slopes and main plat-
form during the moisture transport from the oceans up to the
TP, resulting in these regions of dense cloud covers shown in
Fig. 6. It is noteworthy that the high cloud amounts are zon-
ally concentrated between the steep southern plateau slopes
and the shear line of the plateau low vortex over the TP (up-
per panel of Fig. 4; Fig. 6) with the monthly mean cloud
cover fractions up to 90%, which could resulted from the
“CISK-like mechanism” for building the TP’s atmospheric
“water tower” (Fig. 3). Over the large TP platform with rela-
tively plain terrain, the monthly mean cloud covers of around
45% are mostly observed on the central-eastern region with
less cloud cover over the northwestern TP, depending on the
moisture transport across the TP. The plateau low vortex over
the TP and the southward air flows with less moisture on the
north of the shear line could lead to the less cloud cover in
the northwestern platform of TP (upper panel of Fig. 4).
The observed cloud distribution over the TP confirms that
the “CISK-like mechanism” is an important mechanism sus-
taining the atmospheric “water tower” over the TP. Connect-
ing with the cloud and precipitation in the atmospheric “wa-
ter tower”, the plausible hydrological cycles could be real-
ized between tropical oceans and the TP.
4 Conclusions and discussions
The present analyses clearly indicate that the TP presents it-
self as a “warm–wet island”. The surface heating over the
plateau leads to a low-pressure center causing flow conver-
gence at low levels of the plateau and subsequently triggers
vertical motion. This convective system will result in plateau
clouds and precipitation, which would explain abundant wa-
ter storage in the atmosphere over the TP and its surrounding
regions.
The classic Asian summer monsoon theory elucidated an
“air pump” mechanism in relation to the TP. The warm–moist
air from the low-latitude oceans is drawn toward the plateau
by this air pump. Our analysis on the relationship between
the “heat source column” over the TP and warm–moist air
transport in the present study further reveals a CISK-like
mechanism on water vapor suction up the plateau. An ap-
preciable portion of warm–moist air converges at the foot of
the south rim of the plateau. The convergence of the warm–
moist air ascends along the plateau’s slope and diverges at
CISK(1)
Interaction
region
CISK(2)
Tibetan
Plateau
Ocean
Q1&Q2
Q1&Q2
·V 0
·V 0
·V 0
·V 0
Figure 7. A diagram of the summary on two ladders of CISK-like
processes with two couplings of heat source Q1and moisture sink
Q2over the TP’s southern slopes and main platform in forcing wa-
ter vapor flows climbing up the TP, which is marked with soil color.
about the altitude of the plateau top. This divergence flow
enforces the convergence at the heated low-pressure center
over the TP and feeds in the convective system with warm–
moist air, which results in the clouds and precipitations for
the atmospheric water tower over the TP.
These dynamic and thermodynamic processes depict a
coupling of two CISK type systems, both with convergence
at low levels and divergence at upper levels, but the sys-
tems are horizontally contiguous as well as vertically stag-
gered. The two systems display a mutually supportive mech-
anism with the mechanical and thermal TP impact between
the southern slopes and the platform of the TP in the interac-
tion region marked in Fig. 7. It is this coupling that ladders
the moist air up to the plateau building the atmospheric “wa-
ter tower” over the TP.
In this study, the mean climate of air vapor transport to the
TP is investigated based on the summertime averages over
the past years, and a two ladder “CISK-like mechanism” is
identified as a key process sustaining the atmospheric “wa-
ter tower” over the TP. The role of intraseasonal variability,
synoptic-scale system activities and diurnal variation in the
atmospheric heat source and moisture over the TP (Sugimoto
et al., 2008; Fujinami and Yasunari, 2004) will be considered
in a future study on the warm–moist air transport up to the
plateau. It should be emphasized that considering the quality
of reanalysis data over and around the TP, a comparison be-
tween NCEP/NCAR and some other reanalysis data sets such
as JRA-25, ERA-Interim, or MERRA is necessary in further
work. Furthermore, the two CISK type system revealed from
this observational analysis need to be further studied with
numerical models to understand the mechanism to work.
www.atmos-chem-phys.net/14/11287/2014/ Atmos. Chem. Phys., 14, 11287–11295, 2014
11294 X. Xu et al.: An important mechanism sustaining the atmospheric “water tower”
Acknowledgements. This research was jointly supported by the
projects of China Special Fund for Meteorological Research in
the Public Interest (GYHY201406001), Nature Science Fund of
China (41130960), and Priority Academic Program Development
of Jiangsu Higher Education Institutions (PAPD).
Edited by: L. Zhang
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