Download pdf

Meteorol Atmos Phys 100, 217–231 (2008)
DOI 10.1007/s00703-008-0305-8
Printed in The Netherlands
1
State Key Laboratory for Severe Weather, Chinese Academy of Meteorological Sciences,
China Meteorological Administration, Beijing, P.R. China
2
International Pacific Research Center and Department of Meteorology,
School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI, USA
3
Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC, USA
Data analysis and numerical simulation of moisture source
and transport associated with summer precipitation
in the Yangtze River Valley over China
X. D. Xu1 , X. Y. Shi1 , Y. Q. Wang2 , S. Q. Peng3 , X. H. Shi1
With 14 Figures
Received 20 November 2007; Accepted 14 January 2008
Published online 14 August 2008 # Springer-Verlag 2008
Summary
In this study, the moisture source=sink and long-distance
moisture transport associated with the summer precipitation
in the mid-lower reaches of the Yangtze River Valley (YRV)
over China have been investigated based on both data analysis and numerical simulations using the regional climate
model RegCM3. The study focused on both a case of the
1998 summer flood events and the interannual variation of
summer precipitation during 1961–2005 in the YRV. The
results show that (1) the low latitudinal band from the Bay
of Bengal, Indochina Peninsula, South China Sea to the
Philippine Island is the major moisture source region for
the summer precipitation in the YRV; (2) an eastward moisture transport from the Bay of Bengal and a westward moisture transport from the Philippines Sea meet at the South
China Sea, where a main water vapor channel forms and
advances north-northwestward and turns northeastward in
the vicinity of the eastern Tibetan Plateau and South China
to the YRV; and (3) the long-distance moisture transport
from the eastern Indian Ocean-South China Sea to the YRV
contributes to more than half of the summer precipitation
in the YRV. Furthermore, it is shown that RegCM3 driven
by the NCEP=NCAR reanalysis is able to reproduce the
Correspondence: Dr. Yuqing Wang, IPRC=SOEST, Rm 409G,
POST Bldg., 1680 East–West Road, University of Hawaii at
Manoa, Honolulu, HI 96822, USA (E-mail: [email protected])
overall climatological water vapor transport in East Asia
and the features of the water vapor flux for flooding and
drought years in the YRV as well as simulate the YRV flood
events in the summer of 1998 reasonably well.
1. Introduction
Asian summer monsoon flow in the lower troposphere is rich in water vapor, which brings moist
air from the adjacent low latitude oceans to East
Asia and contributes largely to early summer precipitation in South China and to Meiyu rainfall
in the mid-lower reaches of the Yangtze River
Valley (YRV, outlined with dashed rectangular
area and denoted as area A in Fig. 2). It is generally believed that the Bay of Bengal, South
China Sea, and the tropical western Pacific are
the major water vapor source regions (5–15 N,
80–130 E, outlined with dashed rectangular area
and denoted as area B in Fig. 2) for the summer
precipitation in the YRV (e.g., Ding 1992;
Simmonds et al. 1999). To reveal the water vapor
transport, especially the long-distance transport,
is critical to the understanding of precipitation
218
X. D. Xu et al.
processes and the anomalous monsoon rainfall
in the inland areas of East Asia, such as the
YRV in China, where the local evaporation is
far enough to explain the observed precipitation
amount.
The presence of Tibetan Plateau, the largest
and highest in the world, makes the East Asia a
very special region of which the climate is
quite distinct from other regions in the globe.
Tibetan Plateau plays a critical role in channeling the summer monsoon flow to its southern
and eastern flakes and thus the path of water
vapor transport (Tao and Chen 1988; Xu et al.
2002, 2003a, b, c). Xu et al. (2003a, b) indicated that the eastern Tibetan Plateau acts as a
‘‘platform’’ of water vapor transport to the YRV
from low latitude oceans, such as the South
China Sea, eastern Indian Ocean, and Bay of
Bengal. As a result, there exists a ‘‘Large Delta’’
region, namely the area where the complex interaction occurs between the Tibetan Plateau and
the Asian monsoon, where water vapor is transported from the low latitude by the southerly
flow and then transported east-northeastward to
the YRV.
Previous studies have shown that anomalous
summer rainfall in China is mainly caused by
anomalous water vapor transport associated
with the Asian and East Asian summer monsoon
and northward propagating intraseasonal oscillation (Ding and Hu 2003; Wang et al. 2003;
Zhou and Yu 2005; He et al. 2007). For example, Zhou and Yu (2005) found two anomalous patterns associated with typical anomalous
summer rainfall patterns in China, which are
different from those patterns associated with
normal monsoon rainfall. In a recent study with
numerical experiments for the case of 1998
heavy rainfall events in China, Chow et al. (2007)
showed two main water vapor sources to the early summer precipitation in South China: one is
the tropical western Pacific and the other is the
Indian Ocean (including the Bay of Bengal) and
South China Sea. The latter is shown to contribute about half to the total early summer rainfall
over China.
The main objective of this study is to identify
the paths of long-distance water vapor transport
associated with the summer precipitation in the
YRV of China and to reveal the associated teleconnection pattern of water vapor flux. Wang
et al. (2004) demonstrated that regional climate
model can be used to understand the regional
climate processes. Therefore, in addition to the
diagnostics based on the reanalysis and daily station rainfall data, we also conducted numerical experiments using a regional climate model (RCM)
to demonstrate the robustness and the significance of the teleconnection between water vapor
transport over the eastern Indian Ocean and
South China Sea and the summer rainfall anomalies in the YRV of China. Sensitivity experiments also help quantify the contribution of the
water vapor transport from the key water vapor
source regions to the total summer rainfall in the
YRV of China.
The rest of the paper is organized as follows.
The next section describes the data and methodology. Section 3 discusses the moisture source=
sink associated with the summer precipitation in the YRV based on the analysis for the
1998 YRV flood events and the correlation between the column-integrated water vapor and
the precipitation in the YRV during 1961–2005.
Section 4 discusses how the associated moisture
is transported to the YRV from the moisture
source regions. Results from a series of numerical experiments with an RCM are presented in
Sect. 5. Main conclusions are drawn in Sect. 6.
2. Data and methodology
The National Center for Environmental Prediction=National Center for Atmospheric Research (NCEP=NCAR) reanalysis data during
the period 1961–2005 were used in this study.
The 6-hourly variables used to estimate the water
vapor transport and atmospheric circulation include zonal and meridional wind components,
specific humidity, temperature, and geopotential
height at 17 standard pressure levels and the surface pressure fields. Monthly precipitation data
from 600 weather stations in China from 1961–
2005 were utilized to examine the interannual
variability.
For the case study of 1998, surface daily precipitation data from 656 weather stations over
China and the GAME (GEWEX Asian Monsoon
Experiment) regional reanalysis data were also
used. The latter has a horizontal resolution of
0.5 lat. by 0.5 long. at 17 standard pressure levels. The GAME reanalysis was done based on a
Data analysis and numerical simulation of moisture source and transport
data assimilation system with observational data
from the GAME-IOP (Intensive Observation
Period, April 1 to October 31, 1998), GTS (Global Telecommunication System), and data from
several other field experiments, including TIPEX,
GAME=Tibet and HUPEX, and also soundings
and wind profiler data from Japan, Korea, Indonesia, Malaysia, Thailand, Vietnam et al. The
data from GAME=Tibet include east–west and
north-south cross-sections for boundary layer
observations.
The column-integrated water vapor and both
zonal and meridional water vapor transports are
219
calculated based on the following mathematical
formula:
ð
1 Ps
qðx; y; p; tÞdp;
ð1Þ
Qðx; y; tÞ ¼
g 300
ð
1 Ps
qðx; y; p; tÞuðx; y; p; tÞdp; ð2Þ
Qu ðx; y; tÞ ¼
g 300
ð
1 Ps
Qv ðx; y; tÞ ¼
qðx; y; p; tÞvðx; y; p; tÞdp; ð3Þ
g 300
where g is gravity, u=v the zonal=meridional
wind component, q the specific humidity, and
Ps the surface pressure.
Fig. 1. Scatter diagrams of
the column-integrated water
vapor (top), zonal water vapor
flux (middle), and meridional
water vapor flux (bottom)
from GAME versus NCEP
reanalyses. The linear
regression is shown in each
panel with line segments
together with the linear
regression equation and
correlation coefficient
220
X. D. Xu et al.
To see the quality of the NCEP=NCAR and
GAME reanalyses over East Asia, we compared
the column-integrated water vapor, and both the
zonal and meridional water vapor transports calculated from the two reanalysis datasets. In order
for a direct comparison, the calculated quantities
from NCEP=NCAR reanalysis were interpolated
into 0.5 by 0.5 lat.=long. grids, the same grid
system as the GAME reanalysis. As we can see
from Fig. 1, the column-integrated water vapor
and its zonal and meridional transports based
on the two datasets are highly correlated, indicating that the NCEP=NCAR reanalysis is comparable to the GAME reanalysis in describing
water vapor transports in East Asia and thus
can be used in this study as a supplemental data
source to GAME reanalysis. In the following
sections, the GAME reanalysis will be used for
the 1998 case study and the NCEP=NCAR reanalysis will be used in the analysis of interannual variability and also used to force a regional
climate model.
The RCM used in this study is the RegCM3,
which is described in some details in Pal et al.
(2007). RegCM3 is a limited area, hydrostatic,
primitive equation model with sigma as the vertical coordinate. The Grell scheme with Fritsch
and Chappell closure is used for deep convection parameterization (Grell 1993). The radiation is included with the package adopted from
the Community Climate Model version 3 (CCM3,
Kiehl et al. 1998). The subgrid vertical mixing
is parameterized based on the non-local scheme
of Holtslag and Boville (1993). The BiosphereAtmosphere Transfer Scheme (BATS) of
Dickinson et al. (1993) is used as the land surface
model. The NCEP=NCAR reanalysis at every 6hour interval is used as both the initial and lateral
boundary conditions for RegCM3. The experimental design will be discussed in the relevant
section below.
3. Moisture source regions associated
with the summer precipitation in the YRV
To study the moisture sources of the heavy
rainfall in the YRV of China, the 1998 GAME
reanalysis and daily precipitation observations
at 656 weather stations in China are used.
Figure 2 shows the spatial distribution of the
correlation coefficient between the column-integrated water vapor and the daily rainfall averaged in the YRV during June 1–July 31, 1998.
Besides the YRV-East China Sea and the eastern
Tibetan Plateau, the Bay of Bengal, Indochina
Peninsula, and South China Sea-Philippine
Islands are three centers with significant high
positive correlation coefficients (with significance levels of confidence 0.1) which are
far to the south of the YRV heavy rainfall re-
Fig. 2. The spatial distribution of the
correlation coefficient between the daily
precipitation in the YRV and the columnintegrated water vapor in East Asia during
June 1–July 31, 1998. The areas with
correlation coefficients greater than 0.21
or less than 0.21 at 0.1 significance level
and above are shaded. The two dashed
rectangular area denote the YRV
(27.5–32.5 N, 110–122.5 E area A)
and the Bay of Bengal-South China Sea
(5–15 N, 80–130 E, area B), respectively
Data analysis and numerical simulation of moisture source and transport
221
Fig. 3. Interannual variation
of summer precipitation in
the YRV (27.5–32.5 N,
110–122.5 E) and the
column-integrated water vapor
in the Bay of Bengal-South
China Sea (5–15 N,
80–130 E) during 1961–2005
gion. This indicates that the Bay of Bengal to
the Philippine Islands could be the major moisture source regions of the summer precipitation
in the YRV.
To examine the robustness of the moisture
sources identified for the summer of 1998, we
further analyzed the correlation between the annual summer precipitation in the YRV (area A)
and column-integrated water vapor averaged in
the Bay of Bengal-South China Sea (area B)
during 1961–2005 (Fig. 3). It is clearly seen that
overall, the interannual variation of the summer
precipitation over the YRV is positively correlated with the column-integrated water vapor
in the Bay of Bengal-South China Sea region.
The correlation coefficient is 0.346 (with significance levels of confidence 0.05). The significant positive correlation between the summer
precipitation of the YRV and the moisture in
the Bay of Bengal-South China Sea region on
interannual timescales further indicates that the
moisture source identified previously for the
summer of 1998 is robust. Therefore, the Bay
of Bengal-South China Sea is the main moisture
source region of the precipitation in the YRV
and plays an important role in causing the
flooding events during the Mei-Yu season in the
YRV.
The correlation between the YRV summer precipitation and the column-integrated water vapor
in the Bay of Bengal-South China Sea shown
in Fig. 3 only indicates the climatological water
vapor source regions of the YRV precipitation.
It is interesting to examine whether the relation-
ship is applicable to the extreme flood=drought
events. For this reason, we calculated the anomalies of the column-integrated water vapor flux
in summer for flooding years (1980, 1983, 1991,
and 1998) and drought years (1978, 1981, 1985,
and 1986) using NCEP=NCAR reanalysis data,
which is shown in Fig. 4. It can be seen that
the anomalies of water vapor flux for flooding
years (Fig. 4a) show an anticyclone pattern, i.e.,
strong westward (negative anomalies) flux in
the low latitudes (area B), strong eastward (positive anomalies) flux over the YRV (area A) and
the eastern China sea, and northward=northeastward (positive anomalies) flux along 110 E
and 90 E from low latitudes to middle latitudes.
The anomalies of water vapor flux for drought
years (Fig. 4b) are opposite to those for flooding
years, i.e., showing a cyclone pattern. This significant contrast of results between flooding and
drought years indicates that a strong transport
of water vapor from the Bay of Bengal and
South China Sea to the YRV resulted in heavy
summer precipitation in the YRV, while a weak
transport of water vapor accounted for the
drought in the YRV.
4. Moisture transport associated
with summer precipitation in the YRV
To further trace the paths of the moisture transport associated with summer precipitation in
the YRV, we performed a correlation vector
analysis (Xu et al. 2003a). The composite correlation vector can provide an overall view of the
222
X. D. Xu et al.
Fig. 4. The anomalies of the columnintegrated water vapor flux in summer
for (a) flooding years (1980, 1983, 1991,
and 1998) and (b) drought years (1978,
1981, 1985, and 1986) using NCEP=NCAR
reanalysis data
trajectories of water vapor transport and is defined as
~
ð4Þ
Rðx; yÞ ¼ ~iRu ðx; yÞ þ~jRv ðx; yÞ;
where ~
R is the composite correlation vector, Ru is
the component correlation with Qu and Rv is the
component correlation with Qv. Figure 5 shows
the composite correlation vector field, which is
obtained based on the correlation between the
area-averaged daily precipitation over the YRV
and the column-integrated water vapor flux during June 1 and July 31, 1998. We can find that
during June and July, the water vapor source for
the flooding events in the YRV can be traced
back to two branches.
The first one started from the South China Sea
to the west and then turned north to the YRV. The
second branch started from the Bay of Bengal to
the east and then crossed the Indochina peninsula
and finally entered the South China Sea. These
two moisture transport channels merged into one
major channel toward the northeast to the YRV.
The warm, moist air met the continental cold air
from the mid-high latitudes in the YRV and
resulted in persistent heavy rainfall events. Our
results are consistent with Yi and Xu (2002), who
Data analysis and numerical simulation of moisture source and transport
223
Fig. 5. The composite correlation vectors
between the YRV daily precipitation and
the water vapor flux during June 1–July 31,
1998, showing the ‘‘trajectories’’ of water
vapor transport to the YRV
Fig. 6. Time-latitude crosssections of integrated water
vapor flux (vectors, g cm1 s1 )
averaged between 110–130 E
and the daily precipitation
with values larger than 5 mm
(shaded area) averaged
between 110–122.5 E for
June of 1998
found that the water vapor associated with the
1998 heavy summer rainfall in the YRV stemmed
from the westward transport from the South China
Sea and eastward transport from the Arabian Sea.
The latter path climbs the Tibet Plateau from
the south and turns eastward and merges into
the water vapor path from the South China Sea.
As a result, a moisture flux confluence belt appears along the YRV, forming an anomalous
moisture transport path and resulting in the abnormal torrential rainfall events in the YRV.
The time evolution of moisture transport and
the corresponding precipitation can be clearly
seen in Fig. 6, which shows the time-latitude
cross-section of integrated water vapor flux averaged between 110–130 E and the daily
precipitation averaged between 110–122.5 E
for June of 1998. At the beginning of June, precipitation occurred to the south of the YRV
where a strong northeastward moisture transport
was located. This precipitation band moved
northeastward with time until June 13 when a
southeastward water vapor flux met the northeastward water vapor flux at the YRV. Later on,
the precipitation band stayed at the YRV until the
end of June.
To further confirm the moisture source of summer precipitation over the YRV identified in the
summer of 1998, we show in Fig. 7 the interannual variations of the monthly total summer
precipitation in the YRV and the balance of column-integrated water vapor in the regions of the
Bay of Bengal and South China Sea (area B,
5–15 N, 80–130 E) during 1961–2005. The in-
224
X. D. Xu et al.
Fig. 7. Interannual variations
of summer precipitation in
the YRV (27.5 –32.5 N,
110 –122.5 E) and the
balance of column-integrated
water vapor flux in the Bay
of Bengal-South China Sea
(5 –15 N, 80 –130 E)
during 1961–2005
terannual variation of summer precipitation over
the YRV is positively correlated with the balance
of water vapor in the Bay of Bengal-South China
Sea regions with a correlation coefficient of
0.436 (with significance levels of confidence
0.05), especially for the flooding years of
1969, 1980, 1983, 1991 and 1998. The significant positive correlation between the YRV summer precipitation and the Bay of Bengal-South
China Sea moisture balance on interannual timescales further confirms that the long-range moisture transport identified previously for 1998 is
robust. Therefore the Bay of Bengal and South
China Sea are the main moisture source regions
and the northward transport of water vapor by the
summer monsoon flow plays an important role in
causing the flooding events over the YRV.
Figure 8 shows the anomalies of the water vapor balance over East Asia during June 1–July
31, 1998. Note that values on the boundaries
of each grid cell are anomalies of the eastward=northward (positive) or westward=southward (negative) water vapor flux, while that at
the center of each cell is the net gain (positive)
or loss (negative) of water vapor within the grid
cell. As we can see from the figure, a westward
water vapor transport occurs at low latitudes over
the Bay of Bengal and South China Sea (area B)
and an eastward transport appears in the mid-latitude region over the YRV (area A). Two significant channels transporting extra water vapor
from the low-latitude to the mid-latitude can be
seen for cells between 80 and 90 E and those
between 100 and 110 E. The westward, north-
Fig. 8. The anomalies of the water
vapor balance over East Asia during
June 1–July 31, 1998 relative to the same
period climatology of 1961–2005. The
values (in kg s1 ) marked near the boundary
of each grid cell are anomalies of the
eastward=northward (positive) or
westward=southward (negative) water
vapor transport, while those on the center
of the cells are the net gain (positive) or
loss (negative) of the water vapor within
the grid cell
Data analysis and numerical simulation of moisture source and transport
225
Fig. 9. As in Fig. 8, but for the summers
of all flooding years (1969, 1980, 1983,
1991, 1993, 1996, 1998, and1999) in the
YRV during 1961–2005. The anomalies of
the water vapor balance were calculated by
subtracting the climatological mean of
1961–2005 from the mean flooding years
ward, and eastward transports form an anticyclone path, bringing water vapor from the Bay
of Bengal and South China Sea (area B) to the
YRV and resulting in heavy rainfall in the YRV in
the summer of 1998.
To see whether such the pattern of moisture
transport shown in Fig. 8 is robust for all flooding
events in the YRV or only holds for the case of
1998, the anomalies of the water vapor balance in
summer over East Asia for all flooding years
during 1961–2005 were calculated and plotted
in Fig. 9. A similar anticyclone path of the water
vapor transport is found as in Fig. 8. Therefore,
the Bay of Bengal and South China Sea (area B)
are indeed the main water vapor source regions
for the heavy rainfall in YRV and the moisture
was transported from the source region to YRV in
an anticyclone path consistent with the monsoon
flow.
5. RCM simulation
In this section, results from a series of numerical
experiments using the regional climate model
RegCM3 described in Sect. 2 will be discussed
to further understand the moisture source and
transport associated with summer precipitation
in the YRV. Two sets of numerical experiments
were conducted. The first set of experiments is
multi-year simulations with a relatively coarse
horizontal resolution. Namely, the model was
run at 80 km grid spacing with a domain size of
120 by 80 grid points centered at 25.5 N, 110 E.
The other aspects of the model are given in
Sect. 2. The NCEP=NCAR reanalysis at 6-hour
interval was used as both the initial and lateral
boundary conditions for RegCM3. The model
was integrated from May 15 through September
1 for 25 years from 1976 to 2000. This set of
experiments was designed to examine the model’s capability of reproducing the climatology of
the observed water vapor source=sink and water
vapor transport in East Asia.
Figure 10a and b shows the scatter diagram of
the zonal and meridional water vapor transports,
respectively, averaged in the domain 5–40 N,
80–130 E from reanalysis versus those from
model simulation. Note that in order for a direct
comparison, here the model results were interpolated to the 2.5 by 2.5 lat.=long. grid before
the area-average was made. It is evident that,
although the water vapor transport components
calculated from the reanalysis and the model
simulation are scattered, they are highly correlated. Note that there seems a systematic bias for
the zonal water vapor transport, where the slope
of the linear regression is somewhat less than 1,
indicating that the zonal transport of water vapor
from the model simulation is larger than the observed (Fig. 10a). Nevertheless, the overall water
vapor transports from the reanalysis and the
model simulation are comparable, indicating that
the model is able to reproduce the water vapor
transport in East Asia.
Figure 11a and b shows the simulated anomalies of the column-integrated water vapor flux in
226
X. D. Xu et al.
Fig. 10. Scatter diagrams of
the column-integrated zonal
(top) and meridional (bottom)
water vapor flux components
averaged in the box of
(5–40 N, 80–130 E) from
NCEP reanalysis versus
RegCM simulation for 25
summers (from May 15 to
September 1) from 1961 to
2000. The linear regression
is shown in each panel with
line segments together with
the linear regression equation
and correlation coefficient
summer for flooding years (1980, 1983, 1991,
and 1998) and drought years (1978, 1981,
1985, and 1986), respectively. Similar to the
results from the NCEP reanalysis data (Fig. 4),
the simulated anomalies of water vapor flux for
flooding years (Fig. 11a) also show an anticyclone pattern while those for drought years show
a cyclone pattern over the regions of South China
Sea, the Bay of Bengal, Tibetan Plateau, the YRV
and the eastern China Sea-western North Pacific.
Compared with Fig. 4, the model reproduced the
overall patterns of moisture transports in East
Asia for the summer flooding and drought years
of the YRV.
The second set of experiments was conducted
to quantify the contribution of water vapor transport from the Bay of Bengal and South China Sea
to summer precipitation in the YRV. This was
done with several sensitivity experiments for
the YRV flooding event during the 1998 summer.
The center of the model domain is at 30 N,
105 E. There are 120 and 100 grid points in
the east–west and north–south directions with
a horizontal resolution of 60 km. The model
has 18 vertical levels with the model top at
50 hPa. The model domain covers the main area
of East Asia (Fig. 12) and the Bay of BengalSouth China Sea. The model also represents
the orography reasonably well in the region
(Fig. 12). Four numerical experiments were performed to demonstrate the importance of longdistance moisture transport to the heavy rainfall in the YRV in 1998. The control experiment
was run with all default parameters, while the
other three sensitivity experiments (denoted as
st1, st2, and st3) were run with reduced moisture
flux along the lateral boundaries over the Bay of
Bengal-South China Sea. Namely, the water vapor flux across the eastern, western, and southern boundaries surrounding the Bay of Bengal
and South China Sea (5–15 N, 80–130 E) was
reduced by 1=4, 1=2, and 7=8 in st1, st2, and
st3, respectively. Since there is a net gain of
water vapor in the regions of the Bay of
Bengal and South China Sea (as shown in
Figs. 8 and 9) due to water vapor flux along
the eastern boundary, the reduction of water vapor across the eastern, western, and southern
boundaries results in a proportional reduction
of water vapor gain in these regions.
Data analysis and numerical simulation of moisture source and transport
227
Fig. 11. The simulated anomalies of the
column-integrated water vapor flux in
summer for (a) flooding years (1980, 1983,
1991, and 1998) and (b) drought years (1978,
1981, 1985, and 1986) from the numerical
experiments using RegCM3 (unit: g cm1 s1 )
Figure 13 shows the observed and the control
experiment simulated total precipitation during
June 12–28, 1998. The model simulated rainfall
center in Southwest China is to the north, while
that in South China is to the east of the corresponding observed. The simulated rainfall amount
in the YRV is weaker than the observed, in particular the maximum rainfall amount is largely
underestimated. This is partly due to the relatively coarse model resolution used and partly due to
the discrepancy in model physics. Although the
model has considerable biases, it reproduced the
major precipitation pattern in South and East
China. We can consider that overall the model
has the ability to simulate the severe precipitation
events in East Asia during June 12–28, 1998.
This allows us to conduct three sensitivity experiments to quantify the effect of long-distance water vapor transport from the Bay of Bengal-South
China Sea on precipitation in the YRV.
Figure 14 shows the distribution of the model
simulated precipitation from three sensitivity
experiments (left panels) and the corresponding
difference from the control one (right panels).
One can see that although the overall spatial distribution is similar to that in control experiment,
both the area and magnitude of main precipitation centers were reduced considerably in proportion to the reduction of water vapor transport
from the Bay of Bengal-South China Sea in the
228
X. D. Xu et al.
Fig. 12. The regional climate
model domain (shaded) and
topography (contours in
meters) used for numerical
experiments for the case of
the 1998 summer
Fig. 13. Total rainfall (mm) during June 12–28, 1998 from observation (left) and control simulation (right). The areas
with rainfall greater than 200 mm are shaded
sensitivity experiments. For example, when the
water vapor flux was reduced by 1=4, 1=2, and
7=8 in st1, st2, and st3, the overall rainfall was
reduced by 25–30%, 40–50%, and 50–60%, respectively in most areas in South China and the
YRV. The reduction in total precipitation in these
regions results directly from the reduction of water vapor available to these regions. This is mainly due to the fact that the Bay of Bengal and
South China Sea are dominated by the southwesterly monsoon flow in the lower troposphere,
and thus reducing the water vapor near the lateral
boundaries as designed in the sensitivity experiments reduces the water vapor transport from the
Bay of Bengal and South China Sea to South and
East China and thus results in a significant decrease in precipitation in the region.
Table 1 lists the area-averaged rainfall over
the YRV (25–30 N, 110–120 E) from observation and model simulations, the difference between the sensitivity and control experiments,
and the relative reduction in percentage from
the control simulation. Consistent with what we
see from Fig. 14, the area-averaged rainfall over
the YRV shows a proportional decrease with the
reduction of water vapor transport from the Bay
of Bengal and South China Sea. For example,
when the water vapor near the lateral boundaries
was reduced by 7=8 in st3, the area-averaged
precipitation over the YRV was reduced by about
half. This is consistent with the results of Xu et al.
(2003a), Qian et al. (2004), and the latest study
by Chow et al. (2007). The latter found that the
water vapor transport by the southwesterly asso-
Data analysis and numerical simulation of moisture source and transport
229
Fig. 14. Total rainfall (mm) during June 12–28, 1998 from three sensitivity experiments st1 (top), st2 (middle),
and st3 (bottom) (left) and the corresponding difference from the control experiment (right panels). The areas with rainfall
greater than 200 mm are shaded in the left panels and the negative values are shaded in the right panels
Table 1. The area averaged rainfall amount during June
12–28, 1998 over the YRV (25–30 N, 110–120 E)
from observation and model simulations, the difference
between the sensitivity and control experiments and
observation, and the relative reduction in percentage
from the control simulation
Observation
ctl
st1
st2
st3
Rainfall
amount
(mm)
Difference
from control
experiment (mm)
Percentage
in reduction
(%)
366.7
313.7
244.7
209.6
158.7
69.0
104.1
155.0
22.0
33.2
49.4
ciated with the Indian summer monsoon contributed up to half of the early summer precipitation
over China. Our results from the sensitivity experiments further confirm that water vapor transport from the Bay of Bengal and South China Sea
accounts for at least half of the summer precipitation in the YRV and thus Bay of Bengal-South
China Sea are the main moisture source region to
the heavy precipitation in the YRV.
Note that the rainfall in Northeast China increased as the water vapor transport from the
eastern Indian Ocean and South China Sea was
reduced (Fig. 14). This is due to the fact that the
precipitation decrease in the YRV reduces the
230
X. D. Xu et al.
convection and latent heat release in the YRV.
This heating source otherwise would have a feedback onto the large-scale convergence in the lower troposphere. The reduction of this atmospheric
heating source thus reduces the moisture convergence from the higher latitudes to the north and
northeast. As a result, the water vapor available
to and thus the precipitation in Northeast China
would increase as the water vapor transport from
the eastern Indian Ocean and South China Sea is
reduced. Therefore, in addition to the direct reduction of moisture transport to the YRV and South
China, there is an indirect effect of the reduced
precipitation over the YRV on the large-scale circulation and thus precipitation in Northeast China
in the sensitivity experiments.
6. Conclusions
In this study, we have investigated the moisture
source=sink and long-distance moisture transport
associated with the summer precipitation in the
mid-lower reaches of the Yangtze River Valley in
China based on both data analysis and numerical
simulation using a regional climate model driven
by the NCEP=NCAR reanalysis. Although detailed analysis is mainly based on the extreme
case for the summer of 1998, the findings are
robust as we have shown for the interannual variations of summer precipitation in the YRV and
the moisture source in the eastern Indian OceanSouth China Sea and its total water vapor budget
during 1961–2005. In particular, the contribution
by the long-distance moisture transport to the
summer precipitation in the YRV is quantified
with sensitivity numerical experiments using the
regional climate model RegCM3. Our main conclusions are summarized below.
(1) The summer precipitation in the YRV is
highly correlated with the column-integrated
water vapor in the low latitudinal band from
the Bay of Bengal, Indochina Peninsula, and
South China Sea to the Philippine Islands.
They are found to be the major moisture
source regions for the summer precipitation
in the YRV.
(2) The water vapor transport from the source
regions to the YRV is mainly through a pathway associated with the southwesterly monsoonal flow. An eastward transport from the
Bay of Bengal and a westward transport
from the Philippine Islands meet over South
China Sea and form a main water vapor
channel to the north which turns northeastward in the vicinity of the eastern Tibetan
Plateau and South China to the YRV.
(3) The regional climate model RegCM3 driven
by the NCEP=NCAR reanalysis is able to
reproduce the overall climatological water
vapor transport in East Asia and the contrast
features of the water vapor flux between the
flooding and drought years in the YRV. The
model also shows a good performance in reproducing the precipitation pattern of the
YRV flooding events in the summer of
1998, although the simulated precipitation
amount is generally weaker than the observed and there also are some biases in
reproducing the location of heavy precipitation centers in the region.
(4) Our numerical sensitivity experiments show
that the moisture transport from the eastern
Indian Ocean-South China Sea to the YRV
contributes to more than half of the summer
precipitation in the YRV. We also found that
in addition to a direct reduction of the summer precipitation in the YRV, a reduction of
the water vapor transport from the Bay of
Bengal-South China Sea results in a decrease
in the atmospheric heating source, which in
turn has a feedback to the large-scale flow in
the lower troposphere and thus indirectly affects precipitation in Northeast China.
Although we have identified the moisture
sources and long-distance moisture transport associated with summer precipitation in the YRV,
detailed studies are still needed to further quantify the moisture flux through moisture budget
analysis. Such an analysis should include multiple years and provide contrasts between climate
extremes, such as the drought and flood years. It
is not clear yet whether the moisture sources to
the YRV experience any interannual or longer
time-scale variations. Future studies may focus
on this aspect in order to have a deep understanding of the water cycle in East Asia. Another important area is to identify the role of Tibetan
Plateau in the water cycle in East Asia. Previous
studies have shown the importance of Tibetan
Plateau to the water vapor transport to the YRV
Data analysis and numerical simulation of moisture source and transport
qualitatively. Data analysis and modeling studies
could help quantify the effect of Tibetan Plateau
on the water cycle in the East Asian monsoon
system.
Acknowledgments
This study was jointly supported by the China and Japan
intergovernmental cooperation program (JICA); the social
commonwealth research program of Ministry of Science
and Technology of the People’s Republic of China under
grant 2005DIB3J057; and the Chinese National Natural Science Foundation under grant 90502003.
References
Chow KC, Tong H-W, Chan JCL (2007) Water vapor sources
associated with the early summer precipitation over
China. Clim Dyn; DOI: 10.1007=s00382-007-0301-6
Dickinson RE, Henderson-Sellors A, Kennedy PJ (1993)
Biosphere-atmosphere transfer scheme (BATS) version 1e
as coupled to the NCAR Community Climate Model.
NCAR Technical Note. NCAR=TN-387 þ STR, NCAR,
Boulder, CO, 72 pp
Ding Y, Hu G (2003) A study on water vapor budget over
China during the 1998 severe flood periods. Acta Meteor
Sinica 61: 129–45 (in Chinese)
Ding Y-H (1992) Summer monsoon rainfalls in China. J
Meteor Soc Japan 70: 373–96
Grell GA (1993) Prognostic evaluation of assumptions used
by cumulus parameterization. Mon Wea Rev 121: 764–87
He J-H, Ju J-H, Wen Z-P, Lu J-M, Jin Q-H (2007) A review
of recent advances in research on Asian monsoon in
China. Adv Atmos Sci 24: 972–92
Holtslag AAM, Boville BA (1993) Local versus nonlocal
boundary-layer diffusion in a global climate model. J
Climate 6: 1825–42
Kiehl JT, Hack JJ, Bonan GB, Boville BA, Williamson DL,
Rasch PJ (1998) The national climate center for atmospheric research community climate model (CCM3). J
Climate 11: 1307–26
Pal JS, Giorgi F, Bi X, Elguindi N, Solomon F, Gao X,
Francisco R, Zakey A, Winter J, Ashfaq M, Syed F, Bell
JL, Diffanbaugh NS, Kamacharya J, Konare A, Martinez
231
D, da Rocha RP, Sloan LC, Steiner A (2007) RegCM3 and
RegCNET: regional climate modeling for the developing
world. Bull Amer Meteor Soc 88: 1395–409
Qian JH, Tao WK, Lau KM (2004) Mechanisms for torrential rain associated with the Mei-Yu development during
SCSMEX 1998. Mon Wea Rev 132: 3–27
Simmonds I, Bi D, Hope P (1999) Atmospheric water vapor
flux and its association with rainfall over China in summer. J Climate 12: 1353–67
Tao S, Chen L (1988) A review of recent research on the
East Asian summer monsoon in China. In: Chang C-P,
Krishnamurti TN (eds) Monsoon meteorology. Oxford
University Press, Oxford, pp. 60–92
Wang Y, Sen OL, Wang B (2003) A highly resolved regional
climate model (IPRC_RegCM) and its simulation of the
1998 severe precipitation events over China. Part I: Model
description and verification of simulation. J Climate 16:
1721–38
Wang Y, Leung LR, McGregor JL, Lee D-K, Wang W-C,
Ding Y-H, Kimura F (2004) Regional climate modeling:
Progress, challenges and prospects. J Meteor Soc Japan
82: 1599–628
Xu X, Tao S, Wang J, Chen L, Zhou L, Wang X (2002) The
relationship between water vapor transport features of
Tibetan Plateau-monsoon ‘‘Large Triangle’’ affecting
region and drought-flood abnormality of China. Acta
Meteor Sinica 60: 257–66 (in Chinese)
Xu X, Chen L, Wang X, Miao Q, Tao S (2003a) The sourcesink structure of water vapor transport of Mei-Yu belt
in Changjiang River basin. Chinese Sci Bull 48: 2288–94
(in Chinese)
Xu X, Miao Q, Wang J, Zhang X (2003b) The water vapor
transport model at the regional boundary during Mei-Yu
period. Adv Atmos Sci 20: 333–42
Xu X, Zhou L, Chang S, Miao Q (2003c) Characteristics of the
correlation between regional water vapor transport along
with the convective action and variation of the Pacific
subtropical high in 1998. Adv Atmos Sci 20: 269–83
Yi Q, Xu X (2002) The propagation and development of
cloud cluster systems and severe precipitation event in
1998. Climatic Environ Res 6: 129–45
Zhou T-J, Yu R-C (2005) Atmospheric water vapor transport
associated with typical anomalous summer rainfall patterns in China. J Geophy Res 110: D08104; DOI:
10.1029=2004JD005413