Data analysis and numerical simulation of moisture source and transport associated with summer precipitation in the Yangtze River Valley over China

Transcription

1 Meteorol Atmos Phys 100, (2008) DOI /s 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. Xu 1,X.Y.Shi 1, Y. Q. Wang 2, S. Q. Peng 3, X. H. Shi 1 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 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 ( yuqing@hawaii.edu) overall climatological water vapor transport in East Asia and the features of the water vapor flux for flooding and drought years in the YRVas 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, 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

2 218 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 YRVof 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 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 Data and methodology The National Center for Environmental Prediction=National Center for Atmospheric Research (NCEP=NCAR) reanalysis data during the period 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 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

3 Data analysis and numerical simulation of moisture source and transport 219 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 calculated based on the following mathematical formula: Qðx; y; tþ ¼ 1 g Q u ðx; y; tþ ¼ 1 g ð Ps 300 ð Ps 300 ð Ps qðx; y; p; tþdp; qðx; y; p; tþuðx; y; p; tþdp; ð1þ ð2þ Q v ðx; y; tþ ¼ 1 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 P s 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

4 220 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 Biosphere- Atmosphere Transfer Scheme (BATS) of Dickinson et al. (1993) is used as the land surface model. The NCEP=NCAR reanalysis at every 6- hour 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, 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, 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 ( N, E area A) and the Bay of Bengal-South China Sea (5 15 N, E, area B), respectively

5 Data analysis and numerical simulation of moisture source and transport 221 Fig. 3. Interannual variation of summer precipitation in the YRV ( N, E) and the column-integrated water vapor in the Bay of Bengal-South China Sea (5 15 N, E) during 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 (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 (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 relationship 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

6 222 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 ~Rðx; yþ ¼~iR u ðx; yþþ~jr v ðx; yþ; ð4þ where ~R is the composite correlation vector, R u is the component correlation with Q u and R v is the component correlation with Q v. 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, 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

7 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 cm 1 s 1 ) averaged between E and the daily precipitation with values larger than 5 mm (shaded area) averaged between Efor 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 E and the daily precipitation averaged between E for June of 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 YRVuntil 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, E) during The in-

8 224 X. D. Xu et al. Fig. 7. Interannual variations of summer precipitation in the YRV ( N, E) and the balance of column-integrated water vapor flux in the Bay of Bengal-South China Sea (5 15 N, E) during 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 (with significance levels of confidence 0.05), especially for the flooding years of 1969, 1980, 1983, 1991 and 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, 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 The values (in kg s 1 )markedneartheboundary 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

9 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 The anomalies of the water vapor balance were calculated by subtracting the climatological mean of 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 YRVand resulting in heavy rainfall in the YRVin the summer of 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 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 YRVin 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 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, 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

10 226 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, E) from NCEP reanalysis versus RegCM simulation for 25 summers (from May 15 to September 1) from 1961 to 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 Bengal- South 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 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, 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.

11 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 cm 1 s 1 ) Figure 13 shows the observed and the control experiment simulated total precipitation during June 12 28, 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, 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

12 228 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, 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-

13 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, 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 Rainfall amount (mm) Difference from control experiment (mm) Observation ctl st st st Percentage in reduction (%) 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

14 230 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 YRVon 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 Ocean- South China Sea and its total water vapor budget during 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

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