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The Upstream "Strong Signals" of the Water Vapor Transport over the Tibetan Plateau during a Heavy Rainfall Event in the Yangtze River Basin


doi: 10.1007/s00376-016-6118-7

  • A heavy rainfall event that occurred over the middle and lower reaches of the Yangtze River Basin (YRB) during July 11-13 2000 is explored in this study. The potential/stream function is used to analyze the upstream "strong signals" of the water vapor transport in the Tibetan Plateau (TP). The studied time period covers from 2000 LST 5 July to 2000 LST 15 July (temporal resolution: 6 hours). By analyzing the three-dimensional structure of the water vapor flux, vorticity and divergence prior to and during the heavy rainfall event, the upstream "strong signals" related to this heavy rainfall event are revealed. A strong correlation exists between the heavy rainfall event in the YRB and the convective clouds over the TP. The "convergence zone" of the water vapor transport is also identified, based on correlation analysis of the water vapor flux two days and one day prior to, and on the day of, the heavy rainfall. And this "convergence zone" coincides with the migration of the maximum rainfall over the YRB. This specific coupled structure actually plays a key role in generating heavy rainfall over the YRB. The eastward movement of the coupled system with a divergence/convergence center of the potential function at the upper/lower level resembles the spatiotemporal evolution of the heavy rainfall event over the YRB. These upstream "strong signals" are clearly traced in this study through analyzing the three-dimensional structure of the potential/stream function of upstream water vapor transport.
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  • Boos W. R., Z. M. Kuang, 2010: Dominant control of the South Asian monsoon by orographic insulation versus plateau heating. Nature, 463( 7278), 218- 222.10.1038/nature0870720075917904464c9510fc5093d645e7febe9b3aehttp%3A%2F%2Fwww.nature.com%2Fabstractpagefinder%2F10.1038%2Fnature08707http://med.wanfangdata.com.cn/Paper/Detail/PeriodicalPaper_PM20075917The Tibetan plateau, like any landmass, emits energy into the atmosphere in the form of dry heat and water vapour, but its mean surface elevation is more than 5 km above sea level. This elevation is widely held to cause the plateau to serve as a heat source that drives the South Asian summer monsoon, potentially coupling uplift of the plateau to climate changes on geologic timescales. Observati...
    Chen B., X.-D. Xu, S. Yang, and W. Zhang, 2012: On the origin and destination of atmospheric moisture and air mass over the Tibetan Plateau. Theor. Appl. Climatol., 110, 423- 435.10.1007/s00704-012-0641-y02aab151f4eb993d551c72e793db7ee3http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00704-012-0641-yhttp://link.springer.com/article/10.1007/s00704-012-0641-yThe Tibet Plateau (TP) is a key region that imposes profound impacts on the atmospheric water cycle and energy budget of Asia, even the global climate. In this work, we develop a climatology of origin
    Chen B., X.-D. Xu, and T. L. Zhao, 2013: Main moisture sources affecting lower Yangtze River Basin in boreal summers during 2004-2009. Int. J. Climatol., 33, 1035- 1046.10.1002/joc.3495ecf5a4e62bfdf2d39e2f60040e52d163http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fjoc.3495%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/joc.3495/fullThis study presented a quantitative climatology of atmospheric moisture affecting the Yangtze River Basin (YRB) region in boreal summer season, as well as the spatial evolution of atmospheric moisture passage towards the target regions. A three‐dimensional Lagrangian particle dispersion model FLEXPART was driven by the National Centers for Environmental Prediction Final Operational Global Analysis data combined with a backtracking scheme, and the transport contributions to the moisture budget over the YRB region was identified through the continuous calculation of changes in specific humidity along the FLEXPART back trajectories of all air masses residing over this region for a period of six summers of year 2004–2009. The back trajectory analysis revealed four major moisture sources contributed to the YRB summer water vapour with different transport timescales: the East China Sea (17.5%), South China Sea (26.6%), Indian peninsula and the Bay of Bengal (20.5%) and Arabian Sea (13.6%). The properties of moisture sources and its transport processes are dominated by the Asian Summer Monsoon regimes. The Tibetan Plateau also acts as an effective barrier for the meridional moisture transport, leading to distinct moisture sinks at the southern slope. In contrast to the previous results, the tropical western Pacific only plays a minor role in the water vapour contributors. The importance of the four source areas varies over the summer: East and South China Sea sources persist throughout the summer, whereas the Indian peninsula, the Bay of Bengal and Arabian Sea sources reach the strongest moisture supply to the YRB region only in high summer (July), showing a close association with the March of Asian monsoon. The further evaluation shows that the inter‐annual variability of precipitation over YRB is strongly related to the moisture sources in the Bay of Bengal and Arabian Sea. Copyright 08 2012 Royal Meteorological Society
    Chen L.X., E. R. Reiter, and Z. Q. Feng, 1985: The atmospheric heat source over the Tibetan Plateau: may-august 1979. Mon. Wea. Rev., 113( 10), 1771- 1790.10.1175/1520-0493(1985)1132.0.CO;2c9198782cfda56e2d1712e4d53801307http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F1985MWRv..113.1771Chttp://adsabs.harvard.edu/abs/1985MWRv..113.1771CNot Available
    Chen S. X., 1982: The definition and index of summer monsoon in the south of China. Chinese Tropical Geography, 2( 2), 10- 14. (in Chinese)
    Chow K. C., H. W. Tong, and J. C. L. Chan, 2008: Water vapor sources associated with the early summer precipitation over China. Climate Dyn., 30, 497- 517.10.1007/s00382-007-0301-6865dac28d3977f38cbafc080b045c9b9http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00382-007-0301-6http://link.springer.com/article/10.1007/s00382-007-0301-6This study investigates the water vapor sources for the early summer precipitation over China in association with the Asian summer monsoon, based on the sensitivity experiments performed by a regional climate model for the year 1998. It is found that the northern South China Sea (NSCS) is an important region for the early summer precipitation over China, particularly the south China region. The evaporative water vapor flux or sea surface temperature over the NSCS could significantly affect the southwesterly water vapor transport towards the NSCS. This in turn may significantly change the water vapor transport from the NSCS to China and so changes the precipitation there. The results of the experiments also show that the precipitation over China does not particularly depend on the water vapor transports from some distant sources by the large-scale flows. Most of the required water vapor could be obtained from the ocean within the monsoon region. The results suggest that the water vapor transport over China is basically a combination of the southeasterly water vapor transport associated with the north Western Pacific subtropical high and the southwesterly water vapor transport associated with the Indian summer monsoon. Without the latter, the early summer precipitation over China could be reduced by up to half of the original amount.
    Gao G. D., P. M. Zhai, 1993: Water vapour transport during two wet/drought summers over the Yangtze river valley. Advances in Water Science, 4, 10- 16. (in Chinese)60f4a50768731a7f9f1d1fc2cbb4b45chttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-SKXJ199301001.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-SKXJ199301001.htmBased on the radiosonde data observed two times a day for 125 stations in China, the total transport, eddy transport and divergence fields of water vapour are calculated and analyzed for the whole atmospheric layer over the Yangze River Valley during two typical wet/drough summers (1980/1985), when there were three currents that carried water vapour steadily from the southwest, northwest and southeast respectively, and then joined together over the Yangtze River Valley, high precipitation would appear near a convergence belt, yielding floods and water-loggings. On the other hand, during drought period, the three currents are weak and unstable, so that the condition of convergence belt formation cannot be met. Eddy transport also has similar feature. Water vapour transport due to a strong and stable current from the southwest, is a main source of precipitation.
    Guan Z. Y., J. Han, and M. G. Li, 2011: Circulation patterns of regional mean daily precipitation extremes and their linear trends over the middle and lower reaches of Yangtze River during boreal summer. Climate Research, 50, 171- 185.10.3354/cr0104539186021f734d8b30e3f0d53c7a9d5e0http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F274419772_Circulation_patterns_of_regional_mean_daily_precipitation_extremes_over_the_middle_and_lower_reaches_of_the_Yangtze_River_during_the_boreal_summerhttp://www.researchgate.net/publication/274419772_Circulation_patterns_of_regional_mean_daily_precipitation_extremes_over_the_middle_and_lower_reaches_of_the_Yangtze_River_during_the_boreal_summerABSTRACT By employing the composite analysis and using daily data from the National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis and precipitation records at 743 stations in China, the anomalous circulation patterns for the regional mean daily precipitation extreme (DPE) events over the middle and lower reaches of Yangtze River (MLRYR) in June, July and August of the boreal summer during the period from 1979 to 2008 were investigated. This analysis determined that there have been 93 DPE events in the past 30 yr. Two types of anomalous circulation patterns were revealed in association with these DPE events. A Type I circulation pattern demonstrates an anomalous cyclone over MLRYR and an anomalous anticyclone in the South China Sea (SCS) and tropical northwestern Pacific Ocean (NWP) in the lower troposphere. An anomalous anticyclone in the upper troposphere exists in the region south of the MLRYR. The water vapor is transported not only from the Bay of Bengal, the SCS and the NWP, but also from areas northwest of the MLRYR. The apparent heating anomalies are favorable for intensifying the anomalous vertical meridional circulations in East Asia. The pattern of sea surface temperature anomalies (SSTAs) in the Indo-Pacific sector looks similar to the SSTA pattern during the maturing and decaying phases of El Nino episodes. Of the 93 DPE events 16 were induced by a Type II circulation pattern. This pattern looks largely different from the Type I pattern in aspects of distributions of anomalous winds, divergence, water vapor sources, thermal forcings and SSTAs. These results will help us understand the occurrences of DPE events in the MLRYR.
    Hu G. Q., Y. H. Ding, 2003: A study on the energy and water cycles over Changjiang-Huaihe river basins during the 1991 heavy rain periods. Acta Meteorologica Sinica, 61( 3), 146- 163. (in Chinese)10.1007/BF02948883bdb8fab087818b3ff5d6b5741849f1cahttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QXXB200302001.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-QXXB200302001.htmAfter analysis of the global water vapor background during the 1991 heavy rain over Changjiang-Huaihe (Jianghuai) River Basins (simply called JRB), the energy and water budgets over the JRB heavy rain region are given attention, and main conclusions are gotten as follows: (1)The mechanism of water vapor transport is that, on one side, plentiful water vapor in stationary eddy mode transports to JRB from Bengal Bay and South China Sea, on the other side, the JRB moisture transports to north in transient eddy mode which may be related to meso- scale and meso- scale systems occurring frequently in JRB, and they make part of the JRB plentiful moisture influx drop in precipitation and the others transport to high latitude region (lesser moisture region) in order to maintain the global water vapor balance. (2)During the rainfall, the local evaporation term is very important in the water vapor supplies and recycles, which is 1/3-1/2 of the precipitation. This is the same as in 1998. (3)During the rainfall, the water vapor comes mostly from the southern and western boundaries of the heavy rain region. When the rainfall is strong, inflow of water vapor from the south is stronger than from the west, and the reverse is true in a weak rainfall event. The outflows of water vapor go through the eastern and northern boundaries and mainly the east. Both inflow and outflow of water vapor happen principally in the low and middle layers. (4)During the five rainfall processes, the big value regions of the apparent heat source and the apparent moisture sink correspond to the strong precipitation regions, indicating that the water vapor condensation is the main effect in the air heating. During the whole Meiyu period, it is mainly convection precipitation which is stronger and stronger with the season moving. In the rainfall, vertical ascending motion occurs and when the rainfall is stronger the vertical ascending motion is stronger with higher level ascending center, and these are related to the positive feedback between the release of cumulus condensation and vertical ascending motion.
    Jian M. Q., H. B. Luo, 2001: Daily variation of heat sources over the eastern Qinghai-Xizang plateau and surrounding areas and their relationship to the circulation over the Tibetan Plateau. Plateau Meteorology, 21( 2), 25- 30. (in Chinese with English abstract)d5c360b8be036ca390c507c206b2f0c6http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTotal-GYQX200201004.htmhttp://en.cnki.com.cn/Article_en/CJFDTotal-GYQX200201004.htmUsing the twice daily routine rawinsonde data in the domain 90°~130°E, -5°~45°N from 1 May to 31 August 1998, the heat source and moisture sink are computed, and the diurnal variation characteristics of heat sources and moisture sinks and their relationship to the circulation over the eastern Qinghai-Xizang Plateau are analyzed. The results show that the heat sources over thearea from Indo-China peninsula to the eastern Plateau are stronger at 12:00 than at 00:00, while the heat sources over the area from the central South China Sea to the central China are strong at 00:00, but weak at 12:00. The moisture sink has a similar diurnal variation to the heat source. The ascending motion over the eastern Qinghai-Xizang Plateau becomes stronger significantly in the evening and the local meridional monsoon circulation also intensifies. Meanwhile, the ascending motion over the middle reaches of Yangtze river at 00:00 becomes weaker and turns to descending motion in the evening. Thus a local thermal diurnal zonal circulation exists with the up branch over the eastern Plateau and the down branch over the middle reaches of Yangtze river. The heavy rainfall in summer, leading to a flooding disaster, occurmainly early in the morning.
    Jin Z. H., 1981: The budget of moisture in the summer of 1979 in China South Sea. The Tropical Summer Monsoon Conference. Yunnan People Press, 151- 164. (in Chinese)
    Li J. P., 2012: The study of characteristics, circulation situation and vapor track of low vortex on the Qinghai-Tibet Plateau. PhD dissertation, Lanzhou University, 31- 47. (in Chinese)
    Liu Y. Y., Y. H. Ding, 2009: Influence of the western north Pacific summer monsoon on summer rainfall over the Yangtze River Basin. Chinese Journal of Atmospheric Sciences, 33( 6), 1225- 1237. (in Chinese with English abstract)10.1016/S1003-6326(09)60084-4b0d52eb3-f8ef-4db0-a66d-ba3f0b8946c5482532009336218d153dbf675bb01f06535df6dc19a22http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK200906010.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK200906010.htmBased on the NCEP/NCAR reanalysis circulation data and the precipitation data from 1979 to 2005, the relationship between the western North Pacific summer monsoon (WNPSM) and the summer rainfall over the Yangtze River basin has been discussed from both time and space aspects by the analysis of the precipitation, atmospheric circulations, water vapor transport and intraseasonal oscillations in the summer monsoon period. The results show that: (1) There is a remarkable negative correlation between WNPSM and the precipitation over the Yangtze River basin. When the WNPSM strengthens, the western Pacific subtropical high (WPSH) is abnormally northern and the southerly current along the western flank of WPSH is weaker than normal, which induces divergence of the low-level circulations anomalies and the water vapor transport anomalies over the Yangtze River basin, as a result, the precipitation over the Yangtze River basin is reduced. While in the weak WNPSM years, the WPSH is abnormally southern and western, and the strong southerly water vapor transport anomalies form in the Yangtze River basin and its southern area, which induces convergence of the southern and northern wind anomalies over the Yangtze River basin, the atmospheric ascending motion over it is exceptionally active, which is favorable for the precipitation over the Yangtze River basin.(2) The East Asian local Hadley circulation also displays obvious distinctions in the strong and weak monsoon years. When the WNPSM is strong, the local Hadley circulation in East Asia is quite weak, with the anomalous subsidence occurring over the Yangtze River Basin, which makes the rainfall over the Yangtze River basin decline, while the situation is just the opposite in the weak monsoon years.(3) There is a significant climatological intraseasonal oscillation (CISO) in the WNPSM region. In the period of the weak WNPSM, the Yangtze River rainfall is simultaneously influenced by the westerly CISO from the tropical western Pacific and the easterly CISO from the tropical Indian Ocean together, which gives rise to more rainfall than normal there. In the strong monsoon years, however, only the westward propagating CISO from the western Pacific exerts impact on the Yangtze River basin, so it is not easy to stimulate the precipitation.
    Shen R. G., G. S. Huang, 1981: The relation between circulation of tropical summer monsoon and water vapor transport in the south of China. The Tropical Summer Monsoon Conference. Yunnan People Press, 116- 128. (in Chinese)
    Tao S. Y., 1980: Heavy Rainfalls in China. Science Press, 225 pp. (in Chinese)
    Tao S. Y., 1987: A review of recent research on the East Asian summer monsoon in China. Monsoon Meteorology, C. P. Chang, and T. N. Krishnamuti, Eds., Oxford University Press, 60- 92.7a7cf2cfdb1d11184ad32b44ecf07d62http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10012388648http://ci.nii.ac.jp/naid/10012388648A review of recent research on the East Asian summer monsoon in China TAO S. Y. Monsoon Meteorology, 1987
    Wang J. Q., S. P. Zheng, Y. Q. Cui, S. P. Xu, and W. G. Chen, 1991: An overview of the regional distribution of heavy rainfall in China. Hydrology, 1991( 3), 1- 7. (in Chinese)
    Wu G. X., Y. S. Zhang, 1998: Tibetan Plateau forcing and the timing of the monsoon onset over south Asia and the South China Sea. Mon. Wea. Rev., 126, 913- 927.10.1175/1520-0493(1998)1262.0.CO;2962b26850b5f2203cd21fef2c436af5fhttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127342%2Fhttp://ci.nii.ac.jp/naid/10013127342/Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May, then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs. Finally, the timing of the onset of the Asian monsoon in 1989 was explored. It was shown that the onset of the Asian monsoon occurs when the warm or rising phase of different low-frequency oscillations reach the East Asian monsoon area (EAMA) concurrently. These include the warm phase of the eastward propagating two-to three-week oscillation (TTO) of the upper-layer temperature in middle latitudes, the rising phase of the northward propagating Madden-Julian oscillation of the southern tropical divergence, and the rising phase of the westward propagating TTO of the western Pacific divergence. It was concluded that the timing of the Asian monsoon onset is determined when the favorable phases of different low-frequency oscillations are locked over the EAMA.
    Xu, X. D., Coauthors, 2002: A comprehensive physical pattern of land-air dynamic and thermal structure on the Qinghai-Xizang Plateau. Science in China Series D: Earth Sciences, 45, 577- 594.10.3969/j.issn.1674-7313.2002.07.001d00796fb74ca4de8966c373db7675975http%3A%2F%2Flink.springer.com%2Farticle%2F10.1360%2F02yd9060http://d.wanfangdata.com.cn/Periodical_zgkx-ed200207001.aspxAccording to the boundary layer observations of three stations (Garze, Damxung and Qamdu) and relevant earth satellite, radiosonde and surface observations during the intensive observational period (IOP) of the second Tibetan (Qinghai-Xizang) Plateau Experiment of atmospheric science (TIPEX), the land-air physical process and dynamic model on the Tibetan Plateau were comprehensively analyzed in this study. The dynamic characteristics of boundary layer and the rules of turbulent motion on the plateau were illustrated. The characteristics of distributions of wind speed and direction with mutiple-layer structure and deep convective mixed layer on the plateau, the strong buoyancy effect in turbulent motion on the plateau on which the air density is obviously smaller than on the plain, and the Ekman spiral and its dynamic pump effect of the plateau deep boundary layer have been found. The local static distribution of water vapor and the horizontal advection of water vapor in the plateau boundary layer were studied. The abnomal thermodynamic structure on the plateau surface and boundary layer, including the plateau strong radiation phenomenon and strong heating source characteristics of the middle plateau, was also analyzed. The authors synthesized the above dynamic and thermodynamic structures of both surface and boundary layers on the plateau and posed the comprehensive physical model of the turbulence and convective mixture mechanism on the plateau boundary layer. The characteristics of formation, development and movement for convective cloud cluster over the plateau influencing floods in the Yangtze River area of China were studied. The conceptual model of dynamic and thermodynamic structures of turbulent motion and convective plume related to the frequent occurrence of "pop-corn-like" cloud system is given as well.
    Xu X. D., L. S. Chen, 2006: Advances of the Study on Tibetan Plateau Experiment of Atmospheric Sciences. Journal of Applied Meteorological Science, 17( 6), 756- 772. (in Chinese)21d50eeb082a05a43f2af840ca33b5f6http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F288676075_Advances_of_the_study_on_Tibetan_Plateau_experiment_of_atmosphere_scienceshttp://en.cnki.com.cn/Article_en/CJFDTOTAL-YYQX200606012.htmA review of the research work on Tibetan Plateau for the recent 50 years is given.Especially,the important results for the first atmospheric science experiment on Tibetan Plateau in 1979(QXPMEX) and the second one in 1998(TIPEX) are suggested.It is found the long-frenquency oscillation which is inherent in Tibet Plateau tropospheric circulation and marked by out-ward propagation and the characteristics of Ekman spiral of PBL can be seen over Tibetan Plateau.The height of PBL is found to be as high as 2200 m over Tibetan Plateau,it is much higher than those in the plain areas.Dynamic and thermodynamic structures,as well as the characteristics of turbulence and convective clouds in the Plateau are discussed.A comprehensive physical pattern of convective structure of PBL is also given.It is found that under the condition of proper cloud cover,extremely high values of global solar radiation,effective radiation and surface net radiation are measured and the rain storm and flooding associated with the initial convective cloud system can be tracked to the Tibetan Plateau area.The regional impacts of heat source and heat sink associated with changes of the surface albedo over Tibetan Plateau are discussed.And the seasonal scale change of mid-long wave in the atmosphere is under the influence of the regional and seasonal change that is brought by the heat source and sink.The feedback of annual scale change of snow cover on the Tibetan Plateau is also emphasized in the research result,it shows that the planetary scale circulation on the Tibetan Plateau,the anomaly of SST and their interactions can be influenced by the snow cover.There are remarkable evolutions of the interactions of Tibetan Plateau and Asian monsoon.It is found that the availability of sensitive heat pump(SHAP) induces the abrupt air circulation from winter to summer and the South Asian High jump to the north,and maintains the period of the monsoon.It is found that Tibetan Plateau and its eastern areas "large triangle" are key areas for the transportation of water vapor flow.It is a very important rule to form the rain storm and the flooding in the Yangtze River during Meiyu period.The characteristics of matter transfer and ozone anomaly on Tibetan Plateau are found.There is a low value center of ozone on Tibetan Plateau in summer and the descending trend of ozone in Lhasa is notable than that of the east of China at the same latitude.Lhasa locates in the areas of ozone low value center.
    Xu X. D., C. G. Lu, X. H. Shi, and S. T. Gao, 2008: World water tower: An atmospheric perspective. Geophys Res Lett,35(20), doi: 10.1029/2008GL035867.10.1029/2008GL035867e75a9dff9dfdf432e8fad5a90ef9afbbhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2008GL035867%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2008GL035867/fullA large amount of water is stored in the world's highest and largest plateau, the Tibetan Plateau, in the forms of glaciers, snowpacks, lakes, and rivers. It is vital to understand where these waters come from and whether the supply to these water resources has been experiencing any changes during recent global warming. Here we show the maintenance of water content in the atmosphere over the Tibetan Plateau, the atmospheric circulations and transports of water vapor to this part of the world, and the trend of the water vapor supply. The Tibetan Plateau serves as a role of ``the world water tower'', and its land-ocean-atmosphere interaction provides a profound impact on the global natural and climate environment. The analyses of a half-century time series of atmospheric water vapor, precipitation, and surface temperature indicate that the atmospheric supply to this water tower presents an increasing trend under recent global warming condition.
    Xu X. D., X. H. Shi, and C. G. Lu, 2012: Theory and Application for Warning and Prediction of Disastrous Weather Downstream from the Tibetan Plateau. Environmental Science,Engineering and Technology Series. Nova Science Publishers, Inc., 111 pp.9d391cec33de1837eddfccd533aaf108http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F288432627_Theory_and_application_for_warning_and_prediction_of_disastrous_weather_downstream_from_the_Tibetan_Plateauhttp://www.researchgate.net/publication/288432627_Theory_and_application_for_warning_and_prediction_of_disastrous_weather_downstream_from_the_Tibetan_PlateauThe dramatic rise of the Tibetan Plateau's land surface exerts a particular thermal and mechanical force on air flows above and surrounding this highland, which gives rise to many unique weather patterns and climate environments. A case in point is the Asian summer monsoon, the largest monsoonal system on Earth. Apart from its special role in the Asian summer monsoon, the Tibetan Plateau also imposes significant influences on global and regional weather and climate systems. This book unravels several aspects of land-water-atmosphere interaction over the plateau, and describes most recent advances in scientific research and technological developments related to the Tibetan Plateau.
    Xu X. D., C. G. Lu, Y. H. Ding, X. H. Shi, Y. D. Guo, and W. H. Zhu, 2013: What is the relationship between China summer precipitation and the change of apparent heat source over the Tibetan Plateau. Atmospheric Science Letters, 14( 5), 227- 234.10.1002/asl2.444644745e4d4b0be2339bdd0601a879484http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fasl2.444%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1002/asl2.444/fullAbstract Top of page Abstract 1.Introduction 2.Data and data quality control 3.Anomalies and long-term trends in spring heating over the Tibetan Plateau 4.Anomalous patterns in China summer precipitation 5.Change of monsoonal moisture transport in response to anomalous heating over the Tibetan Plateau 6.Long-term trend of precipitation distribution over China 7.Conclusion and discussions Acknowledgements References It is well documented that the East Asian summer monsoon has been experiencing a steady weakening trend in recent decades. Because the Asian summer monsoon (including both East Asian monsoon and South Asian monsoon) is the largest and most pronounced monsoonal system in the world, its change in strength may exert a profound impact on global weather and climate systems, especially on the rainfall pattern in South and East Asia. On the other hand, as a vast elevated landmass, the Tibetan Plateau forms a huge heat source protruding into the free atmosphere. Setting against the backdrop of global climate change, whether or not does the change of this heating affect the change of Asian summer monsoon and thus rainfall distribution? Here we show that the apparent heat source over the Tibetan Plateau is closely correlated with the East Asian summer monsoonal circulation, and that the weakening of the East Asian summer monsoon is closely associated with the decreasing trend of the Tibetan Plateau apparent heat source. Further analysis indicates that the change of rainfall pattern in China in recent decades is consistent with the decreasing of the East Asian summer monsoon .
    Yanai M., C. F. Li, and Z. S. Song, 1992: Seasonal heating of the Tibetan Plateau and its effects on the evolution of the Asian summer monsoon. J. Meteor. Soc.Japan, 70( 1B), 319- 351.10.1175/1520-0469(1992)049<0256:PAPIAP>2.0.CO;205428d0802b7cc6c4be6e6800db6178ahttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F40000634880http://ci.nii.ac.jp/naid/40000634880Using the objectively analyzed FGGE II-b upper-air data, the large-scale circulation, heat sources and moisture sinks over the Tibetan Plateau and surrounding areas are examined for a 9-month period from December 1978 to August 1979. In addition to the FGGE data, special soundings obtained during the Chinese Qinghai-Xizang (Tibet) Plateau Meteorological Experiment (QXPMEX) from May to August 1979 are also used in the objective analyses. The evolution of the large-scale flow patterns, temperature, outgoing longwave radiation (OLR) and vertical circulation is described in order to identify the distinct seasonal changes from winter to summer that lead to the onset of the Asian summer monsoon. The Tibetan Plateau maintains a large-scale thermally driven vertical circulation which is originally separated from the planetary-scale monsoon system. The rising motion exists only on the western Plateau in winter and then spreads to the whole Plateau as the season progresses. The monsoon onset over Asia is an interaction process between the Plateau-induced circulation and the circulation associated with the principal rainbelt migrating northward. During winter the Plateau is a heat sink, but it is surrounded by regions of more intense cooling. In spring the Plateau becomes a heat source, but the cooling in the surrounding areas continues. The sensible heat flux from the surface provides the major source of heating on the Plateau. However, additional contribution from condensation heating is observed in the western Plateau during all seasons and, more significantly, in the eastern Plateau during summer. The sensible heating of the elevated Plateau surface and the radiative cooling in the environment maintain the horizontal temperature contrast that drives the thermally direct vertical circulation. The detailed examination of the warming process of the upper troposphere during two transition periods, i. e. , the onset of the Southeast Asian monsoon in May and that of the Indian monsoon in June, reveals that the temperature increase over the eastern Plateau during the first onset was mainly the result of diabatic heating, whereas that over the Iran-Afghanistan-western Plateau region leading to the second onset was caused by intense subsidence. There are large diurnal variations in the boundary layer and vertical circulation over the Plateau. As a result of diurnal heating of the surface, a deep mixed layer of nearly uniform potential temperature exists over the Plateau in the evening (1200 UTC), suggesting the role of thermal convection in the upward transport of heat. However, moisture is not well mixed vertically and there is a large horizontal temperature gradient in the boundary layer. From late spring to summer the boundary layer becomes more stable for dry convection. On the other hand, the vertical distributions of equivalent potential temperature in late spring and afterwards show a conditionally unstable stratification for moist convection with the increase of moisture of surface air.
    Yang K. M., B. G. Bi, Y. A. Li, and L. Q. Dong, 2001: On flood-causing Torrential Rainfall in the Upstream District of Changjiang River in 1998. Meteorological Monthly, 27( 8), 9- 14. (in Chinese)b934aea7db789e97b4d2fe4cbd31a94fhttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QXXX200108001.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-QXXX200108001.htmBy using conventional observation data, HLAFS grid data and GMS cloud data, physical mechanism affecting weather system and rain formation, interaction and inter impact between low and middle latitude weather system are analyzed and diagnosed. The results indicate that several strong rain procedures occur in the large scale background of middle high latitude in Eurasia. The low eddy which forms in the east area of Qingzang plateau and develops in the Sichuan Basin and its shear line are the main weather systems. The rain intensification is closely related with interaction between middle and low latitude systems, specific construction of plateau eddys.
    Yu S. H., 2008: New research advances of the Tibetan plateau vortex in summer. Torrential Rain and Disasters, 27( 5), 367- 372. (in Chinese)ffac95479eccf5555a51f3c035cc1dafhttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-HBQX200804019.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-HBQX200804019.htmTibetan Plateau Vortex is one of the major heavy rain systems in China during summer.The advances about studying Tibetan Plateau Vortex in the current and the previous QXPMEX are simply summarized.The obtained results in studying Tibetan Plateau Vortex in TIPEX and recent years are emphatically summarized in the activity characteristic,the mechanics,the macroscale conditions of development and moving eastward,and the structure of Tibetan Plateau Vortex.In this paper,the study limitations and the research direction are pointed out for the furth research.
    Zhou Y. S., S. T. Gao, and G. Deng, 2005: A diagnostic study of water vapor transport and budget during heavy precipitation over the Changjiang River and the Huaihe River basins in 2003. Chinese Journal of Atmospheric Sciences, 29( 3), 195- 204. (in Chinese)a2702efe7b1adb1d729975b123bd68d8http%3A%2F%2Fwww.jourlib.org%2Fpaper%2F1556817http://www.jourlib.org/paper/1556817By analyzing the features of the atmospheric circulation systems from 21 June to 11 July 2003, which corresponds to the heavy precipitation over the Changjiang River and the Huaihe River basins in 2003, the water vapor transport vector is decomposed into the sum of its nondivergent (rotational) and divergent (irrotational) components in terms of the streamfunction and potential, and the water vapor budgets are calculated in three regions of the Meiyu front system. The results indicate that the Changjiang River and the Huaihe River basins are the strongest water vapor sink over the global from 21 June to 11 July 2003. The minimum potential region (the maximum convergent region) of water vapor transport vector corresponds to strong precipitation region. The Indian monsoon circulation and the South China Sea monsoon in summer play important roles in the moisture transportation. Besides the vertical ascending motion transporting the moisture up to the middle levels, the moisture, which comes from low latitudes and turns to west when it goes through the Tibetan Plateau, can increase the humidity content in the middle levels over the Changjiang River and the Huaihe River basins and is favorable for the forming of the severe precipitation.
    Zou J. S., M. H. Wang, and W. Zhang, 1987: A preliminary study on the demarcation of rain storms in China. Acta Geographica Sinica, 42( 3), 151- 163. (in Chinese)4cd8aec5919f1ddaf79106d3e5cad82fhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F304893886_A_preliminary_study_on_the_demarcation_of_rain_storms_in_Chinahttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DLXB198702005.htmBased on the average and extreme values of maximum 24-hr rainfall for the last 30 years at hydrological and meteorological stations in China, the demarcation of severe rainstorms in China is investigated. The intensities and seasonal distribution features of severe rainstorms, weather systems and the direction of water vapour transfer and some geographical factors, including topography, elevation and the contrast nature of sea and continent, are comprehensively considered for demarcation. First of all, based on synoptic climatology, ten regions, where rainstorms occur, may be preliminarily delimitated. Secondly, by calculating the areal rainfall index of each region, the correlation coefficient of the areal rainfall index with annual rainfall (or precipitation in May-Aug.) at every station in a given region can be computed. If the correlation coefficient reaches the level of significance (a = 0.05) it means that the station belongs in the same climate region. Finally, ten homogeneous climate regions of severe rain storms and four subregions are delimitated. The genesis of rainstorms and its characteristics in each of the regions are briefly dis-cribed.
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    [13] Yuanchun ZHANG, Jianhua SUN, Shenming FU, 2017: Main Energy Paths and Energy Cascade Processes of the Two Types of Persistent Heavy Rainfall Events over the Yangtze River-Huaihe River Basin, ADVANCES IN ATMOSPHERIC SCIENCES, 34, 129-143.  doi: 10.1007/s00376-016-6117-8
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    [15] JIAN Maoqiu, QIAO Yunting, YUAN Zhuojian, LUO Huibang, 2006: The Impact of Atmospheric Heat Sources over the Eastern Tibetan Plateau and the Tropical Western Pacific on the Summer Rainfall over the Yangtze-River Basin, ADVANCES IN ATMOSPHERIC SCIENCES, 23, 149-155.  doi: 10.1007/s00376-006-0015-4
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Manuscript received: 04 May 2016
Manuscript revised: 10 July 2016
Manuscript accepted: 12 July 2016
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The Upstream "Strong Signals" of the Water Vapor Transport over the Tibetan Plateau during a Heavy Rainfall Event in the Yangtze River Basin

  • 1. Nanjing University of Information Science & Technology, Jiangsu 210044, China
  • 2. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China

Abstract: A heavy rainfall event that occurred over the middle and lower reaches of the Yangtze River Basin (YRB) during July 11-13 2000 is explored in this study. The potential/stream function is used to analyze the upstream "strong signals" of the water vapor transport in the Tibetan Plateau (TP). The studied time period covers from 2000 LST 5 July to 2000 LST 15 July (temporal resolution: 6 hours). By analyzing the three-dimensional structure of the water vapor flux, vorticity and divergence prior to and during the heavy rainfall event, the upstream "strong signals" related to this heavy rainfall event are revealed. A strong correlation exists between the heavy rainfall event in the YRB and the convective clouds over the TP. The "convergence zone" of the water vapor transport is also identified, based on correlation analysis of the water vapor flux two days and one day prior to, and on the day of, the heavy rainfall. And this "convergence zone" coincides with the migration of the maximum rainfall over the YRB. This specific coupled structure actually plays a key role in generating heavy rainfall over the YRB. The eastward movement of the coupled system with a divergence/convergence center of the potential function at the upper/lower level resembles the spatiotemporal evolution of the heavy rainfall event over the YRB. These upstream "strong signals" are clearly traced in this study through analyzing the three-dimensional structure of the potential/stream function of upstream water vapor transport.

1. Introduction
  • The Tibetan Plateau (TP) is the highest plateau in the world with the most complex terrain. Its average elevation is about 4000 m above mean sea level. It makes up about 1/4 of China's and 1/6 of Asia's total land. (Tao, 1980) pointed out that China and some other countries that are influenced by monsoon systems are prone to flooding in the summer. (Tao, 1987) also evidenced that the TP, located in the north of the Indian monsoon area and west of the East Asia monsoon area, plays a key role in the modulation of weather and climate of China and East Asia, even in global climate. The impact of the TP's terrain on regional and global atmospheric circulation and hydrological cycles, particularly the Asian monsoon, remains a hot topics of debate within the scientific community (Wu and Zhang, 1998; Boos and Kuang, 2010).

    In particular, the relationship between heavy rainfall in the Yangtze River Basin (YRB) and the dynamic and thermal effects of the TP upstream has long been a key focus for meteorologists and atmospheric scientists (e.g., Yanai et al., 1992; Wu and Zhang, 1998; Xu et al., 2008, 2013; Chen et al., 2013). A number of previous studies have shown that the dynamic effect of the TP has crucial influences on the downstream weather and climate over the Yangtze-Huai drainage areas (Chen et al., 1985; Yanai et al., 1992; Chen et al., 2012). (Chow et al., 2008) pointed out that the water vapor accounting for early summer rainfall in China is mainly transported via the southwest of China from the Indian monsoon. They suggested that a weak Indian monsoon will bring less rainfall in China, whereas a stronger Indian monsoon will cause flooding. (Xu et al., 2008) proposed a "world water tower" model for the special atmospheric circulation and water vapor cycle over the TP. During the heavy rainfall events over the YRB in 1998, the frequent development of convective clouds could be observed over the central and eastern TP. These convective clouds moved successively eastward in clusters. According to field experiments on the TP, it was found that the mesoscale convective clouds in the east of the TP moved eastward along the YRB (about 30°N). Satellite data analysis also suggested that the immediate cause of the floods in 1998 was low-level lows or shear lines, and some of these low-level lows could be tracked back to the TP.

    The weather systems related to the rainfall in the YRB include regional lows, tropical cyclones and frontal cyclones. The development of these systems involves multi-scale interactions under large-scale circulation conditions. However, the cause of heavy rainfall in East China, especially the water vapor transport and its corresponding circulation structure, has a close relationship with the lows over the TP. For example, (Li, 2012) demonstrated that both the plateau low vortex and the northwest low vortex can give rise to heavy rainfall in East China. The eastward movement of the plateau low vortex takes place mainly in July, while it is in May for the northwest low vortex. And there were more eastward moving low vortexes from the plateau in 1998, 1999 and 2003, resulting in severe flooding over the YRB. There is no doubt that the low vortex from the TP is one of the most important weather systems impacting upon summer rainfall in China. However, in view of the diverse explanations about the relationship between the heavy rainfall over the YRB and the dynamic and thermal effects of the TP, as well the debate in terms of how to connect the upstream dynamical and hydrological "strong signals" and the downstream heavy rainfall, we hope the results of this study will shed some light on this controversial topic.

2. Data and methods
  • The meteorological field data from the NCEP Final Operational Global Analysis dataset, with a resolution of 1°× 1° and 26 levels vertically extending from the surface to 10 hPa (see http://dss.ucar.edu/datasets/ds083.2/data/) are used in this study. We also use observed hourly precipitation data (July 2000) from 2400 gauge stations over China, which are from the National Meteorological Center of the China Meteorological Administration observation archives. The precipitation data are controlled for quality before being released. The low-level cloud-cover data from 753 observational stations over China for 1961-2010 are also adopted.

  • 2.2.1. Stream function and velocity potential analysis

    Considering the dynamic impacts of bypassing and cross-mountain flow associated with the TP's terrain on water vapor transport, it is convenient to use the velocity potential and stream function to represent the atmospheric flow fields. The formulas for velocity potential and stream function are: \begin{eqnarray} \label{eq1} &&\frac{\partial v}{\partial x}-\frac{\partial u}{\partial y}=\zeta ;\quad (1) \\ \label{eq2} &&\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}=D ;(2) \\ \label{eq3} &&\nabla^2\phi=\zeta ; (3) \\ \label{eq4} &&-\nabla^2\chi=D . (4)\end{eqnarray} The u component represents the east-west component of the horizontal wind (x-direction), while the v component represents the north-south component (y-direction). ζ and D represent horizontal vorticity and divergence, respectively. The symbol 2 is the Laplacian operator. Here, we solve the Poisson equation, Eq. (4), to obtain lines with a constant value of the stream function φ. Similarly, we solve the Poisson equation, Eq. (5), to obtain lines with a constant value of velocity potential χ.

    2.2.2.Correlation vector analysis

    In order to investigate the relationship between the formation and development of the heavy rainfall and the upstream water vapor flux, the method of correlation vector analysis is adopted. The correlation vector is defined as \begin{equation} \label{eq5} {R}(x,y)={R}_u (x,y)+{R}_v(x,y) , (5)\end{equation} where R is the correlation vector between the rainfall and total water vapor flux; and Ru(x,y) and Rv(x,y) are the zonal and meridional components of the correlation vector, respectively. Equation (6) can also be easily derived with the potential or stream function.

3. Correlations between summertime rainfall over the YRB and low-level clouds
  • (Hu and Ding, 2003) showed the structure of the inflow and outflow of water vapor during heavy rainfall in the Yangtze-Huaihe River basin. It appeared that both the inflow and outflow occur at the lower levels. Specifically, the inflow is in the south and west of the rainfall region, while the outflow is in the east and north. Xu et al. (2002, 2012) suggested that the TP plays two roles in water vapor transport. First, as a heating source, the plateau acts as an air pump that attracts low-latitude warm and moist air coming up towards the TP. Second, due to the high-rise nature of the plateau, it blocks and deflects a large amount of water vapor to the east. Therefore, in this sense, the TP plays the role of a water vapor "re-channel station". (Xu and Chen, 2006) also characterized the convective clouds moving successively eastward from the TP and pointed out that these low-level clouds are the precursors of the convective systems responsible for the heavy rainfall over the YRB. During the second TP Atmospheric Scientific Experiment (TIPEX), the analysis results with temperature black body satellite data and water vapor cloud images showed that the formation and development of convective systems could account for the heavy rainfall in Wuhan in late July.

    Figure 1 shows the correlation coefficient distribution of summertime rainfall during July 1961-2010 over the YRB and the low-level cloud cover for the same time period. It is notable that the areas of significant positive correlation (with a confidence level of 95%, or correlation coefficient R>0.28) represent a typical banded area extending from the TP to the YRB. These results reflect the close relationship between the banded structure features of the local or eastward-moving convective cloud activity with heavy rainfall within this area. The results also further verify the conclusion obtained in TIPEX that the clouds responsible for the heavy rainfall of the YRB are from the TP. And a key fact that the intense convective clouds originate from the TP is also proven.

    Figure 1.  Correlations between average rainfall (units: mm) in July and low-level cloud cover for the period 1961-2010. Color shading indicates the topography (units: m). The red and blue solid circles indicate stations exceeding the 90% confidence level (R>0.23 or R<-0.23).

4. Heavy rainfall over the YRB during 11-15 July 2000
  • Among all heavy rainfall zones in China, there are two main zones with frequent extreme heavy rainfall events. One is the middle and lower reaches of the Yangtze River, and the other is North China (Zou et al., 1987; Wang et al., 1991). Considering the dynamic and thermal effects of the TP on convective systems and heavy rainfall downstream, we select a heavy rainfall event that occurred in the Yangtze-Huaihe River Basin during 11-15 July 2000 for this study. The corresponding accumulated precipitation from 2000 LST 11 July to 2000 LST 15 July is shown in Fig. 2.

    We choose this event because it is a highly typical heavy rainfall case. The distribution of daily rainfall from during 11-15 July 2000 presents an obvious banded pattern. The rainfall started from the TP, moved eastward, and finally reached the middle and lower reaches of the YRB; and an obvious banded pattern for the rainfall was formed (figure omitted). Figure 2 also shows this typical heavy rainfall case was characterized by two maximum rainfall centers. One was located at the junction of Sichuan, Gansu, Chongqing and Shanxi provinces, and the other at that of Hubei, Henan, Jiangsu and Anhui provinces. Both maximum centers had accumulated rainfall of more than 50 mm. Compared with Fig. 1, we can see that the accumulated rainfall distribution of this case in Fig. 2 resembles the pattern of correlation between the heavy rainfall over the YRB and low-level cloud cover upstream. This further emphasizes the important role of the moving cloud system in heavy rainfall downstream.

    The rainfall in the major regions of the YRB fluctuated during the period, with an increasing trend (Fig. 3), and reached its peak at 2000 LST 13 July. Figure 3 also depicts the features of the rainfall variation during this period, especially the continuous heavy rainfall in the latter days of this period. But how is the rainfall related to the eastward-moving system and associated water vapor transport? In the following sections, we continue to examine the dynamic structure and characteristics of the water vapor transport associated with the heavy rainfall over the YRB.

    Figure 2.  Accumulated rainfall (units: mm) of day 5 (from 2000 LST 11 July to 2000 LST 15 July). The blue solid outline represents the key rainfall area selected in this study (29°-35°N, 104°-122°E).

    Figure 3.  Time series of average rainfall (units: mm) in the key rainfall area (29°-35°N, 104°-122°E) from 2000 LST 5 July to 2000 LST 15 July (6-h interval). The red double-headed arrow indicates the heavy rainfall process, as shown in Fig. 2.

5. The heavy rainfall downstream from the TP and the structure of the related water vapor transport
  • The dominant flows associated with the Indian monsoon and South China Sea (SCS) monsoon create the major water vapor transport channel during summer (Zhou et al., 2005). (Liu and Ding, 2009) suggested that the monsoon flow from the Indian Ocean and the Pacific have significant impacts on summer rainfall over China. The water vapor source for the rainfall in East China is mainly the SCS, followed by the Bay of Bengal (BOB) and West Pacific (Jin, 1981; Shen and Huang, 1981; Chen, 1982). (Xu and Chen, 2006) further revealed that the "big-triangle" area surrounding the TP is the key region of water vapor transport resulting in the rainfall in the middle and lower reaches of YRB. The water vapor channels in the south and east of the TP play a critical role in the rainfall of the YRB. By analyzing the heavy rainfall over the YRB in 1998, (Jian and Luo, 2001) argued that the variation of the strength of the meridional monsoon flow follows the diurnal cycle of the upward motion of the air over the TP. The upward motion is stronger at night, which indicates the monsoon meridional flow is stronger and more water vapor is transported.

    Knowing that a high correlation exists between the rainfall over the YRB and low-level convective activities, we further examine the structure of water vapor transport using correlation vector analysis. In order to analyze the evolution of the water vapor transport prior to and after this event, lagged correlations are calculated from 2000 LST 5 July to 2000 LST 15 July (with a temporal resolution of 6 hours). Figure 4a shows the lagged correlation vectors between the rainfall in the key regions of the YRB and the total water vapor flux two days prior to the rainfall. It clearly shows the confluence of two water vapor channels from the SCS and the BOB, respectively. The water vapor transport comes from the BOB, turns to the YRB after passing the southeast corner of the TP, which approximately coincides with the maximum rainfall on the same day. Figure 4b shows the lagged correlation vectors one day prior to the rainfall. Compared with Fig. 4a, we can see that the major rainband moves eastward, and the eastern edge of the rainband even extends to the middle and lower reaches of the Yangtze River. The simultaneous correlation vectors indicate that the center of maximum rainfall was located in the middle and lower reaches of the YRB at that time; the water vapor was transported northward from the BOB and turned to the middle and lower reaches of the Yangtze River along the northeastern edge of the TP. The water vapor starting from the SCS continuously transported the moisture to the YRB. The lagged and simultaneous correlation analysis clearly show the convergence/confluence zone of the water vapor transport, which coincides with the eastward migration of the maximum rainfall.

    Figure 4.  The (a) two-day lagged, (b) one-day lagged and (c) simultaneous correlations between the total water vapor flux (units: g m-1 s-1) and the rainfall in the key region, represented as correlation vectors (arrows). Color-shaded areas on land represent the corresponding daily rainfall (units: mm) measured at 2000 LST (a) 11 July, (b) 12 July and (c) 13 July 2000. The blue solid lines denote the confidence level of ≥99%. The red circle indicates the precipitation area.

6. Coupled structure of "low level convergence-upper level divergence" in the potential function during the eastward movement of the heavy rainfall
  • From the perspective of eddy transport, (Gao and Zhai, 1993) demonstrated that the eddy transport of water vapor is in the same direction as the gradient of the water vapor content. (Guan et al., 2011) suggested that extreme rainfall in the middle and lower reaches of the Yangtze River is related to anomalous local circulation systems. When extreme rainfall occurs, there is usually a cyclonic circulation in the middle-lower troposphere over the middle and lower reaches of the Yangtze River and an anticyclonic circulation to the south. We calculate the two-day and one-day lagged correlation, as well as the simultaneous correlation, between the rainfall in the major regions and the velocity potential of water vapor flux, and obtained the corresponding water vapor flux structure (Fig. 5). The two-day lagged correlation map (Fig. 5a) shows a clear water vapor convergence area in the central and eastern TP (Zone A) at 500 hPa, which coincides with the east-west orientated rainband that starts from the central and eastern TP and extends to the YRB. Figure 5b shows that, for the one-day lagged correlation, the area with water vapor convergence in Fig. 5a has moved to the upper and middle reaches of the Yangtze River (Zone B), and coincides with the heavy rainfall in Hubei Province. Figure 5c shows the simultaneous correlation, from which it can be seen that the area with water vapor flux convergence has moved to the middle and lower reaches of the Yangtze River. The heavy rainfall area covers the whole of the middle and lower reaches of the Yangtze River and expands to Anhui, south of Henan and Jiangsu provinces. By analyzing the structure of water vapor transport during and prior to the heavy rainfall (Figs. 5a-c),we find there is also an eastward-migrated water vapor convergence region corresponding to the eastward migration of the heavy rainfall.

    Figure 5.  As in Fig. 4, but for the correlations between the rainfall (units: mm) and the potential function of water vapor flux (units: g cm-1 s-1 hPa-1) at 500 hPa. The blue solid lines denote the confidence level of ≥90%. The red circle indicates the precipitation.

    The correlations between the rainfall and velocity potential of water vapor flux at 200 hPa and 500 hPa two days prior to and after the rainfall are also compared, to further examine the relationship between them (Fig. 6). The difference between the two-day and one-day lagged correlations shows a clear divergence center of velocity potential of water vapor flux at 200 hPa (Fig. 6a), and a convergence center at 500 hPa (Fig. 6b), in the central and eastern TP. This "lower-level convergence and upper-level divergence" coupled system moved eastward and reached the middle and lower reaches of the Yangtze River, which can be easily identified from the difference between the one-day lagged and simultaneous correlations (Figs. 6c and d).

7. Dynamic structure of eastward moving eddies over the TP
  • The dynamic and thermal effects of the TP usually result in the formation of a low pressure system over the TP. These low pressure systems, called Plateau lows, evolve into severe weather systems and deliver abundant rainfall in downstream regions. (Yu, 2008) demonstrated the tremendous impacts of the eastward-moving plateau vortexes on the rainfall over the Yangtze River and reaches of the Yellow River-Huaihe River. (Yang et al., 2001) demonstrated that there was a close relationship between the eastward-moving plateau vortexes and the extreme heavy rainfall over the YRB in 1998.

    Given the "lower level convergence-upper level divergence" coupled structure of the eastward-moving plateau vortex, we calculate the correlations between the rainfall in the major regions of the YRB and the component of the velocity potential function.

    Cross-section maps (along 33°N) of correlation vectors and vertical speed are plotted and shown in Figs. 7a-d. According to the cross section of the correlation vectors of the potential function, the area with upward motion (rectangular region in Fig. 7a) was initially located in the central and eastern TP (98°-105°E) 48 hours prior to the rainfall in the major regions of the YRB, and the upward movement center was located in the same area. At 36 hours prior to the rainfall, that area with significant upward motion had moved to somewhere in the east of the TP (rectangular region in Fig. 7b). The correlation between the rainfall and vertical speed field shows a positive center in this area between 500 hPa and 250 hPa. At 24 hours prior to the rainfall, the upward motion area had partially moved out of the TP to cover an area of the plains. At 12 hours prior to the rainfall, the upward motion had reached the middle and lower reaches of the Yangtze River. The results shown in Fig. 7 may verify that the accompanying significant upward motion was the dynamical cause of the heavy rainfall in the middle and lower reaches of the Yangtze River. The above cross-section maps of the velocity potential clearly depict the upward motion corresponding to the "low level convergence-upper level divergence" coupled structure, as well as the dynamic mechanism of generation, development and eastward movement of the convective systems.

    Figure 6.  Differences in the correlations between the rainfall (units: mm) in the key rainfall region of the YRB and the potential function of water vapor flux (units: g cm-1 s-1 hPa-1) from 2000 LST 5 July to 2000 LST 15 July 2000. Differences in the correlations between the two-day and one-day advanced correlations at (a) 200 hPa and (c) 500 hPa, and between the one-day advanced and simultaneous correlations at (b) 200 hPa and (d) 500 hPa. The blue lines show the difference field of the correlation between two consecutive days. The solid (dashed) lines indicate positive (negative) correlation. The red circle indicates the precipitation area.

    Figure 7.  Correlations between the rainfall (units: mm) in the key rainfall region of the YRB and the components (u, ) of the potential function, as well as the cross-section maps of correlations with vertical movement () (along 33°N). Panels (a-d) are the cross-section maps of 48-hour, 36-hour, 24-hour and 12-hour advanced correlations, respectively. The blue solid rectangle represents the eastern rainfall region focused upon in this study.

8. Conclusions
  • In this study, by analyzing the three-dimensional structure of the water vapor transport in the TP prior to a widespread heavy rainfall event over the YRB, we identified the "strong signals" of the dynamical and hydrological features of the synoptic systems in the TP. The results obtained in the study may shed some light on understanding the relationship between the heavy rainfall over the YRB and the water vapor transport and eastward-moving systems. The following are the main conclusions:

    (1) The convective clouds over the TP play a key role in the occurrence and development of heavy rainfall over the YRB. The results from this study reveal a close relationship between the heavy rainfall over the YRB and the eastward-moving low-level clouds from the TP. The pattern of the low-level cloud cover coincides with the distribution of the accumulated rainfall over the YRB, which also confirms its role.

    (2) Lagged correlation analysis shows the existence of a "convergence" area of water vapor transport and its eastward movement coincides with the corresponding maximum rainfall over the YRB.

    (3) A "lower-upper level" coupled structure, represented by the velocity potential of water vapor flux, originating from the TP, also plays a key role in rainfall over the YRB. This specific structural configuration is critical in leading to the occurrence of heavy rainfall downstream.

    (4) The structure of the velocity potential/stream function and the eastward migration of the strong upward motion area is also one of the upstream "strong signals" of heavy rainfall. The three-dimensional water vapor transport (flux, vorticity and divergence) prior to the occurrence of a heavy rainfall event in the middle and lower reaches of the Yangtze River.

    (5) By adopting the method of three-dimensional correlation analysis between the heavy rainfall and the velocity potential/stream function of water vapor flux, it is possible to clearly reproduce the scenario that the eastward movement of water vapor transport eventually results in heavy rainfall in the middle and lower reaches of the Yangtze River. The analytical method of the velocity potential/stream function used in this paper could be adapted to a trace analysis of the generation, development and eastward movement of other heavy rainfall or mesoscale convective systems.

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