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The Impact of Water Vapor Transport on the Summer Monsoon Onset and Abnormal Rainfall over Yunnan Province in May

CHEN Yan~(1,2),DING Yi-Hui~(3),XIAO Zi-Niu~(4),and YAN Hong-Ming~(2)1 Department of Atmospheric Sciences,Nanjing University of Information Science and Technology,Nanjing 2100442 Yunnan Institute of Meteorological Science,Kunming 6500343 National Climate Center,China Meteorological Administration,Beijing 1000814 National Meteorological Center,China Meteorological Administration,Beijing 100081  
Although the partial westerlies prevails perennially at low level over Yunnan Plateau,it exhibits prominent dry and wet seasons influenced by the monsoon climate.The conspicuous fluctuations of precipitation tend to cause severe disasters,especially in May,the critical early summer period for spring crops growing.As one of the most significant factors contributing to rain,the impact of water vapor transport(WVT) on the summer monsoon onset and the possible relationship between WVT and abnormal rainfall in May are investigated.The primary datasets analyzed in this study include the 19612000 observational precipitation at 120 stations scattered in Yunnan Province of China,NCEP/NCAR reanalysis data,19792000 CPC(the Climate Prediction Center) Merged Analysis of Precipitation(CMAP) and Outgoing Longwave Radiation(OLR).The climatological distribution of vertically integrated WVT vectors in May presents that Yunnan is generally overlaid by southwestern WVT,which mainly consists of the subtropical westerly from the northern part of Indian subcontinent and southwesterly from the tropical Indian Ocean(TIO) and the Bay of Bengal(BOB). In addition,the partial southerly WVT that turns around the west flange of the western Pacific subtropical high is one of the important branches over the eastern part of Yunnan Province.The results show that the average date of the summer monsoon onset in Yunnan is 28 May,identified by a criterion of Yunnan precipitation index.During the summer monsoon onset,the WVT over TIO and BOB greatly intensifies,strengthening the WVT over the western part of Yunnan accordingly.However,the WVT weakens over the eastern part associated with a cyclonic variation over the northwestern South China Sea(SCS).Consequently,the specific humidity over Yunnan increases rapidly,providing necessary moisture condition for the monsoon onset.Meanwhile,the composite result also reveals that cold surges from the mid-latitude may play a crucial role in triggering the monsoon onset.In order to investigate the differences of WVT between floods and droughts in May,six typical flood years and seven typical drought years are figured out.The results of composite analysis indicate that the distinct interannual variation of rainfall in May is closely related to the abnormal situation of large-scale WVT.When the WVT over TIO and BOB is strong,but weak over the SCS and East Asian continent,along with that the cold surges is active over the mid-latitude,it is likely to have more rain,leading to flood in Yunnan Province in early summer,and vice versa.
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