A Diagnostic Analysis On Summer Heavy Rain Events In Yunnan Province,China

It's meaningful to understand the flood and drought in Yunnan province in complicated terrain and unique geographical location,based on the research of the rainfall events and the related circulation features under the global warming.By using daily observations from 9 meteorological stations and the NCEP/NCAR reanalysis data during 1960-2011,a diagnostic analysis is performed to investigate the daily rainfall events and the related atmospheric circulation features under global warming in boreal summer in Yunnan province.The daily rainfall events at the stations were classified into five grades of precipitation intensity,i.e..,small rain,medium rain,large rain,heavy rain and extreme heavy rain events,respectively.Then to discuss the annual change of precipitation and the atmospheric circulation during daily rainfall events at each station.The analysis of summer precipitation at each stations suggests that the large and heavy rain events play an important role in Yunnan's summer precipitation variations.The similar features of the circulation show that an anomalous divergence circulation pattern at upper level(200hPa)over Eurasian continent that corresponds well to the negative anomaly of the East Asian subtropical westerly jet(EASWJ)intensity during the heavy rain events in boreal summer in Yunnan Province.Meanwhile,anomalous southwest winds at lower level(700hPa)with abundant water vapor originated from the Arabian Sea and Bay of Bengal provides necessarily dynamic and water conditions for middle,large and heavy rain mechanism.However the atmospheric circulation anomalies disappeared during the light rain events.To further understanding the different rainfall events in Yunnan province,the wide range heavy rain events in Yunnan province were analyzed.The results show that the weakening of EASWJ at upper level(200hPa),the interaction of the Burma High(BH)and the Western Pacific subtropical high(WPSH)at middle level(500hPa)and the enhanced water vapor transport at lower level(700hPa)contribute to middle,large and heavy rain events.Because the anomalous EASWJ plays an important role in different rainfall and wide range heavy rain events in boreal summer season,we further investigated the anomaly of EASWJ intensity during the different rainfall events and wide range heavy events in Yunnan province,respectively.The analysis of the temperature anomalies during the Yunnan's different rainfall events reveal that the troposphere warming from surface to 200 hPa in northern mid-latitude(30�-45�N)induces the decreases of meridional air temperature gradient,leading to the negative anomaly of EASWJ intensity by the thermal balance during the middle,large and heavy rain events in boreal summer.The precipitation intensity is closely related to the intensity of the warming.And there are weak changes in temperature from surface to 200 hPa during the light rain events.The analysis of the temperature anomalies during the Yunnan's wide range heavy rain events illustrate that the stronger decreases of meridional air temperature gradient than that for Yunnan's heavy rain events at stations lead to the stronger weakening of the EASWJ(25�-42�N)due to the same physical mechanisms,which provides necessarily dynamic conditions.