Abstract:
During the period of June to August,2011,Yunnan province experienced the severest drought weather in summer since 1961.Most regions received little precipitation.A large amount of spring crops were affected,while water storage of reservoirs and ponds were seriously insufficient.Using NCEP / NCAR reanalysis data,NOAA OLR data,and TBB data of FY2D satellite image,we analyzed the causation of abnormal shortage of summer precipitation in Yunnan.Results show that this phenomena was directly related to the South-Asia High,the polar vortex and abnormal circulation over low-middle latitude area.The weak South-Asia high led to weak divergence on upper level,which was unfavorable to upward motion.The weak polar vortex directly brought out weak cold air,while the high pressure over Yunnan and Burma were pretty strong to block the transfer of moist air from Bengal Bay to Yunnan.Consequently,over Yunnnan,the air humidity was extremely low on middle-lower level.The transfer and convergence of water vapor were rather weaker than in normal years,which was the main reason for abnormal shortage of summer precipitation in Yunnan.Meanwhile,the monthly average OLR of Yunnan was rather big that hindered the development of convection,which was also supported by TBB value greater than 0℃ on cloud image over Yunan.Both were disadvantage for rain occurrence.