2011年云南省夏季降水异常偏少成因分析

Analysis of below normal precipitation in Summer 2011 in Yunnan province

2011年6至8月云南省大部地区降水持续偏少,出现了自1961年以来最严重的夏旱天气,全省大春作物大面积受灾,库塘蓄水严重不足,影响较为严重.本文利用NCEP/NCAR再分析资料、NOAA卫星OLR资料和FY2D云图TBB资料,对2011年云南夏季降水异常偏少的成因进行分析.结果表明:云南夏季降水异常偏少与南亚高压、极涡以及中低纬环流异常有直接关系.南亚高压偏弱,导致高空辐散减弱,不利于较强上升运动的产生;极涡偏弱导致冷空气偏弱;滇缅高压偏强不利孟湾水汽向云南输送;云南上空中低层空气湿度异常偏小,水汽输送、汇集较常年异常偏弱是云南夏季降水异常偏少的主要原因.同时云南区域月平均OLR值偏大不利对流发展,云图TBB月平均值大于0℃也反映了云南上空云系偏弱,均不利于降水产生.

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.