报告人: 吴秋霞博士 (中国气象科学研究院灾害天气国家重点实验室)
This paper is the first part that investigates the causes of the persistent precipitation deficiency over southwestern China from September 2009 to March 2010, which led to the severest drought at the region since 1949. The NCEP/NCAR Reanalysis and NASA/CERES EBAF Ed2.6 Top of Atmosphere (TOA) radiative flux are analyzed to investigate effects of internal atmospheric dynamical processes and external thermal forcing on formation of the abnormal warming of the atmosphere, which made the warmer-than-normal air hard to form clouds and precipitation. It is found that the persistent warming of air column was attributed to both the enhanced TOA downward radiative flux (i.e., radiative heating) and the adiabatic heating resulted from the strong descent over the western and central area of the severe drought impact region. The enhanced radiative heating was largely caused by fewer clouds that were in turn in close association with the strong downward motion. Therefore, the strong descent served as a critical factor leading to the persistent warming of air column over the region and thus played an important role in formation of the persistent precipitation deficiency. The strong descent over the region was not an inherent part of descending branches of the Hardley circulation and was directly related to large-scale atmospheric circulation anomaly, causes of which are presented in Part II.