Statistical And Diagnostic Study On Anomalous Precipitation Over South China In June

By using NCEP/NCAR reanalysis data and the observed rainfall over China from 1951 to 2005, this thesis has statistically analyzed the variation of precipitation over South China in June, and further studied the variation characteristics of western Pacific subtropic high (WPSH), sea surface temperature (SST) and the water vapor transport which are related to the abnormal precipitation over South China in June. The WPSH variation in South China during the flood period in June 2005 has also been studied based on a diagnostic equation and the daily NCEP/NCAR reanalysis data. The main results are summarized as follows:Climatologically, the ridges of WPSH at 500hPa are located at 19°N in June in the South China Sea(SCS). The ridges are located far southward (northward) and the WPSH strength is strong (weak) in the cases of abundant (scarce) rainfall over South China in June rather than the normal years with few exceptions.The water vapor flux is strong (weak) over the area from the northern SCS to the southeastern China and the south of Japan in the cases of abundant (scarce) rainfall over South China in June rather than the normal years. However, there exist opposite results for the water vapor flux over the area from the Arabian Sea to the Bay of Bengal.In June, the positive (negative) SST anomalies in the SCS and the subtropical western Pacific and the negative (positive) SST anomalies in the northwestern Pacific are accompanied with positive (negative) anomalies of geo-potential height over the region, which related to the abundant (scarce) rainfall over South China in June.At the lower troposphere, the boundary effect and the vorticity term (zeta term) play a key role in the variation of the geo-potential height over South China, the SCS and the vicinity during the flood period of 14-24 in June, 2005. The beta term, turbulent friction term, vertical advection of divergence, as well as other terms related to horizontal advection of momentum play a secondary role. The boundary effect, zeta term, the turbulent friction term and the term of meridional transport of v-momentum have positive contributions to the variation of geo-potential height over South China and its vicinity. The variations of the geo-potential height fields forced by the zeta term and the turbulent friction term are consistent very well with the observed height field over South China and its vicinity area in June, 2005.At the middle troposphere, the primary factors for the variation of geo-potential height over South China and its vicinity area are the boundary effect, vorticity term (zeta term), beta term and the wind-shear term. The variations of the geo-potential height fields forced by the zeta term and the beta term are consistent very well with the observed height field over South China and its vicinity area in June, 2005.Compared with other former studies, the equation used in this study provides more diagnostic performances and the results are more precise. We can calculate the contribution percentage of each factor to the variation of geo-potential height over South China, and then evaluate their relative importance.