Analysis Of Flood-resulting Rainstorm Cause In Guangxi Liujiang Valley

Statistic is made regarding flood-resulting rainstorm process and it'ssynoptic systems in Liujiang valley from 1951 to 2005. The result show thatflood-resulting rainstorm mainly occur at June or July and the frequency andintensity of flood-resulting rainstorm boost up in this area after 90s of the20th,the result also show that Liujinag flood is mainly come from east-northarea and west area of Liujiang valley. The correlation between Liujiang riverlevel and total process precipitation is most important, 70mm total arearainfall can cause Liujiang flood. The study also find that synoptic systemscausing flash-flood-producing rainstorm are upper-air trough,lowvortex,low-level shear,surface static front,low-level jet,Subtropical Highect,they can be devided as low trough-shear,low vortex-shear and low leveljet-shear three types.We made physical diagnosis about a flood-resulting rainstorm process inLiujiang area on 19-20 July 2004 and found that "2 ridge 1 trough"circulation background,strong low-level jet and low-level shear are thecharacteristic of meso-scale and lagre-scale circulation during thisprocess. The advantaged condition of moisture,heat and dynamic are directfactor which cause this process. Cumulonimbus generate frequently in thisarea,this bring flood-resulting rainstorm. We made numerical simulation teston this process by using MM5 model,by analyse the model ouput we findthat meso-scale vortex pole is builded and maintained is one of factors offlood-resulting rainstorm,heavy rain has relationship with vortex polestate,when vortex pole incline towards south,rainfall weaken. B meso-scalecyclone offer direct dynamic power for this process. We also found thatmechanism of CISK is the internal factor of this process,it forces deepconvection and maintain. The coupling of upper and lower tropospheric jetstreams is advantaged for causing strong convection, this is also one of factorsof flood-resulting rainstorm in Linjiang area.Abnormity of positivePV(potential vortex) in upper air result in interaction of cyclonic circulationin upper air and low air,this positive feekback mechanisim is also one ofimportant mechanisim which lead to this flood-resulting rainstorm process.