A Study on Severe Heavy Rainfall in North China during the 1990s

The general statistical characteristics of heavy rainfall processes occurring in North China during 1960—1999 are analyzed firstly,afterwards,the severe heavy rainfall processes of 1990—1999 are classified into five types and their features of weather pattern are studied further.The statistical studies show that the occurrence frequency of heavy rainfall in the summer of each decade from 1960 to 1999 is very similar.20 heavy rainfall cases occurred in northern Shanxi and northern Hebei provinces in the summer during 40 years,at the same time,60—100 cases in coastal area.There were 39 days of severe heavy rainfall in the summer of 1990—1999.And the mentioned-above cases are classified into five types: 1) the remote interaction between typhoon and westerly trough(or vortex),2) the direct interaction between vortex(or landing typhoon) and westerly trough,3) the standstill or slow-moving of landing typhoon stopped by high pressure in the continent,4) vortices in the lower-middle troposphere and 5) shear line pused by warm air.Generally speaking,among them,the typhoon and vortices are the most important heavy-rainfall-producing systems in North China.Finally,the heavy rainfall occurred in Beijing from 1960 to 1999 are investigated.During 40 years,no heavy rainfall for 7 years and the maximum of 5 cases in one summer.It is revealed that the annual variation of heavy rainfall frequency in Beijing is remarkable.Therefore,the further study on heavy rainfall in Beijing should still be paid more attention.The certain characteristics of heavy rainfall in Beijing are,in some ways,similar to those of heavy rainfall in North China,but there still exist some differences between them,for instance,type 4 and type 2 are the most important weather pattern producing heavy rainfall in Beijing and North China,respectively.In addition,the cold front with trough is also important for the heavy rainfall occurrence in Beijing.