A STUDY ON MESOSCALE CONVECTIVE SYSTEMS OF THE SEVERE HEAVY RAINFALL IN NORTH CHINA BY “9608” TYPHOON

A diagnosis and simulation study have been conducted to reveal the features of the severe heavy rainfall in Hebei province during 3—5 August 1996. The weather pattern of the mesocale systems are diagnosed firstly, and then PSU/NCAR MM5 is used to simulate the process. The simulation results also are employed to study the structure and the development of mesoscale systems. Although the landing typhoon propagating northward is a key system producing heavy rainfall over North China, but the weather pattern of present case is different from most of this kind cases. It is the standstill or slow-moving of landing typhoon stopped by high pressure in the continent, which is rarely happened over North China. During the heavy rainfall process, the interaction of westerly trough and the landing typhoon was not found, but the weak cold air from the southern part of anticyclone in North China invading to the outmost of landing typhoon, which was favorable to the initiation of convection. It is indicated that two mesoscale convective systems were related directly with the severe heavy rainfall in Hebei province. The center of severe heavy rainfall occurred around Shijiazhang area during 20:00 3—08:00 5 August. The analysis of sounding shows that the instability is obvious and the moisture in lower and middle troposphere increases gradually prior to the occurrence of heavy rainfall. The moisture transportation of south low-level jet (LLJ) was very important to the development of convection. The height of LLJ was relatively higher than that in Meiyu front. The most simulation results from MM5 consistent with observation although some deficiency is found. The simulation results reproduce the formation and development of the mesoscale convective systems producing the heavy rainfall. It is shown that the upper-level mesoscale jet formed by the upper level outflow of convection occurred during the evolution of convection. The low-level jet (LLJ) and upper-level jet (ULJ) intensify with the development of convective system. There existed two inflows and two outflows of convection system producing heavy rainfall. The inflow in lower troposphere is from southerly LLJ. The two outflows occur at lower and upper level, respectively. The interaction of LLJ and mesoscale ULJ could maintain and accelerate the development of convective system. It is also shown that the northwards propagation of convection systems was blocked by Taihang Mountain, which was one of the reasons the heavy rainfall persisting over Shijiazhang, Hebei province.