Numerical Studies of Mesoscale Convective Systems Using a Cloud-Resolving Model

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The Baiu season in East Asia is characterized by a very humid climate. In the area, long-lasting and intense precipitation system occasionally develop and cause a heavy rainfall. They are referred to as a mesoscale convective system. In this study, we performed numerical simulation experiments of mesoscale convective systems using CReSS (the Cloud Resolving Storm Simulator) in collaboration with researchers of Taiwan and Korea.In 2002, we improved CReSS ; map projection capability, improvement of the lateral boundary conditions, assimilation of Doppler radar data. Using the improved CReSS, we performed daily weather prediction experiment and simulation experiments of mesoscale convective systems in warm season. We also invited researchers of Taiwan and Korea and made collaborative experiments.In 2003, we performed optimization of CReSS to the Earth Simulator and evaluated its performance. The result showed that vector operation ratio is 99.4 %, parallel operation ratio is 99.985 % and … More sustained efficiency 33 %. Using CReSS on the Earth Simulator, we performed numerical experiments of multitscale Baiu frontal lows, the localized heavy rainfall in Minamata City, a typhoon and snow band cloud over the Sea of Japan.The experiment of the Minamata localized heavy rainfall was performed with the horizontal resolution of 500m using 120 nodes (960CPUs) of the Earth Simulator. The result showed detailed structure of individual convective cells and overall structures of the rainfall system. The multi-scale structures of the precipitation system associated with the Baiu front were also well simulated. The experiment of snow cloud band showed the formation and development of cloud bands or cloud streets over the sea. We also performed the prediction experiment of heavy rainfall caused by Typhoon RUSA and showed that the spiral band intensifies when the typhoon comes to the southern Korean Peninsula. These results show that computation using the cloud resolving model is important for the accurate simulation of a heavy rainfall. Less