上海“0185”特大暴雨的中尺度强对流系统活动特征及其环流背景的分析研究

Meso-scale and Small-scale Circulation Characteristics Analysis of "0185" Heavy Rainfall in Shanghai

通过对常规观测资料、自动站资料、GMS-5卫星云图和多普勒雷达等气象资料分析发现,2001年8月热带低压在福建北部登陆,途经江苏无锡、常熟时,其南侧西南气流与沪浙沿海的东南风形成切变线。受源源不断地水汽输送和切变线动力抬升作用,热带低压南侧不断产生β-中尺度强对流回波带,它们形成后随热带低压气旋性环流向东移动,在上海城区附近辐合形成β-中尺度强对流回波群,在卫星云图上呈现出以此为核心的中尺度对流辐合体(MCC)。分析表明,受水汽凝结、潜热释放这种正反馈机制作用,在热带低压东南侧大尺度切变线中段北侧派生出一个独立、完整的近地面β-中尺度的气旋性环流,它就是MCC的内核,直接造成了上海“0185”特大暴雨的发生。

Heavy rainfall happens on August 5, 2001 in Shanghai, especially in the central urban area with 75.4 mm of 1-hour maximum precipitation and 264 mm of 12-hour cumulated precipitation at Xujiahui. Analyses on the conventional observations, automatic stations data, GMS-5 satellite cloud images and Doppler radar data show that it is closely related with a mesoscale convective compound （MCC） in Shanghai. The number 9 tropical depression moves slowly right to the north of Shanghai because of the adjustment of the long-wave system in the middle latitude and suddenly weakening of the northwest Pacific subtropical high. The MCC is triggered in the shear line between southwesterly current in the southeast of the tropical depression and the strong southeasterly current in Shanghai and Zhejiang, when the tropical depression moves to the north of the Taihu Lake. A slowly eastward-moving circle zone without any echo is found near Wuxi at the night of August 5 in the base reflectivity products of Doppler radar. Some spiral echo bands rotate counterclockwise around the circle zone in the east. The spiral echo bands in the southeast moves to Shanghai under the effect of the shear line and the converge counterclockwise to form a strong echo group of several coordinate meso-β-scale echoes in the central urban zone of Shanghai. This strong echo group in radar products is correspondent to the mesoscale convective cloud above Shanghai, which forms in the northwest of Shanghai and Suzhou initially, and begins to weaken soon after its formation. Subsequently the mesoscale convective cloud develops strongly to be a typical mesoscale convective compound （MCC） about 102600 km^2 in the northwest of Shanghai. Temperature of the cloud top bright core reaches - 81 ℃, and the convective clouds with top temperature of - 63 ℃ cover the whole of Shanghai. A couple of positive and negative speed centers about - 20 and 20 m/s respectively are located in the central urban zone of Shanghai in the radial speed products of Doppler radar. The circle zone without echo in Wuxi is corresponding to the surface convergent center in the stream field, which shows that the circle zone is just the tropical low center. The strong echo group rotating counterclockwise in the central urban zone in Shanghai and the couple of speed centers are also corresponding to another meso-β-scale surface convergent center in Shanghai, which is a meso-β-scale convergent system, named mesoscale cyclone, and is located in the core of the MCC. The meso-β-scale cyclone circulation, as well as the process of its formation and development in the core of the MCC leading to the heavy rainfall at August 5, 2001 in Shanghai, is testified through analyses of automatic weather stations data and Doppler radar data.