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

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 km² 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.