MultiScale Configuration of the 20 July 2016 CycloneInduced Severe Torrential Rain and Its Relationship with the Development of MγCS
Affected by northward cyclone, a rarelyseen severe rainstorm hit the BeijingTianjinHebei Region on 20 July 2016, resulting in a new extremum of daily mean precipitation amount in this region in the past 50 years. Using the FY2F satellite data with 6 min interval and 5 km horizontal resolution, radar data and AWS (automatic weather station) data collected every 10 min, NCEP 1°×1° reanalysis data with 6 h interval as well as the methods of multiple data overlay analysis, TBB gradient computation, retrieval of radar lowlevel wind etc., this paper analyzes the cloud image characteristics of cyclonic rainstorm cloud system in northern China and the organization, dynamic and thermodynamic structures of multiscale systems (synoptic scale and mesoα, mesoβ, mesoγ scale convective systems). Two mesoγ scale convective systems (MγCSs), which contribute to the severe rainstorms in the urban areas of Beijing and Tianjin are taken as examples to analyze the relationship between multiscale ambient configuration and the severe torrential rain events. The results are as follows. (1) The thermodynamic and vapor configurations of different parts of the comma cloud system are: the vortex center (Zone D) coincides with the center of the cyclonic circulation and is a lowvalue zone of the 700 hPa pseudoequivalent temperature (θse). The west side of the tail cloud zone is a dry, cold and cloudless zone. The smooth zone at the edge of the cloud zone corresponds to the energy front zone. The head of the cloud system (Zones A and B) corresponds to the θse highenergy zone of the 700 hPa and the jet stream core of the 850 hPa, while the Zones A and B correspond to northeast and southeast jets, respectively. In the jet zone, the θse of 700 hPa is as high as 78℃, the 850 hPa specific humidity is 18 g·kg-1, the maximum jet core wind speed is 30 m·s-1, and the entire precipitable water amount is 70 mm. (2) The comma cloud system has two MαCSs and one MβCS embeded in the head, which are the major contributors to the severe torrential rains, but the rain intensity is only 20-30 mm·h-1. However the MγCS in Beijing (MγCSBJ) and MγCS in Tianjin (MγCSTJ) both caused extremely heavy precipitation with the intensity getting to 40-70 mm·h-1. The two MγCSs are generated and maintained under different multiscale background conditions: (a) MγCSBJ is located in the vortex center (Zone D), which is the left front convective instability zone of the lowlevel jet, combined with the wind direction convergence line formed by the northerly wind and the east wind. The convergence line appears about 1.5 h earlier than the convective system. (b) MγCSTJ is located in the edge zone between the cloud head and the vortex center. It occurs in the highgradient energy frontal zone of θse and matches the surface wind speed convergence line. (3) The development of MγCSTJ is closely related to the largevalue zone of TBB gradient in the MβCS edge. MγCSTJ has a deep mesocyclone inside, with the axis tilting to northwest, and the maximum rain intensity occurs at the moment when the rotation speed is the fastest.