Analysis of environmental conditions and characteristics of convective system evolution of a Mei-Yu Front heavy rainstorm in southern Jiangsu

Affected by the Mei-Yu Front and the mesoscale vortex on the Mei-Yu Front, a heavy rain event occurred in Suzhou on 6 July, 2020, and caused disasters such as urban waterlogging. In this paper, by using ERA5 reanalysis data from the European Center for Medium-Range Weather Forecasts, as well as various observational data from ground meteorological stations, Doppler weather radar, and wind profile radar, we analyze the weather background, environmental characteristics, convective system evolution, and maintenance mechanism of this heavy rain process. The results show that: (1) The upper westerly trough moved eastward, the cold shear line at the middle and lower levels pressed southward, the convergence of cold and warm air over southern Jiangsu enhanced the convergence at the lower level, the strengthening of the southwest jet at the middle and lower levels and the maintenance of the ground quasi stationary front provided favorable weather background conditions for the heavy rainstorm process. (2) During the strongest rainfall period, the intensity of the radar echo center is 60 dBz and the top height exceeds 14 km. The strong reflectivity factor area corresponds to a differential propagation phase shift of 7°·km^-1. The average vertical extension height of the strong echo is below 6 km, making it a typical low center of mass warm area rainfall. (3) The moderate levels of convective availble potential energy, strong vertical wind shear in the middle and lower levels, and convective instability stratification, and the low lifting condensation level over Suzhou provide favorable environmental conditions for the generation of heavy rainstorm. The continuously increasing low-level jet propagates downwards, to forming an superultra low-level jet. The strong convergence and uplift in the jet exitoutflow area, combined with the rising branches of the secondary circulation above the shear line, provide dynamic uplift and triggering conditions for the generation of heavy precipitation. The strong frontogenesis of the Mei-Yu Front in both horizontal and vertical directions enables the maintenance of heavy precipitation.