Abstract:In this paper, RMAPS-ST prediction results, Doppler weather radar data, surface dense automatic station data were used to analyze a localized sudden short-term meso-γ scale rainstorm which occurred in Tianjin on 22 July 2018. Results show that the rainstorm was a local heavy precipitation process caused by an isolated urban storm which occurred within the control range of 500 hPa subtropical high. It featured a small range (less than 20 km), a short life circle (1-2 h), severe precipitation intensity (62.4 mm·h-1) and complex mesoscale boundary layer environment. The rainstorm happened when the boundary of the cold pool of the upstream precipitation system was still far from the urban area of Tianjin, and it was induced by the combined action of the urban heat island, the weak cold air ahead of the outflow of the upstream cold pool, the systematic northeast wind, and the mesoscale sea breeze which gradually formed in the afternoon. The result of the underlying surface horizontal thermal difference and the corresponding surface energy balance contributed to the formation of significant heat island effect in downtown Tianjin with 2-4℃ heat island intensity, and consequently, the formation and development of the urban heat island warm low pressure accompanied by the heat island effect led to the formation of a mesoscale convergence center in the downtown. Moreover, the pressure gradient between the mesoscale high pressure generated by the upstream precipitation (upstream precipitation area) and the warm low pressure in downtown Tianjin (downstream non-precipitation area) resulted in a northern wind branch, which surpassed the outflow boundary of the cold pool and reached the urban area of Tianjin in advance. This weak cold air in the boundary layer combined with the systematic northeast wind and sea breeze, and converged towards the urban area under the effect of the urban island warm low pressure, which further enhanced the intensity and maintenance time of the urban convergence center. Two asymmetric mesoscale secondary circulations were formed along the vertical direction of the urban area in zonal and meridional directions with ascending branch located in the downtown area of Tianjin. The local heat accumulation accompanied by the heat island effect and the water vapor concentration caused by eastbound migration of mesoscale frontal area, jointly caused the development of local high humidity and energy region in downtown and the increment of vertical instability, which provided favorable mesoscale environmental conditions for the occurrence of the short-time local rainstorm.