天津城区一次γ中尺度短时暴雨的热动力环境分析

The Thermal and Dynamic Environmental Field Analysis of a Meso-γScale Short-Time Rainstorm in Tianjin

利用睿图-短期预报子系统的精细化预报结果、地面加密自动站、多普勒雷达、FY-4A卫星的逐5 min可见光云图以及北京探空等资料,对2018年7月22日发生在天津城区的一次突发性γ中尺度短时暴雨的热动力环境进行了详细分析。结果表明:此次暴雨发生在500 hPa副热带高压控制范围内,是由城区孤立风暴造成的一次局地强降水过程,具有范围小(不足20 km)、生命史短(1~2 h)、雨强大(62.4 mm·h^(-1))、中尺度边界层环境复杂等特点。暴雨是在上游降水系统的冷池边界还远离天津城区时,由城市热岛、上游冷池出流前的边界层弱冷空气、系统性东北风和午后逐渐形成的中尺度海风共同作用造成的。下垫面水平热力差异及地表能量平衡的结果导致天津中心城区形成较为显著的热岛效应,热岛强度达2~4℃,与热岛效应伴随的城市热岛暖低压的形成与发展导致城区形成中尺度辐合中心。上游降水产生的中尺度高压(上游降水区)和天津城市热岛暖低压(下游非降水区)之间的气压梯度导致冷池前沿形成了一支超越冷池出流边界而率先到达天津城区的一支北风,这支边界层弱冷空气与系统性东北风、海风在城市热岛暖低压作用下均向城区汇合,进一步增强了城区辐合中心的强度及维持时间。垂直方向上,沿城区的纬向、经向分别形成了两个方向相反的、非对称的中尺度次级环流,其上升支正好位于天津城区。上游冷池出流前中尺度锋区东移造成的水汽集中以及热岛效应伴随的局地热量累积使城区逐渐发展为高湿高能区,且垂直方向上不稳定度增强,为局地暴雨的发生提供了有利的中尺度环境条件。

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-γscale 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.