天津城市环境对一次局地降水过程影响的模拟研究

Simulation Study of the Impact of Urban Environment on a Local Precipitation Process in Tianjin

采用中尺度气象模式WRF结合自动站观测资料,对天津城区及附近发生的一次局地降水过程进行数值模拟和敏感性试验,研究了城市环境(城市地表、城市大气污染)对降水发生发展的影响。结果表明:城市地表引起的热岛环流,叠加海风环流后,改变了辐合线的位置和强度,直接影响了对流降水的发生及落区。城市大气污染状况可影响对流发展后的强度及降水量,敏感性试验结果显示气溶胶浓度增大后区域平均降水量增加约25%;气溶胶的增雨效应主要表现在可增进对流云中液态水和冰相物质含量,增多的液态水物质被强的上升气流抬升到较高处冻结形成冰晶,这一过程中增多的凝结潜热又可促进对流发展,最终导致地面总降水量增多,研究个例中气溶胶浓度增加可使潜热加热率最大增加110 K·h^-1。

Using mesoscale numerical prediction model WRF combined with dataset from automatic weather stations,focused on a local convective precipitation process which occurred in and near Tianjin urban area,this paper conducts numerical simulations and sensitivity test to investigate the impact of urban environments(surface features,urban air pollution)on the convective precipitation.The results show that the heat island circulation caused by urban surface changed the position and intensity of convergence line after superimposing sea breeze circulation,which directly affected occurrence and falling area of convective precipitation.Once convection develops,the air pollution over cities can affect the intensity and rainfall.The sensitivity test shows that the increase of aerosol concentration makes the regional average precipitation increased by about 25%.The increase of precipitation is related to the formation of more liquid water in convective clouds.The enhancement effect of aerosol is mainly manifested in enhancing the content of liquid water and ice phase substances in convective clouds.The increased liquid water is transported to a higher level by strong updraft and froze to form ice crystals.The increased latent heat release during this process can also enhance the ascending motion and eventually lead to the increase of total precipitation on the surface.In this experiment,the increase of aerosol concentration can increase the latent heating rate by 110 K·h^-1.