一次突发性暴雨的机理分析及短期预报思考

Mechanism Analysis and Rethinking of Short-Term Prediction of a Sudden Rainstorm

2015年9月4日白天,天津出现突发性暴雨过程,利用多种观测资料对此次过程进行分析,结果表明:偏南气流与高压底部东北风相遇,在蓟州山区以南形成辐合,触发雷暴发展;雷暴形成后,回波形成"后向传播"机制,配合中低空的南风脉动,在天津北部形成南北向的"列车效应",导致北部强降水的发生;上游的高空槽降水在近地面形成冷池,其向东辐散气流与偏东风相遇,在降水区下游触发新的雷暴,使得雨带快速东移,且当辐散气流与偏东风相遇后,出现小尺度辐合性气旋式环流,导致下游强降水增幅;当高空槽降水云团主体移过城区后,在γ中尺度辐合流场作用下,触发小尺度对流云团的生成和发展,影响天津城区再次出现强降水。在短期预报过程中,预报员在大尺度模式环境场分析的基础上,对于中尺度模式仅参考了其降水预报,而忽略了对中尺度环境场的分析,分析表明,虽然中尺度模式对此次过程的降水时段预报存在偏差,但其中尺度环境场预报,可以为此次暴雨过程在短期时效内(24h)的预报订正提供参考。

A sudden rainstorm process happened in Tianjin on 4 September 2015.The analysis based on various observation data showed that the southerly flow met with the northeast wind at the bottom of the high pressure,forming the convergence in the mountain front and triggering thunderstorm.After the thunderstorm was formed,the ＂backward propagation＂mechanism formed,matching up the southerly wind in the mid-low level,and forming the south-north＂train effect＂which led to the happening of heavy rainfall.The precipitation caused by upper trough developed into the cool pool near the surface layer.The divergence flow met the easterly flow,triggering the new thunderstorm in the downstream of precipitation area.This＂forward propagation mechanism＂made the rain belt move faster than that of upper trough.And when the divergence flow met with the easterly flow,small-scale convergence cyclonic circulation formed,making the precipitation increase in front of the convergence line.After the upper-trough precipitation moved across the urban area,due to the meso-γscale convergence effect,which triggered the formation of meso-γscale convective cloud,the heavy precipitation once again appeared in the urban area of Tianjin City.During the short-term forecasting,besides the analysis of large-scale environmental background,forecasters only considered the precipitation field from mesoscale model,ignoring the analysis of mesoscale environmental field.The results showed that although there is deviation between the precipitation from model and observed precipitation,the prediction of the mesoscale environmental field could provide reference for the prediction of sudden heavy rainfall in the short period of time（24 h）.