海陆风和地形对一次弱天气背景下暖区特大暴雨的影响分析

Analysis of the influence of sea-land breeze and topography on the extraordinary heavy rain in warm sector under a weak weather background

利用区域气象站、海岛站、测风塔、风廓线和多普勒天气雷达等多种非常规观测以及NCEP/NCAR 0.25°×0.25°再分析资料,对2017年6月22日凌晨广东西部沿海发生的一次预报失败的局地特大暴雨过程的成因进行了综合诊断,重点探讨了局地海陆风和地形(相互)作用对该过程的影响。结果表明:(1)暴雨发生在弱天气尺度环流背景下,华南地区无锋面和高空槽活动,边界层超低空偏南急流为暴雨提供了不稳定能量,不同性质下垫面的热力差异导致天露山山前形成海陆风(偏南)与偏北风的中尺度辐合线,致使初始对流在关键区触发。(2)偏南暖湿气流向北推进受阻后,在天露山地形强迫抬升下对流增强发展成中尺度对流系统(MCS),下垫面热力差异在山前强迫产生的中尺度垂直切变与降水之间可能存在正反馈现象,延长β中尺度对流系统生命史。(3)中尺度辐合线上不断激发的对流云团,形成降水的列车效应,导致了这场罕见的局地特大暴雨过程,凸显海陆风环流对本次暖区暴雨的重要作用。(4)该过程发生前,所有业务客观数值预报模式均未预报出明显降水,数值模式难以做出暖区尤其是弱天气背景下的暴雨以上降水预报,目前的监测和短时临近预警是主要手段。

Using a variety of unconventional observations, such as regional weather stations, island stations, wind towers, wind profiles and Doppler weather radars, and NCEP/NCAR 0.25×0.25° reanalysis data, on the west coast of Guangdong in the morning of June 22, 2017, a comprehensive diagnosis was made for the cause of the local heavy rain event in which the forecast failed, focusing on the impact of local sea-land breeze and topography(mutual) on the event. The results show that:(1) The rainstorm occurs under the weak weather environment.There is no frontal nor high-altitude trough activity in South China. The ultra-low-level southward jet in the boundary layer provides unstable energy for the rainstorm. The thermal difference of the underlying surface with different properties leads to the mesoscale convergence line of the sea-land breeze(southerly) and northerly winds in front of Tianlu Mountain, triggering the initial convection in the key area.(2) When the warm and moist air in the south is blocked to the north, it develops into MCS under forced uplift of Tianlu Mountain topography. The difference in thermal friction between the underlying surface may exist between the mesoscale vertical shear and the precipitation forced by the piedmont. Positive feedback phenomenon extended the life history of β mesoscale convective system.(3) Convective cloud clusters are excited continuously on mesoscale convergence line, forming train effect of precipitation, which leads to this rare local heavy rain event, highlighting the important role of sea-land breeze circulation in this heavy rain in warm sector.(4) Before the occurrence of this event, all the objective numerical forecasting models in service did not predict significant precipitation. The numerical model is difficult to make short-term prediction precipitation with grade above storm in warm zones, especially under the weak weather conditions. Current monitoring and short proximity warnings are the main tools for forecasters.