Analysis of a Heavy Rainfall Event over Beijing During 21-22 July2012 Based on High Resolution Model Analyses and Forecasts

The heaviest rainfall over 61 yr hit Beijing during 21-22 July 2012.Characterized by great rainfall amount and intensity,wide range,and high impact,this record-breaking heavy rainfall caused dozens of deaths and extensive damage.Despite favorable synoptic conditions,operational forecasts underestimated the precipitation amount and were late at predicting the rainfall start time.To gain a better understanding of the performance of mesoscale models,verification of high-resolution forecasts and analyses from the WRFbased BJ-RUCv2.0 model with a horizontal grid spacing of 3 km is carried out.The results show that water vapor is very rich and a quasi-linear precipitation system produces a rather concentrated rain area.Moreover,model forecasts are first verified statistically using equitable threat score and BIAS score.The BJ-RUCv2.0forecasts under-predict the rainfall with southwestward displacement error and time delay of the extreme precipitation.Further quantitative analysis based on the contiguous rain area method indicates that major errors for total precipitation（〉 5 mm h^（-1）） are due to inaccurate precipitation location and pattern,while forecast errors for heavy rainfall（〉 20 mm h^（-1）） mainly come from precipitation intensity.Finally,the possible causes for the poor model performance are discussed through diagnosing large-scale circulation and physical parameters（water vapor flux and instability conditions） of the BJ-RUCv2.0 model output.

北京“7.21”特大暴雨高分辨率模式分析场及预报分析

2012年7月21-22日,61年以来最强降水袭击北京,北京大部分地区出现大暴雨,局部特大暴雨,过程雨量大、雨势强、范围广,造成了严重影响。此次强降水配置较为典型,业务预报提前指示出了此次过程,但预报结果存在强度偏弱,峰值偏晚等偏差。在对此次大暴雨进行综合分析的基础上,利用中国自动气象站与NOAA气候预测中心卫星反演降水资料CMORPH(Climate Prediction Center Morphing Technique)产品融合的逐时降水量网格数据资料作为观测,着重对北京市气象局新的快速更新循环同化和预报系统(BJ-RUC v2.0)的3 km高分辨率模式分析场和预报场进行了检验与分析,以期通过对中尺度模式预报性能的了解,为暴雨可预报性问题提供进一步的参考。研究结果表明,此次特大暴雨过程水汽条件极佳,降水区域较为集中,呈现西南一东北走向的中尺度雨带特征。利用常规检验评分对预报降水的时间序列进行检验发现,预报降水在时间上滞后,降水强度偏弱,存在偏西南的位置误差,并且未能反映降水系统的线状特征。进一步利用检验连续降水区域定量降水预报的CRA(contiguous rain area)方法,对预报误差进行分解表明,整体降水(>5 mm/h)的主要误差来自于位置和形状误差;而在暴雨(>20 mm/h)的预报中,降水强度的偏差占误差的主要部分。最后结合对预报场大尺度环流和物理量的诊断(水汽条件和不稳定条件),分析探讨了此次极端暴雨预报不佳的原因。