“7·21”北京特大暴雨的预报问题

Forecasting issues of the extreme heavy rain in Beijing on 21 July 2012

根据探空资料和国内外网站上实时收集的2012年7月10—22日的数值预报、卫星、雷达和地面资料，对2012年7月21—22日北京特大暴雨的分析和预报过程进行了总结。得到如下结果：（1）“7·21”暴雨属于一次典型的中尺度对流复合体MCC暴雨过程，发生在具有高温高湿的偏南风低空急流左前方、高空急流人口区右侧、深厚的暖平流等有利于MCC发生的环流背景下。f2）各种数值模式均不同程度地报出了这次暴雨，预见期可达3—4d。1～2d的短期预报可以比较准确地预报出暴雨的落区和强度，但对暴雨的开始和结束时间明显偏晚约6h。（3）卫星和雷达监测表明，在大尺度雨带到达之前12h，初始对流已在锋前暖区发生。根据回波的移动、增强和组织化，可以外推第一波雨峰将在中午前后影响北京，从而对数值预报作出及时的修正。回波和地面要素场（风、露点）的配合分析，可以大致确定发生初始对流所必需的静力不稳定条件和初始抬升条件，可为回波外推预报提供一定的天气学依据。

Based on the upper air sounding, NWP model forecasts, satellite, radar and surface weather data, the extreme heavy rain event in Beijing during July 21-22, 2012 is analyzed and its forecasting process is summarized. The results are as follows. （1） The ＂7.21＂ heavy rain was produced by a typical mesoscale convective complex （MCC） that occurred in the left front of a low-level jet in the warm and moist south- erly, on the right of the entrance jet stream, with deep warm adveetion and clockwise wind direction with altitude, which are very favorable for the occurrence of MCC. （2） A variety of numerical models have predicted the heavy rain with skill in various degrees, and the lead time is up to 3-4 d. The short-term forecasting with lead time 1-2 d can be more accurately predict the occurrence area and intensity of heavy rainfall. But the prediction of heavy rain＇ s start and end times is noticeably delayed about 6 h. （3） Satellite and radar monitoring indicates that 12 h before the arrival of a large-scale rain band, the initial warm convection has occurred ahead of the front. According to the movement, en- hancement and organization of radar echoes, it can be extrapolated that the first stage of the heavy rain will affect Beijing at around noon, and thus timely corrections can be made to the numerical predictions. Using comprehensive analysis of radar echo and the surface weather （such as wind, dew point） fields, we can roughly determine the convective instability condition and the uplift condition favorable for the initial occur- rence of convection, which can provide the basis for echo extrapolation forecasts.