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

 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 southerly, on the right of the entrance jet stream, with deep warm advection 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, enhancement 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 occurrence
of convection, which can provide the basis for echo extrapolation forecasts.