Abstract: Based on the NCEP reanalysis data with the spatial resolution of 1°× 1° grid, a heavy rainstorm process on July 24, 2011 in Beijing was simulated by WRF3.3 (Weather Research and Forecasting Model for version 3.3) model, and this process was diagnosed in terms of high resolution data from the model. The results indicate that this process can be well simulated using the WRF model. The heavy precipitation is affected not only by an uplift power generated by the interaction of long-wave trough and the surface convergence in the middle and low level of troposphere, but also by the force of the divergence at  the upper level of troposphere. Under these configurations, the large-scale dynamic uplift in the middle and low levels and strong divergence at the upper level are in a state of vertical coupling, which is favorable to the development of the vertical circulation and convection over the heavy rainfall region. At the same time, the value of relative humidity is greater than 70% below the 500 hPa over Beijing which forms a deep and humid environment for the generation of heavy rain, and provides abundant water vapor for generating, strengthening, and maintaining precipitation. On the basis of atmospheric stability, the K index is high in the whole Beijing, and the peak value is 42.5 ℃. It means that atmospheric stratification is unstable. The analysis of dynamical variables show that the flow field features of upper divergence and lower convergence accelerate the generation of precipitation. The coupling of negative helicity in upper level and positive value in lower level is the dynamical mechanism to trigger and maintain precipitation.

Key words: WRF model, Diagnostic analysis, Helicity