Numerical Simulation And Diagnostic Analysis On A Heavy Rainfall In North China During18-19August2010

Using the conventional surface observations, radiosonde data and the global1°×1°final reanalysis data from NECP, a heavy rainfall case which occurred on June18-19,2010in North China was analyzed with the synoptic and dynamic diagnosis method. It is found that three weather systems including the Mongolia vortex, Ural mountain ridge and the subtropical high, contributed to the generation of the torrential rain process in the large scale circulation. At500hPa of the troposphere, North China was in the control of a low trough with strong cold air intrusion in the back of the trough. The cold and warm flows intersected over North China which was in favor of the occurrence of the rainstorm.The heavy rainfall case of’8.19’is simulated using the mesoscale nonhydrostatic model WRF with different microphysis schemes and different cumulus parameterizations schemes. Comparison results between tests showed that when the grid size was36km, the simulated rainfall location and intensity with the Lin scheme were closer to observed ones; when the grid size was12km, Thompson scheme performed better than other schemes.In order to achieve the best simulation, the real-time radiosonde data and surface observatio n data was assimilated into the initial background field and boundary conditions. Then the improved high resolution numerical stimulations results were used to do the diagnosis of the precipitation system.Dry PV analyses showed that large values of barotropic PV corresponded quite well to the rainfall air in the location and movement. So barotropic PV was more effective in tracing the rainfall area and can be a reference index of the rainfall intensity and the region. Moist potential vorticity analyses showed that heavy rainfall areas located mainly in the front or the right side of MPV1movement direction (MPV,<0and MPV2>0). Both the evolution of MPV1and MPV2were consistent with the lower troposphere weather system. Cold air intrusion from upper troposphere was another reason causing the strong precipitation. Strong precipitation occurred in the period when the warm moist flow of rose and high-altitude cold dry flows descended.Helicity can indicate the evolution of the precipitation effectively. Environmental wind transported cyclonic vorticity and warm moist air of the lower layer to North China. With the rapid increase of helicity, cyclonic vorticity into the system along the flow direction increased which induced stronger convection and thus promote the occurrence and amplification of the rainstorm.