| Based on routine meteorological observation, satellite cloud pictures and Doppler radar data, the synoptic situation and mesoscale systems causing the torrential rain over Liaodong Peninsula on 3 August 2004 were analyzed. It was pointed out that the torrential rain was caused by low level jet (LLJ) and mesoscale depression in the large trough situation. Doppler radar data showed the existence of mesoscale cyclone and LLJ and the relative motion of masoscale echo to its synoptic scale system.By using MM5V3.5 mesoscale model, the torrential rain event was simulated successfully, and the genesis and evolvement of the mesoscale systems was simulated at the same time. With the analysis of the simulation output, it was ascertained that the LLJ and the upper level jet-LLJ coupling played important roles in enhancing the ascending motion and the torrential rain. With the in-depth analysis, the conclusion was drawn that the Bohai Sea had immediate effect on the development of LLJ and mesoscale depression.Numerical sensitivity experiment verified the positive effect of Bohai Sea and Liaodong Peninsula topography on the torrential rain. By comparing the influence of Bohai Sea, desert, city and Liaodong Peninsula on horizontal velocity at different levels, vertical velocity, surface temperature and humidity, the effects of different underlying surface on the local dynamical and thermal structure of the PBL and corresponding circulation was investigated. Consequently, it was revealed that Bohai Sea and Liaodong Peninsula topography can lead to the amplification of rainfall.Diagnostic analysis of moist potential vorticity (MPV) showed that the torrential rain over Liaodong Peninsula occurred in strong convective instability and baroclinic instability conditions. The horizontal moist isentropes turning to very stiff and dense state led to the development of moist slantwise vorticity, which accelerated the ascending motion and facilitated the torrential rain.Diagnostic analysis of MPV based on sensitivity experiment output indicated that changing the underlying surface and lowering the terrain altitude weakened the instability of low troposphere, and the former also impaired the LLJ, the latter also displaced and rotated the LLJ. It was confirmed that Bohai Sea and Liaodong Peninsula topography can strengthen the PBL instability and the LLJ, and consequently make positive contribution to rainfall. |
