ANALYSIS OF NUMERICAL SIMULATION ON A NORTHEASTERN COLD VORTEX HEAVY RAIN IN THE SONGNENG RIVER BASIN

By using MM5 mesoscale non-hydrostatic model, a numerical simulation and contrast test is conducted successfully of a northeastern cold vortex and its shear which caused heavy rain in the Songneng River Basin on August 8, 1998. Results show that the rainstorm was triggered by the positive vorticity advection at the upper level atmosphere. Coupling of two jets at the upper and lower levels led to the strong precipitation within the area of low vortex coverage. The upper jet stream was strong and temporary, while the lower is weak and persistent. Resulted from the down draught in the front of a blocking high, the easterly dry and cold air current at the mesosphere was the crux of shear formation. The wind shear, which formed at the mid-troposphere as a result of the easterly current meeting with a southerly, is the dominant forcing in producing ascending motion. As the blocking high weakened, the easterly current below and the southerly current at the rear weakened. As a result, the precipitation system abated and moved both easterly and southerly. With enhancement of the northwest cold air, the southerly currents at the upper and lower levels strengthened and weakened respectively. As a result, the precipitation intensity enhanced and the precipitation scale enlarged.