Numerical Simulation Analysis on the Meso-β-scale Systems Structure of a Heavy Rain over the Huaihe River

By analyzing and numerical simulation of a heavy rain event, the physics process and spatial structure of the meso-scale convective system are revealed. Such studies help to get ideas beneficial to and then improve heavy rain forecast. A heavy rainfall occurred over the Huaihe River during 29—30 June 2003 is simulated by using AREM model which is the advanced LASG η-coordinates numerical meso-scale heavy rainfall prediction model with horizontal resolution of 37 km and 28 levels vertically. Comparison between hourly simulated results and observed development shows that the main rainfall period and the main precipitation system are successfully simulated. As indicated by the simulation results:① Though southwest low vorticity is not the direct influencing system, the shear line and low pressure trough area stretching out from the low vorticity provide favorable background condition for the development of meso-β-scale system. ② Southwest low-level jet is the main cause of this weather course. The fast movement of the axis of Southwest low-level jet stream enhances the gradient of the wind and the cyclonic shear in the north of the axis of low level jet, bringing sharp transformation of the vorticity. A strong column of positive vorticity leads to the occurrence and development of the meso-β-scale depression and cyclone at lower level near the vorticity column, whereas a meso-β-scale high pressure developes at the midhigh level of the troposphere without the assorted meso-scale anticyclone. ③ The geostrophic deflection caused by the new meso-high changes the wind field near the meso-high and produces the strong meso-β-scale divergence center around the high. It results in the development of a strong updraft combined with the powerful convergence at lower layer. ④ The strong updraft occurs near the south boundary of the strong frontal zone of θ_se at lower layer. Meridional vertical cell across the strong frontal zone of θ_se firstly originates in the boundary layer then develops upwards slowly. And a meso-β-scale cyclone comes into being circling the ascending branch. ⑤ With the development of the meso-β-scale convergence in lower layer and meso-β-scale divergence in higher layer, water vapor in low layer accumulates to the meso-β-scale system area, thereby forms a narrow vapor convergence band, which ultimately leads to the strong rainfall under the effect of strong updraft.