ANALYSIS AND NUMERICAL SIMULATION OF A MESO-β-SCALE VORTEX IN THE MIDDLE REACHES OF THE YANGTZE RIVER

A heavy rainfall process took place in the middle and lower reaches of the Yangtze River during 26-30 June 1999.The analysis shows that several mesoscale cyclone disturbances along Meiyu front are heavily responsible for this intense precipitation process. An east-moving low vortex in 29 June 1999 was simulated by use of PSU/NCAR MMS in this paper. The results show that the slow-moving and stagnation of this east-moving meso-α-scale low vortex in the mid-dle reaches of the Yangtze River provided background condition for the formation of a meso-β-scale vortex. And the meso-β-scale vortex was produced in the southeast boundary layer of the east-moving meso-α-scale low vortex. After its formation, this meso-β-scale vortex moved northeast ward accompanied with strong heavy rainfall. The evolution and propagating of this strong heavy rainfall were verified by the dense observation-al rainfall data. The simulation also shows that the meso-β-scale vortex appeared in the lee side of mesoscale mountain where the westerly flow in the south of the meso-α-scale vortex converged with the southerly flow from warm section. A mesoscale low and strengthened southeasterly flows near surface in the lee side of the mesoscale mountain strengthened the vorticity and convergence near surface,which was helpful to the develop-ment of the meso-β-scale convective system. When the meso-β-scale vortex moved east to Boyang Lake,the easterly flow in the north of vortex was hampered by the Dabie mountain and turned to south,and strengthened the cyclone circulation in the back of the meso-β-scale vortex. Then under the influence of the southerly flow in the boundary layer,severe convergence and upward movement appeared in the meso-β-scale vortex. The cy-clone circulation developed to high levels,and the meso-β-scale vortex developed to be a meso-α-scale vortex system at last.