The Piecewise Potential Vorticity Inversion's Application In The Interaction Between Mei-yu Precipitation And The Western Subtropical High

The present study selects the mei-yu case of June 1999 over Yangtze River valley in China, and performs vorticity diagnosis and Ertel piecewise PV inversion to discuss the mechanism responsible for the interaction between the Mei-yu precipitation and western Pacific subtropical high(WPSH). The main results and conclusions are as following:1. The 10-day mei-yu process is characterized by lasting long, heavy and localized. The rain belt presented a southwest-to-northeast orientation and reached the Japan peninsula, the precipitation center locates over the lower reach of the Yangtze River valley and the Japan peninsula, respectively. Accompanied the beginning of the mei-yu process, the 200hPa South Asia High (SAH) exhibits remarkable eastward extending, followed by the westward extending of the WPSH to inland, which lagged about 1-2 days of the eastward extending of the upper level SAH. The occurring and withdrawing of the precipitation is well corresponding to the east-west movement of the WPSH.2. The enhancement of the upper level westerly jet is responsible for the eastward extending of the 200hPa SAH. The diagnoses for the complete-form vorticity equation shows a secondary counter Hardley circulation caused by the eastward extending of the SAH along the longitude of the low reach of the Yangze River valley, which results in the westward extending of the WPSH at low-middle level. The condensational latent heat released by rainfall creates anticyclonic vorticity in the upper level and the cyclonic vorticity in the low level, which strengthens the SAH and maintains the vertical motion of the secondary circulation. At the middle troposphere, where locating the maximum diabatic heating, the negative vorticity tendency created by meridional gradient of the heating leds to the westward extending of the WPSH. At the lower troposphere, the anticyclonic vorticity created by the condensational latent heat enhances gradually, the anomalous cyclone moves eastward with the west jet, and together with the effects of the cyclone in eastly in lower latitude, results in the eastward withdrawal of the WPSH. After that, the wind shear is no longer maintains over the Yangtze River Valley but moves northward, with the weakening of the vertical ascending motion, the upper level SAH withdraws westward and the mei-yu process terminates.3. The piecewise PV inversion shows the contribution of each level to 500hPa WPSH as following: the first importance is middle-low air forcing, the secondary one is the middle-upper level forcing, the contribution from bottom forcing is the smallest. In different phases and the different locations, the upper-level Rossby forcing has different contribution to the WPSH. During the eastward extending of the WPSH, the condensational latent heat at the north side of the WPSH causes the positive vorticity anomaly, which produces the local cyclonic circulation, prevents the northward of the WPSH. The largest contribution to the westward extending of the WPSH comes from the middle level nor dry nor wet air forcing. During the eastward withdrawing of the WPSH, the upper level anomalous cyclone over the western Pacific shifts westward along the easterly wind, destroys the inner structure of the WPSH, together with the forcing of the anticyclonic voticity in middle latitude, results in the weakening and eastward withdrawal of the WPSH. Meanwhile, the coupling of the strong trough at the upper lever over the Pacific and the cyclone center near the Japan prevents the WPSH moving westward again.