Mechanism Triggering the "03.7" Heavy Rainfall in the Northwest of Hunan Province

The analysis on torrential rain event in Hunan Province on July 7 to 9, 2003 is accomplished.It is the result of the combined use of conventional intensive observational data, GMS satellite cloud picture, Doppler radar data and the numerical simulation result with mesoscale numerical model of the Penn State/NCAR Mesoscale Model 5(MM5). In particular, it emphasizes the output based on the observational facts and the MM5 high resolution data.The mesoscale convective system is discovered which is prone to the existence and maintenance of the heavy rainfall center in Zhangjiajie.In order to reveal the development mechanism and spatial structures of Meiyu front rainstorm mesoscale convective systems, some innovative work about this rainstorm triggering mechanism is done.These important scientific conclusions can be used to seek new forcasting ideas, and ultimately improve the heavy rain forecast.The heavy rain is related to the movement and evolvement of multiple mesoscale convergence disturbances. With the rapid development of the mesoscale rain clusters, the β-mesoscale vortex takes shape, by which this torrential rain is directly affected.The reason of this rainstorm is not the severe multi-cell and super-cell.Due to the atmosphere constantly generation of new convective cells and afterwards vanish in the north, the organized multi-cell storm is the main reason of this heavy rain in Zhangjiajie.The appearance of head wind zone about several kilometers above the district of Zhangjiajie which is reflected in the Doppler radar radial velocity field is the result of small-scale cyclone formation and maintenance. It is closely related to the emergence of continuous heavy rain.In the low troposphere layer, as the Southwest jet velocity fluctuates, jet stream flow and the largest northeast nuclear center are split by it and the development of the β-mesoscale vortex in the northwest of Hunan is triggered.The suction effect caused by the low-level jet disturbance and the divergence on the top-level of troposphere is the triggering mechanism of the heavy rain, and the dynamical and vapor field disturbance induced by the mesoscale topography is the main element of the heavy rain.The β-mesoscale vortex presents intricacy thermal and dynamical structure, the θ_se equiscalar surface tilt is forced by the latent heat release caused by the strong convective movement and the sudden increase of PV on the low-mid level is made.Simultaneous, the strong convective movement takes place in the region of isentropic surface marked by a sunken feature.The PV sliding along the entropy isentropic and the slope vortex development may be the main cause of maintain and development of the β-mesoscale vortex.