Study On The Mechanism Of Early-Summer Rainstorms In Southern China Triggered By Tropical Disturbances

With several observational data, it is carried out synoptic analysis, potential vorticity inversion, dynamic and thermodynamic diagnosis, numerical simulation and sensitivity experiments with meso-scale numerical model (WRF) to study a typical case of early-summer rainstorms in southern China triggered by tropical disturbance and to investigate the dynamic and thermodynamic structure of tropical perturbation and its related deep moist convection, the physics of tropical disturbance development and the mechanism of rainstorm formation. It is found that: 1) the tropical disturbance has a high pseudo-equivalent potential temperature core in the middle-lower level and its convergence and vortex features mainly lie in the low level. Its positive vorticity and diabatic-heating column tilt northward with height, with direct or indirect secondary circulation in its two sides. The low level convergence and upward motion lie in the north or northeast sides of tropical perturbation. Deep moist convection also occurs in this area. The strong upward airflow ascends and tilts from southeast to northwest in the middle-lower level. It diverges as southwest flow when coming to the upper level. The deep convection system moves downstream under the dynamic steering of 700-400hPa mean flows. 2) The interior dynamics in the atmosphere cannot lead to the fast development of tropical perturbation. However, the diabatic heating is the main physic mechanism for the development of the tropical disturbance and meso-scale heavy rain system. The feedback of latent heat release plays the most important role in the evolution of tropical disturbance and the formation of heavy rainfall. The boundary heat fluxes play a second role and the third comes to the surface heat fluxes. The direct radiation heating only has a fairly minor effect, which even can be neglected. 3) The main physic way of the diabatic-process effects on the evolution of tropical disturbance can be concluded as following. The diabatic heating produces potential vorticity in the middle-lower level of the troposphere, which leads to positive geopotential height perturbation in the middle-upper level and negative equivalent in the middle-lower level. Thus a meso-high will develop in the middle-upper level and a meso-low will develop in the middle-lower level. Correspondingly, a cyclonic circulation associated with convergence in the lower level will be coupled to develop with an anticyclone circulation related to divergence in the upper level. Additional upward motion will occur in the area of rainfall and convection, which will lead to further development of tropical disturbance. 4) The physical mechanism of tropical disturbance triggering early-summer rainstorms can be described as following. Under favorable synoptic-scale background, the positive vorticity is separated from the southern trough in the lower troposphere and effect the atmosphere over the South China Sea (or there exits the remains of positive vorticity over the South China Sea), which leads to weak cyclonic perturbation (tropical disturbance) forming in the northern South China Sea. Then the tropical disturbance further develops under the effects of surface-and-boundary heat and moisture fluxes. The synoptic forcing is strengthened to trigger the outbreak of deep moist convections in the tropical disturbance area. There exist positive feedback effects of deep moist convections upon the tropical perturbations. The tropical disturbance moves northward fast to influence Southern China. At this time, low-level jet is intensified and moisture transportation is enhanced. The upward motion by synoptic-scale forcing leads to the formation, development and maintenance of moist convection and heavy rainfall. The dynamically forced lifting in the upslope topography and the mechanically forced convergence in the tromp-like topography enhances heavy rainfall when the deep moist convection moves to the sophisticated topography. Diabatic heating of rainfall condensation exerts a positive feedback effects on the development of tropical disturbance and meso-scale rainstorm system. 5) The points that should be focused in the short-term forecasting of warm-moist-area rainstorms in Southern China are as following. First, it should be paid attention to the lower-level cyclonic perturbation to the south of Southern China and whether or not the strong wind axis of the disturbance system is in superposition with high precipitable water band. Second, the weather background should be analyzed to see whether the perturbation system would develop fast or not. It should be watched out that the system maybe develop fast when there is deep moist convection activity in the disturbance system. Lastly, the forecasters should analyze the direction of environmental flow in the middle troposphere and see whether there is steering mechanism for the northward movement of tropical disturbance and deep moist convection systems.