Study On The Mechanism Of Dynamic Process And Intensification Of Rainstorm

Rainstorm is one of the main disastrous weathers that affect Jiangsu. Regional floods happened in 1954, 1991, 1998 and 2003, and local flood happened in the northeast of Jiangsu at the end of August 2000, etc., were all resulted from rainstorms or heavy rainstorms. By means of synoptic meteorology, statistics, dynamical diagnosis and numerical simulation, this paper analyzed the rainstorm courses affected Jiangsu, which occurred during Meiyu flood season or resulted from typhoons, and discussed the circulation character, meso-micro synoptic systems, dynamic and thermal terms, and intensification mechanism of rainstorm, etc., thus acquired some meaningful results.1. Analyzed synoptic circulation character, dynamic and thermal terms of rainstorm.According to the synthesizing analysis of the rain courses happened in Jiangsu in Meiyu flood season of 2003, came to the conclusion of circulation character, dynamic and thermal terms of rainstorm courses as follow. North China trough exists in middle latitude, the subtropical high of west Pacific origin seasonally lifts to the north and stretches to the west, at 120°E, and its ridge line reaches 26°N, 588 isopotential reaches the estuary of Yangtze Rivers, there is a shallow trough move towards east from the west of Szechwan, the cold and warm air interjunction locates in the south region of Huaihe valley. On rainstorm days, there always are cold fronts or stationary fronts in the surface charts cooperate with the upper systems, and their position is partial to the north(between 31-34°N). On the west side of the subtropical high, a low level jet stream(in southwest or partial to south direction, ≥12m/s) transports warm moisture towards Jiangsu, making change to the vertical structure of temperature and moisture, thus constitutes convection unsteady and forms differential advection of temperature and moisture between upper and low levels, resulting in unsteady energy accumulation. Near the jet stream, ageostrophic wind makes imbalance between pressure gradient force and geostrophic deflecting force, vertical shear increases in the low level wind field, Richardson number(Ri) minishes accordingly. In the vertical cross section of vorticity, divergence and vertical velocity, convergence exists in low layers, divergence exists in upper layers, and nondivergence exists near 400hPa geopotential height. Rainstorm occurs in the deep ascending motion region, in the vertical cross section of vertical velocity along 119°E, it shows that w<0 in Huaihe valley of Jiangsu(32-33°N), from the surface to 500hPa geopotential height, and the minimum is -25 X 10~-3hPa/s. Whereas, in the non-rainstorm courses, descending motion occurs in thesame region , and2. Disclosed the activity character of compound meso-scale convective cells (MCC), and terms of its occurrence and development.On the basis of the data of brightness temperature of infrared cloud top, analyzed the activity character of MCC that resulted in heavy rainstormm course occurred from July 4 to July 5,2003, disclosed the following fact: the MCC affected Jiangsu was comprised of typical meso- β ( γ ) cloud masses, heavy rainstorm area was corresponding to that of T_Bb<70℃, lower T_Bb resulted in stronger precipitation intensity. During MCC maturing stage, the lowest value of T_Bb could get under -75℃ and the convection usually broke through the tropopause. The strong upper level divergence and low level convergence can cause meso scale low system in Meiyu frontal zone, the foregoing two factors plus ascending motion in furthersome circumstance are the important terms of the occurrence and development of MCC. Cyclonic vorticity field can organize and strengthen the convection of cumulus, the development of MCC is corresponding to the deep meso scale vortex tube in the latitudinal vertical cross section of vorticity, the vortex tube developes vertically thus MCC developes strongly. The convective instability triggers the occurrence of MCC. The maintenance of warm and moist instable airflow, the accumulation of vapor and energy a...