The Mechanism Of Development, Intensification And Maintenance Of Severe Rainstorm In The Typhoon Periphery

During the appearance of Prapiroon typhoon (No. 12) in 2000, a torrential rainoccurred in its periphery. Using a nonhydrostatic mesoscale Model (ARPS(V5.2)),with ADAS 3-D data assimilation system, reanaiysis data of NCEP, radar echo andGMS5 satellite image had been applied to simulate the event. Four sensitivityexperiments had been designed. The relationship between deep moist mesoscaleconvective system (DMMCS) and instability, CVV, horizontal and vertical circulationare analyzed, and the mechanism of this torrential rain is revealed. The conclusionsare following:(1) There are three phases during the evolution of the convective clouds:3001-3012BST, 3013-3023BST, and 3100-3106BST. There are four convective beltswhich converge on Xiangshui. The first belt lies in front of upper trough, and movessoutheastward; the second one lies in the southwest, and moves from southwest tonortheast; the third one lies in the south; and the last one lies in the southeast.(2) The middle-low level convective instability is the precondition of theoccurrence of DMMCS, not only baroclinic instability and conditional symmetryinstability lie in the middle-low level, but also lie in the middle-upper level. Becauseof the low-level convergence, the surrounding moisture centralize in the heavy rainarea, the convergence and merging enhancement of convection cells, and theconvective instability energy transporting from the left-front of typhoon, are theimportant mechanism of the rebuilding of convective instability.(3) At the west (east, south, north) of DMMCS, there lies negative (positive,positive, negative) MPV2 column. In DMMCS, there is an alternative distributionbetween inertial instability column and inertial stability column. At the west and south(east and north), there lie negative (positive) CVV columns, and negative CVVcolumn is beneficial to deep moist convection.(4) Four sensitivity experiments show that latent heat, typhoon, upper trough,subtropical high have important effects separately. The convective instability zonecan be expanded, and the convective instability height can be rose. The high energytongue which is from the typhoon transporting to the heavy rain cell is enhanced. More instability energy is transported to the cell. The middle-upper level baroclinityand symmetry instability are enhanced. Being the convergence of low-levelconvergence and the moisture flux divergence are enhanced, the ascending motion isenhanced. Because of the horizontal gradient ofθ_e and wind vertical shear are added,the CW column is deepened.(5) The thermal wind can cause adjustment by latent heat, then drive themeso-scale circulation, so meso-scale convergence line and meso-β-scale vortexformed. Typhoon, upper trough, and subtropical high have important effect on thedevelopment of meso-scaie convergence line and meso-β-scale vortex, so theconvective cells can be organized and intensified. They have important effect on thedevelopment of downshear circulation, which can supply the dry and cold air, nearbythe heavy rain cell. Then, the convective cells can be intensified. They have importanteffect on upper- and low-level jet, and ageostrophy. The coupling of upper- andlow-level jet can be enhanced, and the wind vertical shear can be added. As theupper-level inertial instability and the low-level inertial stability, the upper-leveldivergence and the low-level convergence can be intensified, the positive vorticityappear. Then, the coupling of divergence and vorticity is intensified.