Numerical simulation of a torrential rain in the northeast of huaihe basin partⅡ: instability conditions and the mechanism of intensification and maintenance

By using of the result of the simulation in part Ⅰ of this study, as well as the mechanism of intensification and maintenance, the relationship between deep moist mesoscale convective system (DMMCS) and instability, convective vorticity vector(CVV) are analyzed. Results show that (1) middle-low level convective instability is the precondition of the occurrence of DMMCS. The surrounding moisture centralize in the heavy rain area because of the low-level convergence. The convergence and merging enhancement of convection cells, and the convective instability energy transporting from the left-front of typhoon are the important mechanism of the rebulid of convective instability. (2) Not only baroclinic instability and conditional symmetry instability lie in the middle-low level, but also lie in the middle-upper level of DMMCS. At the west (east, south, north) of DMMCS, there lies negative (positive, positive, negative) MPV2 column. (3) In DMMCS, there is an alternative distribution of inertial instability column and inertial stability column, at the west and south (east and north), there lie negative (positive) CVV columns, while negative CVV column is beneficial to deep moist convection. (4) The strong slantwise convection induced by inertial instability, baroclinic instability and conditional symmetry instability will enhance the upper level southerly component. Because of the appearance of the compensated downdraft at the low level of the south side of DMMCS, the low level southerly is intensified, the enhancement of upper- and low-level cores are in favor of the development of DMMCS. And that will do favor for the reinforcement and maintenance of inertial instability, baroclinic instability and conditional symmetry instability, it is a positive feedback process. (5) There lies a downshear circulation to the east of rainfall cell, shallow convections near this cell absorb the vapor and instability energy coming from the south, and meso-scale convergence line and meso-β-scale vortex organize and intensify convective cells. In DMMCS, there are an alternative distribution of convergence and divergence columns, and the couple between strong divergence and vorticity columns. They are both in favor of the development of DMMCS, and the instability will be intensified and maintained for its developing in depth.