二维多单体雷暴系统中对流单体生成和发展的新机制

A mechanism for the convective cell regeneration and development within a two-dimensional multicell storm.

在多单体雷暴系统的发展和演变过程中,系统内新对流单体的生成和发展是系统维持最为重要的因素。文中利用中尺度数值模式,研究了环境风场存在低层反向切变(U_z<0)情况下,一种新对流单体的生成和发展机制——处于"亚最优切变态"的多单体雷暴系统的维持和发展机制。与以往处于"最优切变态"条件下新对流单体的生成和发展机制不同,由于环境风场存在低层反向切变,此时密度流产生的水平涡度要大于环境风切变产生的水平涡度,密度流向外扩展较快,使两者处于"亚最优切变态",此时多单体雷暴系统中的新对流单体不再是从密度流前沿的阵风锋前上升气流分离出,而是由发展旺盛的主体对流体在其前部中低层激发的垂直上升运动引起。新生成的对流单体因其前部低层湿空气的流入得到快速发展,而其西侧原有的主体对流体因湿空气的流入被切断而开始减弱、消亡并最终被新对流单体所替代。这种新机制的出现与低层环境风反向切变的强度及地表风速有关。在一定的低层反向风切变条件下,这种新机制可与"最优切变态"机制交替作用:当密度流较强时,多单体雷暴系统中新对流单体的生成主要是以"亚最优切变态"机制为主,且新生成的对流单体强度也较弱;当密度流较弱时,多单体雷暴系统中新对流单体的生成主要以"最优切变态"机制为主,此时阵风锋前上升气流得到快速发展,当发展到一定程度时,新对流单体从阵风锋前上升气流中分离出。

Based on the simulations of a two-dimensional multicell storm under the condition of inverse shear（U_z0） in the lower levels by a mesoscale numerical model,a new mechanism for the cell regeneration and development within the multicell storm at the＂suboptimal shear＂state is proposed.In the presence of a low-level inverse shear,the circulation caused by the surface cold pool is not counteracted by that by the ambient wind shear,and the density current extends out faster,making the multicell storm stay at the＂sub-optimal shear＂state.As a result,a new cell is triggered by the strong vertical perturbation ahead of the mature convection,rather than the split-up from the updraft before the gust front at the leading edge of the surface cold pool.The latter is the mechanism at the＂optimal＂state proposed by Lin et al.（1998）.In the new mechanism suggested in this paper,the regenerated cell grows fast owing to the incident warm moist air from the upstream of the multicell storm,and tends to prevent the moist airflow from entering into the mature convection at its western side.Consequently,the mature convection would weaken,be replaced,and eventually decay.Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool.When the circulation of the cold pool is stronger than that of the wind shear,the multicell storm is at the＂suboptimal shear＂state,and the new convective cell is produced by the disturbance ahead of the mature cell.When the circulation of the cold pool is weaker,the cell regeneration is dominated by the mechanism at the＂optimal＂state with the new cell split from the gust front updraft.Therefore,these two mechanisms are not contradictive.With a moderate low-level inverse shear,they can alternately operate within a multicell storm.