The Downdraft in Convective Shower-Cloud under the Vertical Wind Shear and its Significance for the Maintenance of Convective System

Abstract

The downdraft in Convective shower-cloud is simulated by numerical integration of hydrodynamic and thermodynamic equations, the fall of raindrops being included. The behaviors of downdraft in two cases are computed : in the first case, atmosphere is initially at rest and in the second case, horizontal air-flow with vertical shear initially exists.
The results of computation show that though upcurrents follow the preceding downdraft in both cases without and with wind shear, the upcurrent in the latter case differs from that in the former case in the following respects. In the case without wind shear, the upcurrents of weaker intensity take place in both sides of the cloud only as compensating current and gravitational oscillation. In the case with wind shear, stronger upcurrent is formed in front of the cloud. This upcurrent is forced by stronger convergence due to both diverging current originated from the downdraft and the larger horizontal momentum which the downdraft transports from upper. It is concluded from computations that vertical wind shear organizes the release of instability energy in convective system, though the conversion rate of potential energy into kinetic one in the whole region is less than that in the case without wind shear.