Abstract
A precipitating convective cloud is simulated in a two-dimensional model by numerical integration of the hydrodynamic and thermodynamic equations. Cloud physical processes included in the model are condensation, evaporation, coagulation, breaking, and the terminal fall velocity of water drops relative to the air. The size distribution of water drops is described in terms of seven discrete radii at every grid point and at every time step. Cloud and rainwater contents, rainfall intensity and radar reflectivity are computed directly from the size distributions.
The numerical experiments show that precipitating convective clouds may be grouped into three types according to the way the cloud develops. 1) If the atmosphere has sufficient conditional instability and if vertical shear in the ambient wind field is weak, new convective clouds form on both sides of the initial cloud mainly due to the diverging cold downdraft. 2) When the direction of the vertical wind shear is constant with height and the shear is strong, the cloud is inclined considerably downshear and a downdraft forms on the downshear side of the cloud. The cloud is “short-lived,” dissipating in about an hour from its initiation without the formation of any new cloud. 3) When the vertical wind shear changes direction at a certain level the cloud may be erect. The patterns of updraft and of rainwater content are inclined in the direction of the lower level shear and a downdraft forms on this side of the cloud. In this system the supply and release of convective energy are organized. The cloud can then reach a steady state, and so become “long-lasting.” There is a critical range of height within which the shear must change direction for the formation of a “long-lasting” cloud. It is inferred that this range is dependent upon the thermal stability of the atmosphere and upon cloud physical processes.
