DYNAMICAL INTERACTIONS BETWEEN LARGE CONVECTIVE CLOUDS AND ENVIRONMENT WITH VERTICAL SHEAR

Abstract

The physics of convective clouds depends not only upon thermodynamic processes but also upon the interactions with the environmental wind field. When vertical shear is present, a hydrodynamic (nonhydrostatic) pressure field is induced by relative motions near the boundaries of a large convective system which does not move with the ambient winds. This tends to make the cloud tilt away from the vertical, but at the same time vertical gradients of hydrodynamic pressure aid the formation of new convection. Expressions are derived for the accelerations concerned. Quantitative estimates based upon experimental analogies show that vertical accelerations due to the induced pressure field may be of the same order as those associated with ordinary buoyancy forces. These account for the self-propagating nature of squall-line-type thunderstorms and their ability to continue at night when thunderstorms in stagnant air masses tend to die out. The hypothesis accounts for the observed movements of this type of rainstorm, somewhat toward the right of the winds in the convective layer, and is consistent with the preference of severe phenomena, such as tornadoes, for a particular flank of a rain-storm. Distinction is made between large and small clouds as affected by shear.