Movable CISK

Abstract

The analysis of Conditional Instability of the Second Kind (CISK) for a resting atmosphere leads to a maximum growth rate at zero width of the rain area. However, if the process takes place in a uniform current moving relative to the ground with a velocity ?, and the width of the rain area is 2a, an Ekman boundary layer cannot be established in the rain area when 2a<?/f, where f is the Coriolis parameter. It is shown that the incomplete development of the frictional boundary layer in this area leads to a maximum growth rate at a ≈300 km, and this seems to be a reasonable value for the half-width of a cloud cluster in a tropical depression or of the Intertropical Convergence Zone itself. The efolding time is of the order of half the spin- up time f?1E?½ where E is the Ekman number, i.e., about 2 days at 10°.