A note on the cisk mechanism in the tropics and its role in disturbance formation and maintenance

Abstract

The concept of CISK, or "conditional instability of the second kind" has been advanced to explain the growth of cumulus systems and the intensification of tropical disturbances to storm or hurricane intensity. CISK has also been postulated as a basis for parameterization of cumulus processes in large-scale models. Its investigation played a role in the design and planned analyses of the 1974 GATE experiment. The heart of the CISK concept involves frictionally driven convergence in an Ekman boundary layer, which is supposed to intensify convection, which in turn enhances the boundary layer convergence. Many CISK models equate the Ekman friction layer with the layer below cumulus base in the tropics. This paper corrects this error by showing that the frictional boundary layer in the tropics extends to cumulus tops. Several different types of scale interactions in the tropics are examined. Specific suggestions for broadening the CISK concept are made relative both to tropical storm deepening and to the parameterization of cumulus processes.