On the preferred mode of Ekman-CISK

Abstract

The relationship between the vertical partitioning of the diabatic heating and the preferred mode of motion is explored for an Ekman-CISK model. First an unrealistic, but heuristically useful, sinusoidal heating distribution in the vertical is used to establish the link between the growth rate of unstable modes, the Rossby radius of deformation and the diabatic heating profile. The results of the analysis, augmented with those obtained with a more realistic heating distribution, indicate that a rapidly growing, tropical cyclone scale, preferred mode will result from a diabatic heating profile that has a maximum at a low-level. The vertical structure of this mode possesses some realistic features. In particular it has a large vertical gradient of relative vorticity compared with modes associated with mid-tropospheric diabatic heating maximae.