Influence of Surface Sensible Heat Flux on Incipient Marine Cyclogenesis

Abstract

Incipient marine cyclogenesis is investigated as an instability process of a mean baroclinic state with a vertically nonuniform static stability under the influence of a self-induced surface sensible heat flux in the context of a quasigeostrophic model framework. The analytic solution of such a model is first presented. The surface sensible heat flux induces strongly unstable short waves in the meso-α range and an e-folding time of the order of one day under supposedly relevant parameter conditions. Those modes have a shallow vertical structure near the surface with a pronounced westward tilt with height. The latter facilitates a release of enough potential energy from the basic state to overcompensate the destruction of potential energy by the surface heat exchange itself. The diabatic destabilization by surface sensible heat flux is therefore an alternative mechanism for initiating small marine cyclones without upper-level forcing. A potential vorticity interpretation of this instability process is also given. A supplementary numerical analysis confirms that the meso-α unstable waves are robust because they exist when various combinations of low-level static stability and heating profiles are used. The dependence upon the heating profile is somewhat stronger, capable of inducing an additional set of unstable modes between the meso-α and synoptic-scale ranges when it increases substantially with height near the surface.