Insights from Solutions of a Moist Prognostic Balance model for Tropical Cyclone Evolution

Solutions of a prognostic Eliassen balance model for tropical cyclone evolution that includes explicit latent heat release in ascending and descending air are explored. Starting with an axisymmetric vortex in an initially unsaturated but conditionally-unstable atmosphere, the frictionally-induced overturning circulation leads rapidly to condensation aloft and the moist Eliassen equation for the overturning circulation becomes hyperbolic in the convectively-unstable region. The model cannot be advanced forwards beyond this time as a balance model unless the Eliassen equation is suitably regularized to remove these hyperbolic regions. However, regularization removes convective instability, thereby altering the physics of the problem after regularization occurs. A key finding is that, despite the formation of elevated cloud and the accompanying latent heat release, the initial vortex only spins down. Since buoyant convection is suppressed by the need to regularize the moist Eliassen equation, there is no mechanism available to reverse the frictionally-induced outflow in the lower troposphere required to concentrate absolute angular momentum there. The implications of this finding are discussed in relation to several previous balance models proffered to understand the intensification of tropical cyclones.