A Prognostic Axisymmetric Balance Model of Tropical Cyclone Intensification

This talk will revisit the evolution of an idealized tropical-cyclone-like vortex forced by a prescribed distribution of diabatic heating in the context of inviscid and frictional axisymmetric balance dynamics. Solutions will be presented for a range of heating distributions that are allowed to contract as the vortex contracts and intensifies. Interest is focussed on the kinematic structure and evolution of the secondary circulation in physical space and on the development of regions of symmetric and static instability. The solutions are prolonged beyond the onset of unstable regions by regularizing the Sawyer-Eliassen equation in these regions.

The heating induced low-level secondary circulation below the heating maximum has a local component of flow both inwards and upwards across the absolute angular momentum (M-) surfaces towards low values of M. Except in the frictional boundary layer, where M is not materially conserved, the advection of M towards low values of M is accompanied by a local spin up of the tangential wind speed. The intensification rate of the vortex is essentially constant up to the time when regions of instability ensue. This result is in contrast to the previous
suggestions that the rate should increase as the vortex intensifies because the heating becomes progressively more “efficient” when the local inertial stability increases.

The solutions provide a context for re-examining the axisymmetric balance formulation for tropical cyclone intensification in the light of the classical paradigm articulated by Ooyama as well as another widely-invoked intensification paradigm.