Revisiting the physics of tropical cyclone intensification in three dimensions

We present numerical model experiments to investigate the dynamics of tropical-cyclone amplification and its predictability in three dimensions. For the prototype amplification problem beginning with a weak tropical storm-strength vortex deep convective towers growing in the rotation-rich environment of the incipient core amplify the local vertical rotation. These so-called ``vortical hot towers'' (VHTs) are the basic coherent structures of the intensification process, which itself is shown to

be intrinsically asymmetric and chaotic in nature even though the problem as posed is essentially axisymmetric. This lack of predictability is a reflection of the convective nature of the inner-core region and extends to the prediction of intensity itself.

In the experiments presented herein it is the progressive segregation, merger and axisymmetrization of these towers and the convergence they generate that is fundamental to the intensification process, but axisymmetrization is never complete. There is always a prominent low azimuthal wavenumber asymmetry (often wavenumber one or two) of the inner-core relative vorticity.

The wind-induced surface heat exchange (WISHE) feedback mechanism that has been proposed previously to explain tropical-cyclone intensification in an axisymmetric framework has been found to be much less of a constraint in three dimensions because the VHTs are able to extract locally sufficient surface moisture fluxes necessary for their growth without the wind-moisture feedback that typifies WISHE.