WISHE and tropical cyclone intensification

Recent research has shown that the wind-dependence of the surface enthalpy flux may not be necessary for the intensification of tropical cyclones. Specifically, when the wind speed that is used to calculate the surface enthalpy flux in numerical simulations is capped at a specified value, the simulated tropical cyclones still intensify, albeit at smaller rates and to a smaller equilibrium intensity. Here we develop a simple modification of existing potential intensity theory that predicts that the maximum attained wind speed should vary as the two-thirds power of the nominal potential intensity multiplied by the one-third power of the capping wind speed used in the surface enthalpy flux formulation.

When applied to a variety of numerical simulations using both axisymmetric and three-dimensional models, the simple theory usually underpredicts the maximum sustained wind speed. We show that this underprediction is related both to super-gradient wind effects (in simulations with relatively small horizontal turbulent mixing) and to a boost in storm intensity that results from downdrafts just outside the core and/or CAPE in the ambient environment.

Finally, by axisymmeterizing the surface enthalpy fluxes in three-dimensional simulations, we show that vortical hot towers are to some extent powered by the WISHE mechanism, and that they are not necessary for the intensification of tropical cyclones. We also demonstrate that capping one variable in an instability mechanism that relies on two or more variables merely leads to a reduction of the growth rate and amplitude of the perturbations, not to their elimination.