Entropy ventilation in an axisymmetric tropical cyclone model

It has been hypothesized that ventilation of the higher entropy inner core of a tropical cyclone by low entropy environmental air provides a means by which environmental vertical wind shear is able to inhibit a tropical cyclone from intensifying. We study this hypothesis by using a newly developed finite volume, simplified entropy, axisymmetric tropical cyclone model. Parameterized eddy fluxes of entropy are used to represent ventilation by shear induced asymmetries near the radius of maximum winds.

The tropical cyclone's maximum winds weaken in response to the parameterized ventilation, but the degree of weakening is sensitive to the characteristics of the entropy mixing, including the location and amplitude of the mixing. The greatest sensitivity occurs at mid-levels where the entropy deficit between the inner core and the environment is the largest. In contrast, ventilation at upper levels does little to affect the intensity.

The decrease in intensity can be explained through both dynamical and energetic reasoning. Dynamically, the decrease in the radial entropy gradient in the free troposphere must imply a decrease in the tangential wind speed at the top of the boundary layer as shown by a modified thermal wind relation. Energetically, the inner core entropy budget of an unperturbed storm shows there is an approximate balance between surface fluxes, dissipation, radiation, and export. Adding ventilation results in an additional sink that results in a less efficient heat engine.