Minimal conceptual models for tropical cyclone intensification

Abstract

We examine a hierarchy of minimal conceptual models for tropical cyclone intensification. These models are framed mostly in terms of axisymmetric balance dynamics. In the first set of models, the heating rate is prescribed in such a way to mimic a deep overturning circulation with convergence in the lower troposphere and divergence in the upper troposphere, characteristic of a region of deep moist convection. In the second set, the heating rate is related explicitly to the latent heat release of ascending air parcels. The release of latent heat markedly reduces the local static stability of ascending air, raising two possibilities in the balance framework. The first possibility is that the effective static stability and the related discriminant in the Eliassen equation for the overturning circulation in saturated air, although small, remains positive so the Eliassen equation is globally elliptic. The second possibility, the more likely one during vortex intensification, is that the effective static stability in saturated air is negative and the Eliassen equation becomes locally hyperbolic. These models help to understand the differences between the early Ooyama models of 1968 and 1969, the Emanuel, 1989 model, and the later Emanuel models of 1995, 1997 and 2012. They provide insight also into the popular explanation of the WISHE feedback mechanism for tropical cyclone intensification. Some implications for recent work are discussed.