Thursday, 13 February 2003
Studying MPI with numerical models
Theoretical investigation of maximum possible hurricane intensity were started by the efforts of Kleinschmidt (1951) and Malkus and Riehl (1960). Further refinements in what is commonly referred to as maximum potential intensity (MPI) theory have occurred to the present day by various researchers; each potential improvement in theory leading to improved understanding on the factors that influence or are influenced by hurricane intensity. Since none of the extant theories rely on the physics of vortex asymmetries beyond possible parameterization of their net impact on the axisymmetric vortex, numerical simulation of the hurricane in an axisymmetric framework can prove fruitful to test the sensitivities of these MPI theories. Comparison of the theory of Emanuel (1995) with simulations using the Rotunno and Emanuel (1987) model have exposed physics of the hurricane that permit MPI to be exceeded. The incorporation of a second source of heat for the eyewall, namely the large latent heats of the low-level eye, basically confirms the Carnot engine picture of the hurricane, but it also raises other issues. Of particular interest is the role of azimuthal eddies on the inner edge of the eyewall that appear to be the primary advectors of latent heat to the eyewall. These azimuthal eddies exist at the breakdown of an azimuthal vortex sheet. An analogous situation that certainly exists in real hurricanes is the existence of vertical eddies at the inner edge of the eyewall due to the existence of barotropic instability, an instability that cannot be realized in the model due to the constraints of axisymmetry. Further investigation with the axisymmetric model will continue to prove useful, but the study of MPI is soon at the point where three-dimensional considerations cannot be long ignored.