Insights on hurricane intensity

Abstract

The motivation of this thesis is to obtain greater understanding of the controls on hurricane intensity. The results from three different hurricane models are presented: I) a Geophysical Fluid Dynamics Laboratory Hurricane Prediction System simulation of Hurricane Opal (1995) using a reality-based, three-dimensional representation of the atmosphere; 2) the Emanuel (1995a) highly-simplified, axisymmetric hurricane model; and 3) the Rotunno and Emanuel (1987) axisymmetric, cloud-resolving, nonhydrostatic, grid model.

From the Opal simulation, we conclude that the popularly identified features of the environment of the storm (ocean eddies, trough-interaction. jet entrance regions) are not able to explain the modulations of intensification found in the simulation; although vertical shear is a strong candidate for the eventual weakening of the storm. This points to a thermodynamic interpretation of intensification. The Emanuel (1995a) model was designed to test a hypothesis for largely thermodynamic development of a hurricane. While confirming steady state intensity predictions of the Emanuel (1995b) maximum potential intensity theory (E-MPI) to within 5 meters per second, sensitivities of the model to purely numerical parameters argue against use the of this model as an operational forecast model.

With the Rotunno and Emanuel (1987) model, E-MPI can be greatly exceeded. Since this model should be faithful to many of the approximations of E-MPI theory, the remaining assumptions of E-MPI theory can be considered in turn. The model produces a significant flux of heat from the eye to the eyewall. allowing the modeled eyewall to ascend against a slight static stability, and this behavior is the key violation of E-MPI theory. The stabilization is a measure of the transfer of heat to the eyewall, and by accounting for this second source of heat, an ad hoc modification of E-MPI can explain most of the “superintensity” of the modeled storm.

We provide evidence that this active interaction between the eye and eyewall may operate in real hurricanes from published observations and in more-realistic, three-dimensional simulations of hurricanes. We propose that a new MPI should be developed to include this influence on hurricane intensity.