Hurricane Superintensity

Abstract

High spatial and temporal resolution simulations using the Rotunno and Emanuel axisymmetric, cloud-resolving, hurricane model are found to greatly exceed Emanuel?s energetically based upper bound for maximum potential intensity (E-MPI). Using a control simulation similar to that of Rotunno and Emanuel with a sea surface temperature (SST) of 26.13°C, the E-MPI is exceeded after 15 simulation days, after the warming of the eye is able to extend down to the ocean surface. At still higher resolution, the modeled storm greatly exceeds E-MPI more quickly, during initial spinup, and the resulting intensity for the standard numerical and microphysical parameters is found to converge with, respectively, radial and vertical grid spacing of 3.75 km and 312.5 m with maximum tangential winds (Vmax) of ≈90 m s?1. This is notably greater than the energetically based upper bound of Vmax = 55 m s?1. This ?superintensity? occurs only in the presence of an enhancement of low-level eye entropy. The high-entropy air is entrained into the eyewall primarily by a breakdown of an azimuthal vortex sheet at the inner edge of the eyewall. Among the many underlying assumptions of E-MPI, only the violation of the related assumptions that the eyewall is neutral to moist ascent and that no entropy is fluxed from the eye to the eyewall can explain the degree of superintensity observed; other assumptions may be individually violated but their impacts on the intensity estimates are much smaller. The impact of the entrainment of heat from the eye to the eyewall on E-MPI theory is estimated through an ad hoc increase in the effective SST as a way of accounting for a second source of heat. This procedure produces a close estimate of the modeled intensity, but the problem is not closed since the degree of eyewall heating is not known a priori. Published observations and recent three-dimensional, cloud-resolving modeling studies are reviewed that appear to present various aspects of the observed entropy structure and the eye?eyewall interaction of the superintensity mechanism.