Microphysical influences on hurricane track and intensity in idealized simulations

In September, 2005, Hurricane Rita made landfall on the Texas/Louisiana border after having provoked an evacuation of Houston and Galveston Island. The evacuation occurred because the National Hurricane Center's (NHC) 54 hour forecast predicted a landfall just west of Galveston Bay. In real-time simulations of this event, we noted a very substantial impact from cloud microphysical assumptions on hurricane tracks simulated by the WRF model. Indeed, the ensemble spread from an experiment varying only microphysics and cumulus parameterizations in a single model with a common initial condition was comparable to that obtained from NHC's own ensemble which comprises a variety of models of different types and degrees of sophistication. That experiment employed 30 km horizontal grid spacing; similar results were obtained in much higher resolution runs varying only microphysics.

Hurricanes often form and propagate through complex and dynamic environment, complicating analysis of the microphysical impacts on simulated track and intensity. To isolate these influences, we created "Waterworld" (WW), a modified real-data version of WRF which retains Earth's rotation and (optionally) curvature, but has no land, a uniform SST of 29C and a calm, horizontally homogeneous base state based on Jordan's (1958) hurricane season composite. In WW, hurricane-like vortices spin up in response to localized instability added to the initial condition. Once established, the storms evince significant differences with respect to size, strength, track and propagation speed depending on microphysics assumptions. The goal of this talk is to reveal why microphysics can have such a dramatic influence on hurricane motion.

Most of the WW storms are very realistic with respect to size and strength. One set of admittedly unrealistic microphysical assumptions, however, has resulted in the production of extremely intense vortices, with central sea-level pressures as low as 782 mb being reached. These apparent 'hypercanes' are revealing some insights into how hurricanes can exceed their theoretical maximum intensity.