Recent observational and numerical research suggests that the core structure and intensity of a hurricane are affected by large-scale flow in ways that should be predictable. This paper discusses results from a series of numerical simulations of hurricanes within idealized flow fields. The simulations are performed using MM5 (Penn State/NCAR Mesoscale Model, Version 5) with a fine-mesh nested grid and explicit representation of moist processes. The primary focus of the work is on examining the effects of different patterns of vertical shear, mean and storm-relative flow and boundary layer processes on the distribution of winds and rainfall in the core region.
The results indicate that both vertical shear and the effects of storm movement upon the boundary layer cause major asymmetries in the cores of hurricanes. Storms influenced by significance vertical shear tend to exhibit maximum rainfall and increased winds speeds on the downshear-left side of the eyewall. The storms are strongly influenced by even small amounts of vertical shear in the lower and middle troposphere. They are less sensitive to upper-level shear. The primary dynamical processes forcing the asymmetries are different from those that dominate the secondary circulations in dry model simulations of vortices in shear. Both the nature of the asymmetries and the intensity of the simulated storms depend upon the manner in which the moist processes in the eyewall region are parameterized.