Effects of Environmental Flow upon Tropical Cyclone Structure

Numerical simulations of tropical-cyclone-like vortices are performed to analyze the effects of unidirectional vertical wind shear and translational flow upon the organization of convection within a hurricane’s core region and upon the intensity of the storm. A series of dry and moist simulations is performed using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model version 5 (MM5) with idealized initial conditions. The dry simulations are designed to determine the patterns of forced ascent that occur as the vortex responds to imposed vertical wind shear and translational flow, and the mechanisms that modulate the vertical velocity field are explored. The moist simulations are initialized with the same initial conditions as the dry runs but with a cumulus parameterization and explicit moisture scheme activated. The moist simulations are compared to the dry runs in order to test the hypothesis that the forced vertical circulation modes modulate the convection and hence latent heat release in the hurricane core, as well as to evaluate the net effect of the imposed environmental flow on the storm intensity and structure. The results indicate that the pattern of convection in the storm’s core is strongly influenced by vertical wind shear, and to comparable degree by boundary layer friction. In the early stages of moist simulations, typical of the tropical depression stage, the regions of forced ascent and the mechanisms that cause them are similar to those in the dry runs. However, once the moist storm runs deepen enough to develop saturation in part of the eyewall, the patterns of vertical motion and associated rainfall differ between the paired dry and moist runs with identical initial conditions. The dry runs tend to produce a strong, deep region of ascent in the sector of the storm that lies downshear right of the center. The moist runs begin similarly, but as the storms intensify they strongly favor upward motion and rainfall downshear left of the center. It appears that the vertical motion patterns in the dry and moist simulations are dominated by similar adiabatic lifting mechanisms prior to the development of partial eyewall saturation. Once the moist runs reach saturation, this adiabatic lifting mechanism no longer occurs due to the latent heat release within the ascending air. Hence, the patterns of forced ascent in the dry runs should be relevant for understanding patterns of convection in loosely organized systems such as tropical depressions, but not in mature hurricanes. The rainfall patterns produced by the moist simulations are in good agreement with recent observational analyses of the relationships between rainfall distribution and vertical wind shear in Atlantic hurricanes.