Mature Structure and Motion of a Model Tropical Cyclone with Latent Heating by the Resolvable Scales

Abstract

The mature structure of a model tropical cyclone is presented with emphasis on convection in the eyewall and spiral rainbands. Representative patterns of rainband activity occur during the last two days of a 13-day experiment and are shown for 288 h or day 12. These model rainbands seem to be forced by a pair of quasi-stationary spiral bands of upward motion that appear in the low troposphere and boundary layer ahead of and behind the westward moving vortex. These forcing bands, in turn, may be the result of a nonlinear interaction of azimuthal wavenumber one with itself. The convective band elements ahead of the vortex do not reach the outflow layer, but are capped by a mesoscale subsidence band in the upper troposphere. Elements behind the vortex, however, reach the outflow layer and have greater precipitation intensity. During the last two days of the experiment, three times as much rain falls behind the vortex and outside of the inner core as falls in front of the vortex. The mesoscale subsidence band, which caps the convection in bands in front of the vortex, is the direct result of a characteristic asymmetry of the 250 mb layer. This asymmetry forms between simulation hours 60?84 and results from a westerly shear of the initial environment current and the asymmetric distribution of convection in bands outside of the inner core of the vortex. The model vortex structure, in cross-section format, is compared with recent aircraft observations of strong hurricanes, particularly Hurricane Allen on 5 August 1980. The model data represent a time and space average with respect to the real data. When this averaging is taken into account, reasonable agreement is found between the model and Allen. The model updraft maximum occurs inside of the radius of maximum wind and the precipitation maximum is located outside of the maximum wind at low levels, in agreement with the real data. The time-average motion of the vortex is compared with the mass-weighted mean environment current at 990 km radius. For periods of 4 days or more, the vortex speed agrees with the current speed and the vortex path is mostly to the right of the environment current. For 10 h averages, vortex speed deviations ?12% occur and show effects of vortex environment current interactions.