Large eddy simulations of the hurricane boundary layer

In order to provide better understanding of the structure of the turbulent flow in the boundary layers of hurricanes, a series of exploratory large eddy simulations have been conducted using idealized starting conditions based on the strong low-level winds found in mature tropical cyclone cores. The model is integrated for 8 h over a 25 km x 25 km domain 4 km deep, using 100 m and 50 m mesh spacings in the horizontal and vertical, respectively. Maps and statistics of the resulting quasi-steady flow regimes are obtained during the final 2 h. Initial wind profiles are assumed to contain zero shear, and are allowed to range over speeds from 30 to 50 m/s in selected experiments. Deep convective precipitating clouds are not represented in these unsaturated simulations, but lapse rates are set to small values, consistent with the effective small static stability of the moist atmosphere typical of tropical cyclone cores. Capping inversions are not present in these simulations, so that turbulence continues to develop upward slowly with time. The simulations are terminated before significant turbulence kinetic energy reaches the top of the model domain. To account for propagation of perturbation flow structures, the model domain is allowed to translate at a velocity approximately 75% of that of the initial imposed flow, which assures that any coherent circulations that develop are properly sampled within the simulation domain. Surface roughness lengths of 0.05 m are used to represent vegetated land terrain.

Flow statistics exhibit an asymptotic approach to stationarity after 6 h of simulation time. TKE profiles show a gradual decrease of amplitude above a strong peak in the lowest 1 km, and are similar in shape to those seen in recent Doppler profiler observations of spectral width in landfalling hurricanes. TKE amplitudes are much larger than those observed and simulated in any prior published boundary layer studies. Mature boundary layer structures seen in the simulations include intense longitudinal striations in the horizontal wind, aligned approximately with the mean flow in the lowest 500 m. Horizontal wind differences across these structures are almost half the amplitudes of the imposed initial windspeeds, in approximate agreement with recent high-resolution observations made by mobile Doppler radars. Characteristic vertical velocity within these structures is 3 m/s. Despite their simplicity, it appears that these LES simulations can provide much useful insight into the dynamics of the typical tropical cyclone core boundary layer flow.