14A.3 Effects of resolved turbulence in a large eddy simulation of a hurricane

Thursday, 3 April 2014: 2:00 PM
Garden Ballroom (Town and Country Resort )
Richard Rotunno, NCAR, Boulder, CO; and G. H. Bryan

This study examines the effects of resolved turbulence in a large eddy simulation (LES) of an idealized hurricane (dx = dy = 62 m, dz = 31 m). Resolved turbulence is defined here as the model-produced fluctuations from the azimuthal-averaged (i.e., mesoscale) flow of the hurricane. The distribution of resolved turbulence kinetic energy (TKE) compares well with recent airborne-radar-based studies that show a maximum of TKE at the eye-eyewall interface, especially below 2 km ASL and just inward of the radius of maximum winds (RMW). Compared to the parameterized TKE in the model, the magnitude of resolved TKE is typically more than an order of magnitude larger except (as expected) very near the surface. The resolved TKE budget further shows that TKE advection and the pressure-interaction terms are not negligible, as surmised in recent observations-based studies that could not measure these terms.

Budgets of angular momentum (M) and moist entropy (S) are also analyzed and compared with recent modeling studies that rely entirely on parameterized turbulence. The LES output supports the previous studies in the sense that turbulence acts primarily to reduce the radial gradients of both M and S in the eyewall of the simulated hurricane. A curious aspect of this LES is that counter-gradient fluxes can exist in the radial direction in small regions, which is not considered in any mesoscale model turbulence parameterization to our knowledge. The strongest counter-gradient radial fluxes are located outward of the RMW where S contours bow downward in rain-driven downdrafts.

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