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Colloquium

Department of Atmospheric Science

Tues., Apr. 12, 3:10 pm, EN6085

Simulating Tropical Cyclones: Impacts of Air-Sea Flux Parameterizations, and Some Thoughts on Large-Eddy Simulations

Benjamin Green

NOAA/CIRES

Abstract

Tropical cyclones (TCs) are fueled by large fluxes of sensible and latent heat from the air-sea interface. These fluxes (as well as momentum fluxes) cannot be explicitly resolved by numerical weather prediction (NWP) models and therefore must be parameterized. Unfortunately, there is a great deal of uncertainty as to the behavior of air-sea surface fluxes, especially under strong (hurricane-force) winds. Thus, the numerous NWP parameterizations of surface fluxes introduce model error into TC forecasts, which limits the accuracy of predictions of TC intensity. In this talk, the sensitivity of WRF-ARW simulated TCs to parameterizations of the surface exchange coefficients for drag (Cd) and moist enthalpy (Ck) is examined. In agreement with theory, increased Ck yields a stronger TC both in terms of minimum central pressure and maximum 10-m wind speed. The impacts of Cd are not as straightforward: increased drag does reduce the maximum 10-m wind speed (in agreement with theory), but also deepens the minimum central pressure (opposite of what is predicted by theory) – in other words, Cd changes the pressure-wind relationship of simulated TCs. Cd also profoundly impacts TC structure, such that increased drag yields a more compact primary circulation.

The last part of this talk will present results both from a large-eddy simulation (LES) of a highly idealized TC-like boundary layer, and from a “full physics” large-eddy permitting simulation of Hurricane Katrina (2005). Each of these two approaches represents an intermediate step towards the longer-term goal of replacing parameterized boundary layer turbulence with LES in full-physics TC models.

 


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