Extended Abstract (204K)2A.4
Effect of ocean waves on air-sea momentum fluxes and hurricane intensity
Isaac Ginis, University of Rhode Island, Narragansett, RI; and I. J. Moon, B. Thomas, and T. Hara
Recent observational, theoretical and experimental results demonstrate that air-sea momentum flux at high wind conditions strongly depends on the wave field and that the drag coefficient levels off or even decreases at very high winds. Yet bulk parameterizations of air-sea momentum fluxes used in most hurricane research and forecast models are based on extrapolation from field measurements in much weaker wind regimes, predicting monotonic increase of the exchange coefficients with wind speed.
We will present the results of a numerical study focusing on the effect of ocean waves in the hurricane-ocean interaction. In a series of idealized hurricane experiments using a coupled wave-wind model, waves to the right and front of the hurricane track become trapped as the hurricane translation speed becomes faster and waves are exposed to prolong forcing from the wind. As a result, higher, longer and more developed waves are formed to the right and front of the track and yield higher drag coefficients, while lower, shorter and younger waves to the rear and left yield lower drag coefficients. The drag coefficient shows the tendency of leveling-off and decrease at high winds and varies significantly depending on the relative position from the storm center.
Experiments with the GFDL/URI coupled hurricane-ocean model indicate important sensitivity of the hurricane intensity to the momentum flux parameterization at the sea surface. If the roughness length is set constant (capped) for wind speeds > 35 m/s, consistent with the results from our coupled wave-wind model, it leads to a substantial increase of maximum surface wind. The changes of the wind speed extend through the boundary layer. These results strongly suggest that the proper estimation of momentum flux in hurricane conditions can be only achieved by a coupled-wave-ocean model. They also suggest that the GFDL/URI model intensity predictions could be significantly affected by improved parameterization of the air-sea exchange at high wind speeds.
Session 2A, CBLAST II
Monday, 3 May 2004, 10:45 AM-12:00 PM, Le Jardin Room
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