| The USA Office of Naval Research (ONR) in collaboration with the National Oceanic and Atmospheric Administration's Hurricane Research Division (NOAA/HRD) is conducting a 5-year Coupled Boundary Layer Air-Sea Transfer (CBLAST) experiment designed to improve the accuracy of typhoon and hurricane intensity prediction through an improved understanding of the air-sea flux processes at high winds. The specific objective of this experiment is to develop a new surface wave-dependent flux parameterization for the high wind hurricane boundary layer containing secondary (roll-vortex) circulations over fetch limited seas in the presence of sea spray and one or more swell components from an airborne platform. We will test the following hypotheses: 1) that surface momentum exchange coefficients increase with wind speed for moderate winds (>30 m/s), are enhanced by fetch-limited waves or opposing swell, but level off or decrease above a high wind threshold (>45 m/s), especially in quadrants where swell has a significant downwind component, 2) that compensating mechanisms for enhanced surface air-sea enthalpy fluxes over and above current parameterizations must exist for storm maintenance and growth above some high-wind speed threshold, and3) that candidate mechanisms are separable and can be estimated, such as a) enhanced turbulent fluxes due to wave interactions, b) spray evaporation and c) secondary flow circulations (roll-vortex type).To begin the process of examining these hypothesis, instrument testing and calibration as well as aircraft flight plan experimentation were conducted in 2002. Observations on both sides of the air-sea interface were made concurrent with surface wave spectra. Intensive experimental work is planned for the Atlantic hurricane seasons of 2003 and 2004. The airborne in-situ and remote sensing instrumentation suite used for this purpose is briefly described along with the planned array of air-deployed drifting buoys and subsurface floats. Recent tropical cyclone flux estimates are described, and new studies on the variation of air-sea exchange coefficients with wind speed are summarized. |
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