Air-sea fluxes in Hurricane Frances (2004) from dropsonde data and a coupled model

The importance of surface fluxes for tropical cyclone (TC) intensity is widely accepted by the scientific community. The determination of the surface fluxes under high-wind conditions is difficult due to the lack of accurate observations in high winds. The physical processes controlling the exchange coefficients of heat, moisture, and momentum fluxes are not well understood. Recent observations from the Coupled Boundary Layer Air-Sea Transfer (CBLAST) field program provided an excellent opportunity to evaluate coupled model simulations of air-sea fluxes in hurricanes. We calculated the surface heat fluxes using two different flux parameterizations using the Global Positioning System (GPS) dropsondes data from CBLAST and the sea surface temperature (SST) from the SSM/I and TMI Satellite data. The methods are based on budget analysis and the Monin-Obukhov stability theory Furthermore, the momentum and exchange coefficient of momentum flux are computed using these two methods and a third one which assumes that the mean wind profile is logarithmic. The fluxes from a fully coupled atmosphere-wave-ocean simulation of Hurricane Frances (2004) are compared with the air-sea fluxes based on the dropsonde data obtained during CBLAST 2004 field program from 30 August – 2 September 2004. The model used for this study is the coupled atmosphere-wave-ocean model developed at the University of Miami (Chen et al. 2005). The different dropsonde analysis show that the momentum exchange coefficient levels off above hurricane force winds, similar to that of the results of Powell et al. (2003) and Donelan et al. (2004), but are slightly greater than the ones found in both studies. The coupled model also shows that the drag coefficient is higher in the left-rear quadrant of the storm were the waves are younger and roughest. There is a strong asymmetry in the air-sea fluxes associated not only with the variability in the surface waves, but also ocean cooling in the wake of Hurricane Frances. More analysis is currently underway to include more dropsondes in this analysis and make a quadrant analysis to compare with the coupled model.