Air-sea enthalpy and momentum exchange at major hurricane wind speeds
Bell, Michael M.
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Air--sea exchanges of heat and momentum are important elements in understanding and skillfully predicting tropical cyclone intensity, but the magnitude of the corresponding wind-speed dependent bulk exchange coefficients is largely unknown at major hurricane wind speeds greater than 50 m s-1. Since direct turbulent flux measurements in these conditions are extremely difficult, the momentum and enthalpy fluxes were alternatively deduced via axisymmetric angular momentum and total energy budgets. A comprehensive error analysis was performed using both idealized numerical simulations to quantify and mitigate potentially significant uncertainties resulting from unresolved budget terms and observational errors. An analysis of six missions from the 2003 CBLAST field program in major hurricanes Fabian and Isabel was conducted using a new variational technique. This analysis indicates a near-surface mean drag coefficient (CD) of 2.4x10-3 with a 46% standard deviation and a mean enthalpy coefficient (CK) of 1.0x10-3 with a 40% standard deviation for wind speeds between 52 and 72 m s-1. These are the first known estimates of CK and the ratio of enthalpy to drag coefficient (CK/CD) in major hurricanes. The results suggest that there is no significant change in the magnitude of the bulk exchange coefficients estimated at minimal hurricane wind speeds, and the ratio CK/CD is likely less than 0.75 for wind speeds greater than 50 m s-1.
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