  A physical model is proposed for the prediction of the non-monotonic
variation of the drag coefficient, C_d, with wind speed. The model approximates
the effective C_d by the area-weighted averaging of the distinct drag
coefficients associated with the foam-free and foam totally-covered portions of
the sea surface, and identifies the roughness of the sea surface totally
covered by foam with the foam bubble size. Based on the available optical and
radiometric measurements of the foam fractional coverage and the foam bubble
size, the present model yields the effective C_d which is in fair agreement
with that obtained from field measurements of the vertical variation of mean
wind speed (Powell et al., 2003), which showed a reduction of C_d with wind
speed rising to hurricane conditions. The present approach opens new
opportunities for modeling of drag coefficient in hurricane conditions by using
radiometric measurements instead of direct wind speed measurements.
