Air/sea momentum transfer and the microwave cross section of the sea

Measurements of atmospheric fluxes of heat, moisture, and momentum were made simultaneously and coincidentally with microwave backscatter measurements from an airship flown over the Pacific Ocean in 1993. The measurement technique was well suited to measure fluxes at very low wind speeds because the airship required an air speed near 10 m s−1 in order to maintain altitude. The measurements show that very low wind speeds are always associated with very low microwave cross sections and very high air/sea drag coefficients. The occurrence of regions of very low wind speed is not usually correlated with either the sea surface temperature or the air/sea temperature difference. Nevertheless, these regions can remain in place for time periods of several hours. The rate of increase of the microwave cross section at very low wind speeds agrees with that predicted by Donelan and Pierson [1987], but the absolute value of the threshold wind speed appears to be lower than their prediction. The high drag coefficient at low wind speeds is due to the fact that the friction velocity is nearly constant for wind speeds below 4–5 m s−1. Thus, at these wind speeds the increase of the microwave cross section follows the behavior of the wind speed rather than the wind stress. At higher wind speeds, however, the behavior is reversed with the cross section following the wind stress at a constant wind speed. We suggest that this behavior can be understood if momentum transfer across the air/sea interface is supported by both viscosity and the entire spectrum of waves on the surface, as many investigators have indicated.