AIR-SEA EXCHANGE OF WATER-VAPOR AND SENSIBLE HEAT - THE HUMIDITY EXCHANGE OVER THE SEA (HEXOS) RESULTS

Surface layer fluxes of sensible heat and water vapor were measured fr om a fixed platform in the North Sea during the Humidity Exchange over the Sea (HEXOS) Main Experiment (HEXMAX). Eddy wind stress and other relevant atmospheric and oceanic parameters were measured simultaneous ly and are used to interpret the heat and water vapor flux results. On e of the main goals of the HEXOS program was to find accurate empirica l heat and water vapor flux parameterization formulas for high wind co nditions over the sea. It had been postulated that breaking waves and sea spray, which dominate the air-sea interface at high wind speeds, w ould significantly affect the air-sea heat and water vapor exchange fo r wind speeds above 15 m/s. Water vapor flux has been measured at wind speeds up to 18 m/s, sufficient to test these predictions, and sensib le heat flux was measured at wind speeds up to 23 m/s. Within experime ntal error, the HEXMAX data do not show significant variation of the f lux exchange coefficients with wind speed, indicating that modificatio n of the models is needed. Roughness lengths for heat and water vapor derived from these direct flux measurements are slightly lower in valu e but closely parallel the decreasing trend with increasing wind speed predicted by the surface renewal model of Liu et QI. [1979], created for lower wind speed regimes, which does not include effects of wave b reaking. This suggests that either wave breaking does not significantl y affect the surface layer fluxes for the wind speed range in the HEXM AX data, or that a compensating negative feedback process is at work i n the lower atmosphere. The implication of the feedback hypothesis is that the moisture gained in the lower atmosphere from evaporation of s ea spray over rough seas may be largely offset by decreased vapor flux from the air-sea interface.