Abstract. Turbulent fluxes have been measured in the atmospheric surface layer from a boom extending upwind from the Dutch offshore research platform Meetpost Noordwijk (MPN) during HEXMAX (Humidity Exchange over the Sea Main Experiment) in October-November, 1986. We started out to study eddy flux of water vapour, but discrepancies among simultaneous measurements made with three different anemometers led us to develop methods to correct eddy correlation measurements of wind stress for flow distortion by nearby objects. We then found excellent agreement among the corrected wind stress data sets from the three anemometers on the MPN boom and with eddy correlation measurements from a mast on a tripod. Inertial-dissipation techniques gave reliable estimates of wind stress from turbulence spectra, both at MPN and at a nearby ship. The data cover a range of wave ages and the results yield new insights into the variation of sea surface wind stress with sea state; two alternative formulas are given for the nondimensional surface roughness as a function of wave age.
Abstract. Surface layer fluxes of sensible heat and water vapor were measured from 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 simultaneously and are used to interpret the heat and water vapor flux results. One of the main goals of the HEXOS program was to find accurate empirical heat and water vapor flux parameterization formulas for high wind conditions over the sea. It had been postulated that breaking waves and sea spray, which dominate the air-sea interface at high wind speeds, would significantly affect the air-sea heat and water vapor exchange for 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 sensible heat flux was measured at wind speeds up to 23 m/s. Within experimental error, the HEXMAX data do not show significant variation of the flux exchange coefficients with wind speed, indicating that modification of the models is needed. Roughness lengths for heat and water vapor derived from these direct flux measurements are slightly lower in value but closely parallel the decreasing trend with increasing wind speed predicted by the surface renewal model of Liu et al. [ 1979], created for lower wind speed regimes, which does not include effects of wave breaking. This suggests that either wave breaking does not significantly affect the surface layer fluxes for the wind speed range in the HEXMAX data, or that a compensating negative feedback process is at work in the lower atmosphere. The implication of the feedback hypothesis is that the moisture gained in the lower atmosphere from evaporation of sea spray over rough seas may be largely offset by decreased vapor flux from the air-sea interface.
The paper presents an analysis, performed at the Royal Netherlands Meteorological Institute (KNMI), of data obtained at a research platform 9 km off the Dutch coast within the framework of the air–sea gas exchange program ASGAMAGE†. The air–sea transfer velocity of CO2 was determined directly, that is, by observing CO2 fluxes and air–sea concentration differences simultaneously. CO2 fluxes were determined by means of the eddy correlation technique. Special care was taken to avoid the effects water vapour on the CO2 flux measurements. The air and water near the air–sea interface were treated as well‐mixed with respect to CO2. The combination of flux and concentration data allowed the computation of the transfer velocity for CO2 without recourse to other gases. Results for two observation periods, one with downward CO2 fluxes (May) and one with upward CO2 fluxes (October), are consistent. A relation with UN,10, the wind speed adjusted to a height of 10 m and neutral stratification, was determined for the pooled data from the two experimental phases. The relation found was: k660 = 0.54UN,102 cm h−1, with k660 the CO2 transfer velocity normalized to salt water (35‰) at a temperature of 20 °C, and UN,10 in m s−1. The 95% confidence interval of the coefficient extends from 0.46 to 0.63. No relations with other geophysical parameters could be found from the present data set.
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