Reevaluation of the Bulk Exchange Coefficient for Humidity at Sea during Unstable and Neutral Conditions

Abstract

Processes influencing the air?sea exchange of humidity during unstable and neutral stratification were studied using tower measurements from the island of Östergarnsholm in the Baltic Sea. For small air?sea temperature differences, the neutral exchange coefficient for humidity CEN was found to increase with increasing wind speed, attaining a value of approximately 1.8 ? 10?3 at 13 m s?1. The high CEN values were observed during situations when the characteristics of the turbulence structure differed from what would be expected from traditional theory. Results from spectral analysis point to a situation in which the vertical transport of humidity is dominated by smaller-scale eddies. Quadrant analysis showed that these eddies enhance the humidity flux by bringing down drier air from layers aloft. These findings are consistent with recent analyses of the neutral boundary layer in which a change of turbulence regime has been observed. The conclusion is made that this dynamic effect accounts for the observed increase in CEN. Here, CEN was calculated using a wave-dependent normalized wind gradient, which had the effect of reducing the value by about 10% during swell relative to calculations using a non-wave-dependent normalized wind gradient.