Bulk Exchange Coefficient for Humidity at Sea during Unstable and Neutral Conditions: A Re-evaluation based on new Field Data

Analysis of the neutral exchange coefficient for latent heat, C_EN, based on measurements from the Östergarnsholms site in the Baltic Sea has been made.

C_EN was found to be constant with wind speed during periods with an air-sea temperature difference, ΔT, larger than 3 K. However, for smaller ΔT it was demonstrated that C_EN increased dramatically with wind speed.

Through calculations with the spray-flux model of Andreas (2004) it was shown that spray-effects on the turbulent fluxes are relatively small. It was concluded that the increase of the exchange coefficients is due to dynamic effects.

With the aid of spectral analysis it was demonstrated that there is a link between the strong increase of the exchange coefficients and the turbulence structure. During high winds and small ΔT, surface layer scale eddies gain importance on the turbulent transport of latent and sensible heat, at the expense of the larger scale eddies.

The increase in C_EN was found to be closely connected to a similar increase in the roughness length for humidity, z_0q. It was demonstrated that this value deviated strongly from the z_0q values calculated with surface renewal theory during high wind speeds and small ΔT. This result was taken as an indication of the failure of surface renewal theory for wind speeds above 10 ms^-1. It is suggested that this is a result from the increasing importance of wave breaking with a possible link to changes in the turbulence structure.

References

Andreas, E. L, 2004: A bulk air-sea flux algorithm for high-wind, spray conditions, Version 2.0. Preprints, 13th Conf. on Interactions of the Sea and Atmosphere, Portland, ME, 9-13 August 2004, Amer. Meteor. Soc., CD-ROM P1.5, 8 pp.