The impact of spume droplets and wave stress parameterizations on simulated near-surface maritime wind and temperature

Abstract

The influence of ocean gravity waves on the wind and temperature above the surface is investigated using a one-dimensional boundary layer model. The effect of the wave-induced stress is evaluated using three parameterizations: wave age (WaAg), wave steepness (WaSt), and wind action on the wave spectrum (WiAc). It is found that while the WaAg is more effective in reducing the wind for young waves, in the WaSt approach the maximumreduction is for old waves. Onthe other hand, the WiAc is very sensitive to the energy present in high frequencies corresponding to periods less than 2 s, which are found in both young and mature spectra. Since observations show that most of the wave stress is due to the small-period wave energy, in this aspect the WaSt parameterization is not recommended; WaAg is not as accurate; and thus WiAc is the best among the three, although its computational cost is the highest. The droplet load contribution to the total surface stress can be neglected for the droplet spectrum produced by 10-m wind speeds up to 15 m s-1, but its importance increases with the speed and its magnitude becomes about 1/5 of the total stress for wind speed ~30 m s-1. Concerning the latent and sensible heat fluxes accompanying the production of spume droplets by waves, a feasible microphysical formulation for operational use in weather forecasting models is proposed. The droplet spectrum is assumed to be a product of two functions, one depending on the windsea Reynolds number and the other on the droplet radius spectrum. The bulk effect of the latter is analytically evaluated and stored in a table as a function of air temperature Ta, relative humidity R%, and significant wave height Hs. In numerical experiments with initial sea surface temperature 5 K higher than the air surface, latent and sensible heat contributions to the air temperature are computed as a function of the wave spectra. The launched droplet spectrum (which increases the air temperature due to sensible heat) and the relative humidity (which controls the cooling due to the droplet evaporation) define the heat budget and the air temperature evolution. Although in these experiments the sea temperature is much higher than the air temperature, the results show a noticeable dominance of the evaporative cooling in the lower atmosphere mainly for smaller significant wave height. Some air warming is noticeable only from a threshold around Hs ≥ 5 m. © 2010 American Meteorological Society.