Atmospheric boundary layer turbulence closure scheme for wind-following swell conditions

Abstract

ver the ocean, atmospheric boundary layer turbulence can be altered by underlying waves. Under swell conditions, the impact of waves on the atmosphere is more complicated compared that under wind-wave conditions. Based on Large-Eddy Simulation (LES), the wind-following swell impact on the atmospheric boundary layer is investigated through three terms: swellinduced surface momentum flux, the vertical profile of swell-induced momentum flux, and the swell impact on atmospheric mixing. The wave-induced surface momentum flux displays a decreasing trend with increasing atmospheric convection. The swell-induced momentum flux decays approximately exponentially with height. Compared with atmospheric convection, the decay coefficient is more sensitive to wave age. Atmospheric mixing is enhanced under swell conditions relative to a flat stationary surface. The swell impact on the atmospheric boundary layer is incorporated into a turbulence closure parametrization through the three terms. The modified turbulence closure parameterization is introduced into a single-column atmospheric model to simulate LES cases. Adding only the swell impact on the atmospheric mixing has a limited influence on wind profiles. Adding both the impact of swell on the atmospheric mixing and the profile of swell-induced momentum flux significantly improves the agreement between the 1D atmospheric simulation results and the LES results, to some extent simulating the wave-induced low-level wind jet. We conclude that the swell impact should be included in atmospheric numerical models.