A Method for Determining the Small-Scale Variations of the Surface Turbulent Momentum Flux Seaward of the Coast

Abstract

Small-scale variability of the Reynolds wind stress on the surface of the coastal sea, in conditions for which the land is warmer than the sea, is evaluated by idealized numerical simulations. A method for diagnosing the small-scale influences of a coast on the turbulent flux of momentum at the sea surface is proposed. The parameters are defined on the basis of a high-resolution numerical model. This diagnostic method can be used to resolve the surface turbulent momentum flux variations in large-scale models or in the areas with sparse observational coverage. The input data needed are background wind speed and direction, surface temperature contrast at the coastline, and background static stability. The temperature contrast between the land and sea surface introduces horizontal variations in the flux field. The surface turbulent stress at some distance from the coast exhibits an inverse square root dependence on the temperature contrast, not only for offshore but also for onshore wind situations. The turbulent exchange of momentum can be substantially reduced far ahead of the coast, up to a few hundred kilometers in a more stable atmosphere. In general, it is found that the surface stress to the sea near the coast, in a stable marine boundary layer, is almost always smaller than at open sea. The background static stability in general reduces the magnitude of vertical turbulent mixing. Its effect can be introduced in the diagnosis with good confidence. Also the velocity of the cross-coast wind component in the developed sea breeze can be successfully scaled by the atmospheric background stability.