A method for diagnosing the small-scale influences of the 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 static stability. If one or more measurements of the turbulent momentum flux exist at the coastal sea, number of parameters in the diagnostic can be reduced.

The jump in the surface roughness and the temperature contrast between the land and sea surface introduce horizontal variations in the flux field. The surface turbulent stress at some distance from the coast exhibit an inverse square-root dependence on the temperature contrast, not only for off-shore but also for on-shore wind situations. The exchange of momentum can be substantially reduced far ahead of the coast, up to few-hundred kilometers in more stable atmosphere. Generally, it is found that the surface stress to the sea near the coast 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.