Physically-based model of sea state dependency in surface turbulent fluxes

Improvements to a physically based model for surface stress and surface roughness are described. The new insights in this model are related the formulation of the dependence of surface stress on characteristics of surface water waves. Traditionally, if wave characteristics have been considered, they have been applied in the characterization of surface roughness. In the new model, the wave characteristics are considered in two places, the most important of which is a lower boundary condition on wind velocity, replacing the assumption of zero wind speed relative to the current. Velocity, rather than speed, is used in a manner that allows wave directional characteristics (relative to the wind direction) to be considered in the flux model. The second wave-related modification is in the displacement height (a vertical offset of the log-wind profile), which results a improvement in accuracy for high seas. The value and function form of the displacement height follows directly from the first wave consideration: it is not an additional tuning parameter. Both of these considerations have a great influence on the value of Charnock’s constant. One of the great simplifying results of this approach is a value of Charnock’s constant that is independent of wave characteristics (and hence applicable over a very wide range of conditions). Comparisons to observations are used to validate the quality of the modeled surface stress.

The above-mentioned model improvements are related to surface stress, and indirectly related (via friction velocity) to the transfer of moisture and energy (sensible and latent heat fluxes). Another improvement to the model is the use of heat and moisture fluxes parameterized in terms of surface renewal theory. This surface renewal parameterization has an atypically large dependence on the roughness length associated with momentum, creating an additional link between the stress model and fluxes of heat and moisture. Preliminary results indicate substantially reduced difference from observations in modeled energy and moisture fluxes. Of particular interest, is refining the modeled physics, and hence the accuracy of the modeled surface turbulent fluxes, for severe wind and wave conditions.