Integrated model for a wave boundary layer

TL;DR

This paper introduces a semi-phenomenological model to calculate the friction velocity in the wave boundary layer using wave spectrum and wind data, validated with measurements showing 15-20% error.

Contribution

A new integrated model for the wave boundary layer that combines spectrum-based and friction velocity calculations, with validation against experimental data.

Findings

Mean relative error for drag coefficient is 15-20%.

Model effectively links wave spectrum to momentum flux.

Verification performed using simultaneous wave spectrum and friction velocity measurements.

Abstract

In the paper a new version of semi-phenomenological model is constructed, which allows to calculate the friction velocity u* via the spectrum of waves S and the wind at the standard horizon W. The model is based on the balance equation for the momentum flux, averaged over the wave-field ensemble, which takes place in the wave-zone located between troughs and crests of waves. Derivation of the balance equation is presented, and the following main features of the model are formulated. First, the total momentum flux includes only two physically different types of components: the "wave" part TAUw associated with the energy transfer to waves, and the "tangential" part TAUt that does not provide such transfer. Second, component TAUw is split into two constituents having different mathematical representation: (a) for the low-frequency (energy-containing) part of the wave spectrum, the analytical expression of momentum flux TAUw is given directly via the local wind at the standard horizon, W; (b) for the high-frequency part of the wave spectrum, flux TAUw is determined by friction velocity u*. Third, the tangential component of the momentum flux TAUt is parameterized by using the similarity theory, assuming that the wave-zone is an analogue of the traditional friction layer, and in this zone the constant eddy viscosity is realized, inherent to the wave state. The constructed model was verified on the basis of simultaneous measurements of two-dimensional wave spectrum S and friction velocity u*, done for a series of fixed values of W. It is shown that the mean value of the relative error for the drag coefficient, obtained with the proposed model, is 15-20%, only.