  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.
