Impact of waves on air-sea exchange of sensible heat and momentum

The impact of sea waves on sensible heat and momentum fluxes
is described. The approach is based on the conservation of
heat and momentum in the marine atmospheric surface layer.

The experimental fact that the drag coefficient above the sea
increases considerably with increasing wind speed, while the
exchange coefficient for sensible heat (Stanton number)
remains virtually independent of wind speed, is explained by a
different balance of the turbulent and the wave-induced part
in total flux of momentum and sensible heat.

Organised motions induced by waves form the wave-induced
stress which dominates the surface momentum flux. These
organised motions do not contribute to the vertical flux of
heat. The heat flux above waves is determined by how the
diffusivity of turbulence is affected by waves.

The diffusivity of turbulence is altered by waves in an
indirect way. The wave-induced stress dominates the surface
flux and decays rapidly with height. Therefore the turbulent
stress above waves is no longer constant with height. That
changes the balance of the turbulent kinetic energy and of the
dissipation rate and, hence the diffusivity.
The dependence of the exchange coefficient for heat on wind
speed is usually parameterized in terms of a constant Stanton
number. However, an increase of the exchange coefficient with
wind speed is not ruled out by field measurements and could be
parametrized in terms of a constant temperature roughness
length. The large scatter of field data do not allow to
establish the actual dependence.

The exchange coefficient for sensible heat, calculated from
the model, is virtually independent of wind speed in the range
of 3-10 m/s. For wind speed above 10 m/s an increase of 10\%
is obtained, which is smaller than that following from the
'constant roughness lengths' parameterization.