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.

VK Makin, C Mastenbroek. Impact of waves on air-sea exchange of sensible heat and momentum

published, Bound.-Layer Meteorol., 1996, 79

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