On the sea-surface drag and heat/mass transfer at strong winds

A simplified model for the sea surface momentum and enthalpy transfer coefficients are suggested. The main impact of droplets on the boundary-layer dynamics arises from the effect of the ‘spray force’ on the momentum balance of the air–spray mixture. The efficiency of the impact of spray via the ‘stratification effect’ is significantly weaker. The ‘spray force’ effect originates from the action of the vortex force on the marine atmospheric boundary layer (ABL) dynamics; this force results from the interaction of the ‘rain of spray’ with the wind shear. This effect leads to the acceleration of the airflow and the suppression of turbulence in the atmospheric boundary layer above the spray generation layer. It is shown that the drag coefficient 10 D C levels off at a wind speed of around 30 m/s and further decreases with increasing the wind speed as approximately 2 10 U − ∝ . Action of the spray force results also in increase of the turbulent mixing coefficient in the spray generation layer. As a consequence, the vertical gradients of the “scalar” quantities (air temperature and humidity) in this layer are reduced with respect to the reference (no spray effect) gradients and are increased in the entire ABL above this layer. This in turn leads to the enhancement of the bulk transfer coefficient both for the sensible and latent heat. Both, suppression of the drag coefficient, 10 D C , and increase of the enthalpy transfer coefficient, 10 E C , result in rapid rise of the 10 10 / E D C C ratio above the critical value of 10 10 / 0.75 E D C C = at 10 40 U > m/s. Thus suggested model predicts that energy gain from enthalpy exceeds that lost from drag, providing the necessary conditions for tropical storms development.