A theoretical study of the air-sea drag-saturation in very strong winds

TL;DR

This paper provides a theoretical explanation for the saturation of the air-sea drag coefficient in very strong winds, considering the effects of airborne water droplets and capillary wave instabilities.

Contribution

It introduces a hydrodynamic model that accounts for airborne water droplets and their impact on drag saturation in strong winds, linking instability mechanisms to observed phenomena.

Findings

Drag coefficient saturates at high wind speeds due to capillary wave instabilities.

Airborne water droplets form a distinct fluid layer affecting momentum transfer.

Kelvin Helmholtz instability explains the transition to drag saturation.

Abstract

The goal of this note is to provide a theoretical explanation for the saturation of the drag coefficient in strong wind conditions. The hydrodynamic model under consideration takes into account the important effects of airborne droplets of water in a thin layer above the water surface that effectively behave as a different fluid between the water and the air. Above this layer the model is coupled with a log-wind profile for the strong winds blowing above the sea. The main underlying mechanism governing the behavior of the drag coefficient is the Kelvin Helmholtz instability for capillary waves on the water surface and the continuity of shear stress along the intermediate interface.