Dynamic Interaction of Intense Rain with Water Waves

A theory for determining the dynamic effect of intense rain on water waves is established, based on momentum exchange. The theory takes into account the rain intensity, angle of incidence and fall velocity, and the wave amplitude, frequency and water depth. It is found that the rain induces a uniform increase of pressure in the water column and a uniform mass transport in a thin boundary layer affected by the momentum exchange. The rain also induces a fluctuating pressure and shear stress on the free surface. For vertical or near vertical rainfall, these fluctuating free surface forces are responsible for a non-negligible wave amplitude decay, particularly in the high frequency range. In the case of high winds, the rain horizontal velocity component is large and the corresponding stress on the free surface is nearly in phase with free surface slope. Then instead of causing a decay, the rain adds its effect to the wind and enhances the growth of high frequency waves. It is concluded that this effect, previously neglected, should be considered for insertion as a sink-source mechanism in advanced air-sea interaction models.