The Effect of Rain in Calming the Sea

Abstract

The effect of rain in damping surface waves appears to be significant in the estimation of wind speed from the backscatter of radar signal from the sea surface. The radar backscatter depends on the small-scale roughness of the sea surface. This is modified by the generation of capillary and gravity-capillary ripples by raindrops and by the increased damping of the wavelengths of 10-cm scale. The phenomenon is also important in wave generation and wave breaking, since in both cases the short wavelengths play a key role. A laboratory experiment has been performed to investigate the damping of water waves by rain in the absence of wind. Rain of intensity of 300 mm h?1 falls on mechanically generated progressive waves. The wave amplitude is measured before the wave enters and after it exits the rain section of a tank 2.35 m long. On the assumption of exponential damping, it is found that the effect of rain can be described by an eddy viscosity ?E ≈ 0.3 cm2 s?1. The major part of the damping is attributed to turbulence; enhanced viscous decay due to small-scale random and unresolved motions. Short waves, less than 25 cm in wavelength are most severely damped. Rain falling on standing waves produces a similar effect.