Wave damping by rain

Abstract

IT is common knowledge among seafarers that rain "knocks down the sea"^1,2 or that surface-wave breaking is suppressed by heavy rain. Although no quantitative data exist, several mechanisms have been proposed that might account for the attenuation of waves by rain. The most likely seems to be that raindrops produce turbulence in the water which then helps to dissipate the waves. Here we describe laboratory experiments which show that, in the absence of wind, rain preferentially damps short-wavelength gravity waves. The observed damping rates are, however, lower than earlier predictions^3,4. The suppression of short waves has an effect on the sea surface similar to that of an oil film, leading to a reduction in the breaking of long waves. The phenomenon is important because it helps to provide insight into other aspects of wave breaking or wave suppression, and because of its bearing on the detection and estimation of rainfall at sea from ambient noise levels⁵, and of the measurement of winds using radar, as scatterometer estimates of wind speed are known to be biased when it rains⁶. Wind speed is determined from an empirical relation with the observed strength of the backscattered radar signal. Although the signal may be influenced by atmospheric effects in rain, the very-small-scale roughness of the surface caused by rain drops or, as we shall show, the reduction of the 10-cm-scale waves, may also affect the signal and hence the wind-speed estimate.