The attenuation of waves under the action of rain

Abstract

The well known but, until now, unquantified, damping effect of rainfall on water waves is established experimentally. Artificial rain of intensity up to 600 mm hr-1 consisting of raindrops with equivalent diameter 3.2 mm is allowed to fall onto mechanically generated progressive and standing waves in no-wind conditions. The amplitude of the progressive wave is measured before the wave enters the raining section and after it exits. From the amplitude ratio in raining and non-raining conditions the spatial damping due to rain is derived for the progressive waves on the assumption of exponential damping. The resonance curve and the damping of standing waves in raining and non-raining conditions is also studied. The damping due to rain can be described by an eddy viscosity &upsilon;E. All the experiments seem consistent with a value of &upsilon;E of 0.3 cm2sec-1. An estimation of the damping effect of the rain-introduced variable stresses on the water surface is also made. Their effect is calculated to be trivial. The damping of waves with rain can be explained if the wind speed decreases when it starts to rain. The correlation of wind speed with the onset of rain is investigated by the use of 1 min wind speed time series. It is found that for wind speeds greater than 20 knots (10 m sec-1) the wind speed increases with the start of rain. Some possible effects of the wave damping due to rain on wave-growth and wave breaking are discussed.</p