Experimental Study On Rainfall Effect On Wave Growth, Air-sea Interface Turbulence And Their Interaction

Rainfall is a very common phenomenon in daily life; however, in the research of seasurface feature, the effect of rainfall can not be ignored. Especially on the retrieval ofsea surface wind using altimeter data, the change of sea surface roughness during rainwill affect the accuracy of wind retrieval. As a result of wind field and gravity,momentum is transferred between rain and sea water in air-sea boundary layer inhorizontal and vertical directions, sea surface roughness, wind stress and otherfeatures can be altered by rainfall inevitably. In early studies, oceanographers figuredthat rainfall can damp gravity waves. Later studies found dual effect of rainfall onsurface waves: damping in gravity wave range and growth in capillary andgravity-capillary wave range. However, the mechanism behind this phenomenon isstill disputable.Rain effects on mechanical waves and swell and wind waves and turbulence wereinvestigated through experiments in a large windwavetank. Research was focused onthe role of rainfall effects on wind wave and turbulence. For many differentenvironmental conditions, the wind, wave and turbulence were simultaneouslymeasured by Pitot tube, wave gauge and acoustic Doppler velocimetry, respectively.Itis found that the wind waves are damped over full frequency range at low windspeed, but are enhanced at high wind speed. This dual effect of rain on wind wavesincrease with the increasing of rain rate, while the influence of rainfall-area length isnot observable. The variation of mean square slope of water surface is correlated wellwith the wave development. At low wind speed, the corresponding turbulence interms of turbulent kinetic energy (TKE) dissipation rate is significantly enhanced byrain as the waves are damped severely, which is indicated that the energy transfersfrom wave to turbulence. At high wind speed, the augment of TKE dissipation rate issuppressed while the wind waves are enhanced, which is suggested that the energytransfers from turbulence to wave. In the field, however, the rain usually attenuatesthe development of waves, which is more significant with the increasing of windspeed.In order to explain this contradiction of rain effect on waves between laboratory and field, a resonant mechanism about energy exchange between turbulence and wave isproposed. It is hypothesized that the resonance occur only when the spectral peak ofwaves (SPW) overlap in frequency with the inertial subrange of turbulence (IST), inwhich the turbulent energy of random motion transfers to the wave energy of orderedmotion. When SPW and IST are separated away, the energy from ordered waves willtransfer to random turbulence. The farther they depart, the more efficient of energytransferring from wave to turbulence. This mechanism is applied to interpret thedamping phenomenon of gas transfer velocity in laboratory experiments, and thevariation of TKE dissipation rates near sea surface compared with the law of wall.