The effects of rainfall on the ocean surface at low to moderate wind speed

This dissertation presents a series of laboratory investigations of the effects of rainfall on the ocean surface in low to moderate wind speed conditions. Specifically, we examine the role played by rain in enhancing air-sea gas exchange rates, damping surface gravity waves, generating turbulence and the relationships between these phenomena.;Rainfall is believed to participate in the air-sea fluxes through surface waves damping and near surface turbulence generation; however, quantitative studies of the dynamic effects of rainfall entering the ocean are nearly inexistent. Two series of laboratory investigations were completed to examine the combined effect of rain and wind on air-water gas exchange for rain falling on freshwater. We showed that rain and wind combine nonlinearly to enhance air-water gas exchange. In accord with the laboratory results, we developed a non-linear model which was used to extrapolate the results to the field, and we show that rainfall has the potential to dominate the total gas transfer in regions with low winds and intense rainfall such as the tropics. Another two series of laboratory experiments were completed to investigate rain-wave interactions and quantify rain-generated turbulence. These experiments were completed for both fresh- and saltwater conditions. Rainfall is found to damp surface gravity waves downwind of the rain cell but also generate significant high frequency waves beneath the rain; these results have important implications in remote sensing of the sea surface. A wave damping model was updated to include a more realistic profile of the turbulence beneath the surface. The use of this model, however, we point out the need to incorporate the high-frequency rain-generated waves in future models and reveal the underestimation of turbulence intensity levels through wave height damping measurements because of the presence of the high frequency rain-generated waves. Furthermore, rainfall was observed to generate a significant amount of turbulence and mixing in a shallow (O(2--3) cm) surface layer in saltwater at low wind speeds. The turbulence intensities, though, were found only to increase in the lower range of the several rain rates studied, before buoyancy effects actually led to the suppression of turbulent mixing. This near surface turbulence is an integral part of the mechanical energy transfers from the atmosphere to the ocean (drag), but also plays a role in near surface mixing with implications for heat and gas fluxes. The suppression of turbulence at high rain rates by buoyancy implies gas exchange rates will likely be lower than anticipated for high rain, low wind speed conditions.