Abstract:
Acoustical evidence for the formation of subsurface bubble layers generated by extremely heavy rainfall will be presented. These bubble layers are associated with the presence of very large raindrops (over 3.4 mm diameter) within the rain, which in turn, tend to be associated with extremely heavy rainfall. The splash physics of very large raindrops include a turbulent downward propagating jet which has been shown to carry bubbles into the underlying water. These raindrops have also been shown to rapidly deepen a rain-induced mixed layer, suggesting that rain can generate a subsurface bubble layer. The bubble populations are measured by predicting the sound field generated at the surface and then measuring the acoustic extinction well below the surface. Vertically integrated bubble population densities equivalent to the bubble field generated by 10-m/s winds are observed. These bubble populations quickly dissipate when the very large raindrops are no longer present in the rain, suggesting that the bubble layer generated by extremely heavy rain is very thin (tens of centimeters at most). [Work sponsored by NSF and ONR.]