ASA 126th Meeting Denver 1993 October 4-8

3aAO1. Underwater sound produced by rainfall: Secondary splashes of aerosols.

Jeffrey A. Nystuen

Ocean Acoust. Div./AOML, 4301 Rickenbacker Cswy., Miami, FL 33149

Herman Medwin

Naval Postgraduate School, Monterey, CA 93943

Earlier studies have identified three sources of underwater sound production from raindrops: the initial impact, a bubble trapped underwater as the impact crater closes (type I), and a bubble trapped underwater by a turbulent jet associated with the splash canopy formation (type II). For natural rainfall, type I bubbles are associated with small raindrops (0.8--1.1 mm diam), while type II bubbles are associated with large raindrops (diameter greater than 2.2 mm). Predictions for the sound generated by rainfall considering these three sound source mechanisms have been consistently low, suggesting that an additional source mechanism is present. Experimental data will be presented describing a new mechanism of bubble entrapment---namely, bubbles created during the secondary splashes of drop aerosols thrown up during the initial raindrop impact (type III). For the raindrop sizes studied (large raindrops, 3.0--4.7 mm diam), type III bubbles occurred, on average, more than once per impact. These bubbles span sizes, resonance frequencies, and acoustical energy emissions comparable to the type II bubbles. Consideration of type III bubbles improves predictions of the underwater sound generated by rainfall. These predictions are needed to support the inversion of the oceanic ambient sound field to estimated rainfall rate at sea quantitatively. [Work supported by ONR and NRL/TOWS.]