Underwater Sound Generated by Rainfall:
Secondary Splashes of Aerosols

Jeffrey A. Nystuen

Cooperative Institute for Marine and Atmospheric Studies
Rosenstiel School of Marine and Atmospheric Sciences
University of Miami
Miami, Florida

Herman Medwin

Department of Physics
Naval Postgraduate School
Monterey, California

Abstract: Earlier studies have identified three sources of underwater sound production from raindrops: The initial impact, a bubble trapped underwater at the base of the impact crater (type I), and a bubble trapped underwater by a turbulent jet created during the splash canopy formation 50-80 ms after impact (type II). Together these sound sources have been used to predict the underwater sound spectrum produced by rainfall; however, the predictions have underestimated observed sound levels. A new important mechanism of bubble entrapment (sound production) is described-bubbles created during the secondary splashes of drop aerosols thrown up during the initial raindrop impact. These delayed bubbles occur 100-600 ms after the initial impact depending on raindrop size. The average radiated spectral energy incorporating all known sound sources for two large drop sizes (3.2- and 4.7-mm-diameter raindrops) are presented. Improved predictions of the underwater sound produced by rainfall are shown. The predictions form the basis for quantitative inversion measurements of rainfall at sea using underwater sound. A roughened water surface does not appear to significantly change the energy radiated by large raindrops.

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