The Underwater Sound Generated by Heavy Precipitation

Jeffrey A. Nystuen, Charles C. McGlothin and Michael S. Cook

Department of Oceanography
Naval Postgraduate School
Monterey, California

Abstract: The underwater acoustic signature of heavy rainfall is very different from that of light rainfall. During heavy rainfall sound levels are observed to rise with increasing rainfall rate at all frequencies monitored (4-21 kHz) and the 15-kHz spectral peak observed during light rainfall is absent. The sound levels are most highly correlated (r ~=0.8) with heavy rainfall rate for frequencies less than 10 kHz. Lower correlations between sound levels and heavy rainfall rate were observed for frequencies above 10 kHz under several different conditions. When wind speed exceeds 10 m/s, wave breaking mixes bubbles downward and creates a layer of bubbles. This bubble layer attenuates subsequent surface-generated sound (from the raindrop splashes) for frequencies above 10 kHz. Extremely heavy rainfall (total rainfall above 150 mm/h) also generates a subsurface bubble layer. This rainfall-generated bubble layer is evidence of rainfall-induced turbulent mixing of the ocean surface layer and has implications for air/sea exchange processes (momentum, heat, and gas exchange). Finally, previous studies have shown that light rain generates acoustic energy above 10 kHz and that this sound is poorly correlated with total rainfall rate. A simple empirical acoustic rainfall rate algorithm for heavy rain is offered. This algorithm may be site specific. Furthermore, it overestimates rainfall rate early in the rain events examined here (convective rain events), and then underestimates rainfall rate later in the same events. This observation is shown to be consistent with likely changes in the drop size distribution during the lifetime of the rain event. The sound produced by large drops within heavy rain (drop diameter greater than 2.2 mm) is shown to dominate the underwater sound field. The empirical acoustic rainfall rate algorithm is therefore more correctly a measure of the rainfall rate from large raindrops. Fortunately, the rainfall rate from large raindrops is highly correlated with the total rainfall rate, making acoustic monitoring of underwater sound an effective measure of rainfall rate in oceanic regions.

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The underwater sound generated by heavy