Ambient sound in the ocean induced by heavy precipitation and the subsequent predictability of rainfall rate
McGlothin, Charles C. Jr.
Nystuen, Jeffrey A.
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An experiment by the Naval Postgraduate School and the National Data Buoy Center was performed in the Gulf of Mexico to characterize the underwater sound generated by heavy precipitation and to determine if rainfall rates of heavy precipitation can be measured using underwater sound. During this stage of the experiment, twentytwo data sets were recorded with rainfall rates up to 340 mm/hr. For a given rainfall rate, it is found that sound levels from heavy convective precipitation are higher at the beginning of the storm and when the rainfall rate is increasing than at the end of the storm event or when rainfall rates are decreasing. This may be due to changes in the drop size distribution during the life cycle of the storm or to variations in the temperature difference between the raindrop and the ocean surface. Very heavy rainfall (rate > 150 mm/hr) generates near surface bubble layers or bubble clouds which attenuate sound energy at higher frequencies (>15 kHz). The distinctive 15 kHz peak in the sound spectrum for light rain is absent during heavy rain suggesting that the sound production mechanism previously identified for small drops (0.8 - 1.1 mm in diameter) is suppressed by heavy rain even though those small drops are undoubtedly present during heavy rainfall rates. These data show a very high correlation between underwater sound level and the logarithm of the rainfall rate except when high wind speeds (> 10 m/s) and high rainfall rates (> 150 mm/hr) are present. An empirical rainfall rate algorithm for con-vcctive precipitation is proposed suggesting that sound energy is directly proportional to rainfall rate, however any empirical algorithm which does not adjust for changing. storm characteristics should be used with caution.
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