Acoustical Rainfall Analysis: Rainfall Drop Size Distribution using the Underwater Sound Field

Jeffrey A. Nystuen

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

Abstract: Rainfall estimation is difficult, especially in oceanic regions where land-based techniques are unavailable. Fortunately, rain produces a loud and unique sound underwater that can be used to detect and quantify rainfall. Laboratory studies of the sound generated by individual raindrops have provided the basis for a formal inversion of the naturally generated underwater ambient sound field. Field measurements of subtropical rainfall at the Atlantic Oceanographic and Meteorological Lab Rain Gauge Facility are used to demonstrate the forward (predicting the sound field given the rainfall drop size distribution) and the inverse problem (estimating the drop size distribution given the sound field). This acoustical rainfall analysis (ARA) algorithm was tested for several dozen rainfall events spanning six months and was found to provide excellent estimates of rainfall rate, rainfall accumulation, and rainfall reflectivity (the quantity sensed by radars). High temporal resolution (order 5-10s) variations in drop size distribution within the rain can be studied using ARA.

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