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 FEATURED ARTICLE  Rainfall at Sea: Using the Underwater Sounds of Raindrops as a Rain Gauge for Weather and Climate Barry B. Ma, Brian D. Dushaw, and Bruce M. Howe    Rain on Land, Rain at Sea Were you out walking when it starts to rain, you might take shelter in a shed with a tin roof. By listening to the sound of the raindrops plunking on the roof, you could tell that it is raining, how much it is raining, the size of the raindrops, and when the rain has stopped. Listening to raindrops over the ocean surface using a hydrophone is analogous to listening to the rain under a tin roof. Raindrops hitting the sea surface generate loud sounds underwater. The ocean conducts sound very effi- ciently so that the sounds from the sea surface propagate down with little loss of energy; the sound environment of the ocean is much like a large echo chamber. The nature of the sound is unlike the plunking on a tin roof, however. Rain over the ocean sounds like the hiss of white noise underwater (Discovery of Sound in the Sea: Rainfall; see bit.ly/3KoN55m), with frequencies that extend well above the threshold of human hearing. In this article, we describe how those sounds convey considerable informa- tion about the nature of rainfall at sea. Rain is, of course, intermittent in both time and location and has a wide variety of characteristics, ranging from light drizzle to heavy tropical downpours. On land, it is relatively easy to measure: a simple cup placed in the open for an interval of time can be used to determine the rate of rainfall. At sea, however, rain is difficult to mea- sure because the ever-present sea spray can be confused for rain and a rain gauge can be violently disturbed by the confounding effects of waves. These difficulties, together with the obvious underwater sounds from rain, have led to the development of an acoustic rain gauge. Deploying an instrument at sea still faces the perennial problem of requiring some platform to put it on, however. It might appear that the measurement of rain through acoustics would be challenging because there are many contributors to noise in the ocean. The sound environ- ment can be complicated, and all ocean ambient sound is time, frequency, and location dependent. But rain is one of the major natural sources of underwater sound, and when rain is present on the ocean, its sound usually dominates all other sound sources. The dominant sound of rainfall, which occurs in the acoustic frequency range from 1 to 50 kHz, can be used to infer rain rate, accumu- lation, and the size of the drops themselves. Why Listen to Rain? Knowing the distribution of rain is necessary for weather forecasts in day-to-day life and is an essential variable for climate studies. Still, the accurate measurement of rain is an important challenge for climate science. Although it is essential to know the rainfall accurately, the qual- ity of data from the rain gauges on at-sea moorings is poor. Satellite-based remote sensing can also measure the rainfall over the ocean, these measurements provide large-scale coverage of rainfall parameters such as rain rate and accumulation. However, satellite data cannot be used to determine the local variability and details such as drop size distribution. Because of the great value of having data on the rainfall over the expanses of the oceans, novel methods had to be developed to get accurate measures. Through develop- ments described in this article, acoustic rain gauges have been designed for practical, cost-effective deployment on many observation platforms. Over the past quarter century, the oceanographic com- munity has been developing the Global Ocean Observing ©2022 Acoustical Society of America. All rights reserved.  62 Acoustics Today • Summer 2022 | Volume 18, issue 2 https://doi.org/10.1121/AT.2022.18.2.62 


































































































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