Underwater sound energy from large terminal velocity raindrops as a function of drop volume, temperature, and salinity.

Abstract

The principal underwater sound energy radiated by terminal velocity raindrops at sea is due to the bubble oscillations which occur for drops of diameter 0.8 to 1.1 mm (type I) and 2.2 to 4.6 mm (type II). In the absence of bubbles, between 1.1 and 2.2 mm, the impact sound radiation is significant. The type I bubbles radiate at frequencies close to 15 kHz, whereas type II bubbles radiate between 2 and 10 kHz, depending on drop diameter. Therefore type II bubbles, which are common in moderate to heavy rainfall, offer the opportunity to determine rainfall drop distribution and total rainfall rate by remote underwater listening. Type II bubbles radiate almost twice as much energy when the drop and surface temperatures differ by 10 °C. Type II bubbles radiate 45% as much energy in saline water (35 ppt) as in fresh water. The distinctive sound spectral shape for a particular diameter raindrop does not change appreciably with extreme differences of temperature (0 to 22 °C) or salinity (0 to 35 ppt). It is possible, therefore, to condense the data from hundreds of drops acquired in our laboratory into a single relation which gives the average energy radiated by a type II raindrop as a function of drop volume, temperature, and salinity. Using this relation, good agreement is found between measurements at sea and the predicted sound spectrum for a given drop-size distribution. Also, the total rainfall rate and drop-size distribution from sound spectra measured at sea (the inverse problem) are calculated. These early successes lay the groundwork for real-time measurements of total rainfall rate and drop-size distributions in moderate to heavy rainfalls by remote underwater listening. [Work supported by ONR.] a) Lt., USN.