Underwater sound radiation from large raindrops
Jacobus, Peter W.
Nystuen, Jeffrey A.
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The principal underwater sound energy radiated by terminal velocity raindrops at sea is due micro-bubble entrainment and oscillations which occur for drops of the two diameter ranges 0.8 to 1.1 mm (Type I) and 2.2 to 4.6 mm (Type II). In the absence of bubbles, particularly 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 the 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 more energy when the drop and surface temperatures differ, e.g., almost twice as much energy when the drop and surfaces temperatures differ by 10 (o) C. Type II bubbles radiate less energy in saline water, e.g., 45% as much energy at a salinity of 35 ppt as for fresh water. The distinctive sound spectral shape for a particular diameter drop does not change appreciably with extreme differences of temperature (0 to 22(o) C) or salinity (0 to 35 ppt). It is possible, therefore, to condense the data acquired from hundreds of drops 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, we find good agreement between measurements at sea and the predicted sound spectrum for an assumed reasonable drop size distribution. Also, the total rainfall rate and drop size distribution has been calculated from sound spectra measured at sea (the inverse problem). These early successes lay the groundwork for real time measurement of total rainfall rate and drop size distributions in moderate to heavy rainfalls inferred by remote underwater listening.
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