When rain falls onto a large body of water it produces dominating underwater sound over a broad range of audio frequencies. Laboratory studies using more than 1000 single drops, covering the complete size range of actual rain drops at their terminal speeds, have now shown that the complete underwater spectrum of rainfall sound can be dissected into the impact and microbubble sounds produced by four acoustically distinctive ranges of drop diameters D. These are defined as "minuscule" drops (D<0.8 mm), "small" drops (0.8 mm• 2.2 mm). A minuscule raindrop produces only a very weak, almost undetectable, short duration impact noise. A small drop at terminal speed and at local, near-normal incidence, radiates measurable broadband impact sound followed by the very much stronger sound of a "type I" damped microbubble oscillating at frequencies near 15 kHz. A mid-size raindrop radiates only impact sound. Large raindrops, which comprise the major volume of moderate to heavy rainfall, produce an impact sound and a dominating, "type II," "primary" oscillating microbubble of characteristic frequency 2 to 10 kHz depending on the drop diameter. Also, large drops often generate weaker sounds from "secondary" bubbles. The average acoustic energy spectra of large raindrops are distinctive functions of their diameters, the salinity of the surface water, and the temperature difference between the drop and the surface water. When the underwater acoustic intensity spectrum during heavy rain is calculated from the single drop acoustic energy spectra and the drop size distribution, it compares quite well with ocean measurements. The gas injection at the airwater interface is calculated from the probability of bubble formation during a heavy rainfall.

Drop diameters from 2.7 to 4.5 mm are common in heavy rainfall. By using a hydrophone with a flat response up to 300 kHz and A/D conversion up to 1 MHz the components of the sound radiation for these large terminal-velocity drops in the time and frequency domains can be identified. At these high sampling rates it is possible to see notches on the lagging edge of the impulse trace. These notches at 100 kHz and higher frequencies are probably caused by internal drop reflections. Lower frequency oscillations with a range of frequencies less than 10 kHz occur after a time lag of 30 ms or more. Both the frequency of oscillation and the time lag appear to be the functions of drop size. Although the amplitudes of these components are comparable for individual drops the energy radiated by the low-frequency oscillation is greater due to its longer duration. These early results suggest that it may be possible to ascertain the drop spectrum (number of drops as a function of diameter) in rainfall from the spectrum of the underwater sound of the rain. [Research supported by ONR.]

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.

Raindrops are known to produce bubbles by at least two different mechanisms: type I, the pinch-off of the bottom of the crater, and type II, air entrainment from a turbulent water jet formed as a part of the drop splash. Bubble energy is shown to dominate impact energy for the useful range of type II drops. Previous work [Snyder et al., J. Acoust. Soc. Am. Suppl. 1 88, S2 (1990)] has shown a relationship between drop size and bubble frequency of the type II mechanism. The onset of this mechanism is related to the drop kinetic energy at impact, rather than to velocity. As the drop kinetic energy increases above a threshold of 2 × 10−4 J, the likelihood of bubble entrainment increases to approximately 65%. Further work reveals the effects of temperature, salinity, and surface tension on the sound radiated from large raindrops. The relation between the drop diameter and the spectrum of the underwater acoustic energy is examined for terminal velocity drops. This relation makes possible the remote measurement of both drop size distribution and rainfall rate. [Work supported by ONR.]