Characteristics of sound radiation from large raindrops

Abstract

(U) Drop diameters from 2.7 mm to 4.6 mm are common in heavy rainfall. The impact and bubble signals of the underwater sound radiation from these large drops at their terminal velocities have been identified. At a 1 MHz sampling rate, several notches are observed in the approximately 100 microns per second duration impact signal. These notches with time separations of 3 to 4 microns per second between peak and trough, are shown to be caused by internal drop reflections of the impact pressure wave. Frequently observed low frequency oscillations from 2 to 10 kHz, which lag the impact signal by 40 to 55 ms, are attributed to bubbles formed by a complex jet mechanism which has been identified by high speed photography. This mechanism is unlike the bubble formation mechanism for drops of .8 to 1.1 mm diameter previously reported in the literature. Both the frequency of the bubble oscillation and the time lag are functions of drop size. The percentage of drop that generate bubbles is shown to be a function of drop size as well. This peaks at 65% for drops of approximately 4 mm diameter and there are essentially no bubbles for drop sizes in the range of 1.2 to 2 mm in diameter. Although the amplitudes of the impact and bubble signals are often comparable, the acoustic energy radiated by the bubble is greater due to its longer time duration. These results suggest that it may be possible to determine the number of drops as a function of diameter from the underwater sound spectrum of rainfall at sea.