Sound radiation from large raindrops: Dependence on salinity and temperature

Abstract

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.]