Upper ocean diagnostics during rainfall.

Abstract

Laboratory measurements have been made of the sound radiated by the complete size range of acoustically significant, single raindrops, falling at terminal speeds onto the surface of an anechoic water environment. This has provided the source information for the detailed analysis of many effects of light to heavy rainfall at sea. The three acoustically distinctive subranges are defined as ‘‘small’’ drops (equivalent diameter, 0.8–1.1 mm), ‘‘midsize’’ drops (1.1–2.2 mm), and ‘‘large drops’’ (≳2.2 mm). These subranges correspond to drops that are oblate ellipsoids, slightly flat-bottomed ellipsoids, and completely flat or concave-bottomed ellipsoids just before they hit the water surface. Their penetration through the air–water interface results in underwater spectra which, respectively, have peaks around 15 kHz, are very broadband, and have peaks that decrease from 10 to 2 kHz as the drop diameter increases from 2.2 to 5 mm. This knowledge of the acoustical sources at the surface has now made it possible to determine, during rainfall, such ocean parameters as: gas transfer by active microbubbles, bubble populations under the sea surface, rms ocean slope, raindrop size distribution, total rainfall rate, raindrop temperature, and even the type cloud and its base height from which the rain came. Some results will be presented. [Work supported by the ONR.]