Passive acoustic detection and measurement of rainfall at sea

Abstract

It is well recognized that rainfall measurements are needed over the world’s oceans. One method of providing these measurements is to passively listen for the underwater sound signal that is produced by rainfall striking the ocean surface. Since 1998, over 70 buoy-months of ambient sound data have been collected using Acoustic Rain Gauges (ARGs) deployed on deep ocean moorings that form part of the Tropical Atmosphere Ocean (TAO) array in the Pacific Ocean. These data demonstrate the acoustic measurement of oceanic wind and rain. Other ‘‘noises’’ are present in the ocean and need to be detected and rejected. This is accomplished by recognizing the unique spectral and temporal character of rain-generated sound. A quantitative relationship between absolute sound levels and rainfall rate is proposed. The probability of acoustic detection of rainfall events under different weather conditions will be discussed. A quantitative comparison of rainfall accumulation using the acoustic technique with co-located rainfall estimates from on-board R.M. Young rain gauges and from NASA TRMM satellite overpasses (Rainfall Product 3B42) shows promising agreement, but also points out problems associated with each measurement method. [Work supported by ONR—Ocean Acoustics, NSF—Physical Oceanography, and NOAA Office of Global Programs.]