Surface Measurements of Precipitation from an Ocean Mooring: The Underwater Acoustic Log from the South China Sea

Jeffrey A. Nystuen

Applied Physics Laboratory
Univeristy of Washington
Seattle, Washington

Michael J. McPhaden and H. Paul Freitag

Pacific Marine Environmental Laboratory
National Atmospheric and Oceanic Administration
Seattle, Washington

Abstract: Surface measurements of precipitation in oceanic environments have proven especially difficult to obtain because traditional technologies such as tipping bucket rain gauges are unsuitable for deployment from oceanic platforms including ships and moorings. Recently the Pacific Marine Environmental Laboratory (PMEL) of the National Oceanic and Atmospheric Administration (NOAA) has modified a collection gauge, the R.M. Young Rain Gauge, for long-term deployment on deep ocean moorings. This instrumentation package was deployed during part of the South China Sea Monsoon Experiment (SCSMEX). Also deployed on the same mooring were two Acoustic Rain Gauges (ARGs) which monitor precipitation through the interpretation of the high frequency, from 500 Hertz to 50,000 Hertz, underwater sound field. The mooring was located at 20° 22.2' N, 116° 31.2' E and was in place from April 7, 1998 - June 5, 1998. Unfortunately, pirates stole the surface instrumentation on May 6, 1998, limiting data from the R.M. Young Rain Gauge to satellite transmissions prior to the attack. The ARGs survived the attack and reported data throughout the deployment. The acoustic data are interpreted to provide quantification of wind speed; detection, classification and quantification of rainfall; and the detection and quantification of near-surface bubble layers. Percentage of time raining from the two rainfall measurements are in excellent agreement. The acoustic quantification of rainfall amount is adjusted to the R.M. Young rain gauge data and consequently modified acoustic rainfall algorithms are proposed. The acoustic detection of several instances of high near-surface bubble injections during extremely heavy rainfall are described.

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