Wind speed estimates and precipitation detection using ambient sound in the ocean

Schillinger, Douglas J. (2000) Wind speed estimates and precipitation detection using ambient sound in the ocean. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis explores the relationship between ocean ambient sound levels, wind speed and rain. It has long been known that these surface processes generate sound in the ocean, but the development of accurate algorithms has been complicated by the difficulty in obtaining location independent sound levels. Here, absolute source level estimates are achieved by modelling the sources as an infinite field of dipoles at the surface, and accounting for acoustic absorption and reflection from the ocean floor. It is shown that bottom reflections are an important component in elevating sound levels at frequencies below 35 kHz. Knowing absolute source levels, these sound levels can be used to estimate both wind speed and detect the occurrence of precipitation. It is shown that the wind-only generated ambient sound spectrum has a mean slope of -18 dB/decade and ranges from -16 to -20 dB/decade corresponding to wind speeds from 0 to 20 ms⁻¹ for frequencies from 1 to 10 kHz. The spectral slope at frequencies greater than 10 kHz depends upon wind speed. Existing estimation algorithms are shown to overestimate the speed for wind speeds below 10 ms⁻¹ but underestimate wind speeds above 10 ms⁻¹ and that there is a maximum sound level which limits wind speed estimation for frequencies above 10 kHz. A wind speed dependent correction for the existing algorithms is proposed which gives accuracies ±1.3 - 2 ms⁻¹ depending on deployment characteristics and sampling parameters. The accuracy of precipitation identification is limited by the wind speed and the precipitation type. Precipitation classified as Rain by the World Meteorological Organization (WMO) is detectable via acoustic means. Sub-division of the classification of the WMO categories shows that 50% ± 10% of 'Continuous Rain' and 25% ± 12.5% of 'Intermittent Rain' are detectable using the ambient sound spectra.

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