Ocean Ambient Sound Instrument System: Acoustic estimation of wind speed and direction from a subsurface package

Abstract

The Ocean Ambient Sound Instrument System (OASIS) consists of a conventional RD Instruments Acoustic Doppler Current Profilers (ADCPs), modified to allow the recording of high-quality ambient sound in the frequency range from 1 to 75 kHz. In addition to the usual capabilities of an ADCP, this combination of acoustic instrumentation allows wind direction and speed to be inferred from a subsurface platform. The method of wind speed determination from ambient sound levels is explained identifying some of the techniques and limitations. Wind direction is inferred from surface drift velocities using the ADCP data. The design of both the hardware and software required to make combined ADCP and ambient sound recordings is discussed. The capabilities of the system are demonstrated using observations made in the Norwegian Sea at Ocean Weather Station Mike. Using previously published algorithms (and calibration constants) ambient sound-based wind speeds are found that closely match direct wind observations. The typical standard deviations for hourly wind speed estimates is 1.5 m s-1 when using acoustic frequencies less than 10 kHz. Higher acoustic frequencies show greater variance in wind speed estimates. OASIS estimates of the 12-h average wind directions have an error standard deviation of 25°with no mean bias.