Measuring winds from underneath the ocean surface by upward looking acoustic Doppler current profilers

Abstract

Upward looking acoustic Doppler current profilers were deployed in the Gulf of Lions, in the western Mediterranean for winter convection studies during early 1987. The near‐surface Doppler data, which are dominated by sidelobe echo returns from the surface and which are therefore usually discarded for the analysis of subsurface current profiles, were found to contain information useful for determining surface winds. The horizontal Doppler directions were highly correlated with wind direction, measured by a research vessel in the vicinity. Wind speed, while not correlated with the size of the Doppler shifts, was significantly correlated with the acoustic backscatter energy. At 150 kHz frequency and 20° main beam angle the wind speed exponent was determined as 1.32; and at 75 kHz and approximately 24° sidelobe reception angle it was 0.76. At near‐vertical incidence, where more and more energy is scattered sideways with increasing wind speed, the exponent was found to be negative. The wind data were too limited to determine whether a saturation limit of the wind dependence existed. While Bragg scattering seems to be primarily responsible for the surface Doppler shifts, their magnitude cannot be explained by the Bragg effect alone; the modulation of the Bragg scattering ripple waves by longer waves of the surface wave spectrum also appears to be important.