Measurements of atmospheric pressure on wind-generated sea waves

Abstract

Simultaneous measurements of wave elevation and atmospheric pressure on wind-driven sea waves were made using a vertical wave-sensing rod and a small (23 cm diameter) pancake-shaped styrofoam buoy in which was embedded a sensitive pressure transducer; the wave probe constrained the buoy to move with the waves only in the vertical direction. Care was taken to avoid contamination of the pressure signal with dynamic pressures caused by flow distortion around the buoy.Results are presented as power and cross-spectra of wave elevation and pressure, spectra of the fluxes of energy and momentum from the wind to the waves, and spectra of ζ the fractional increase in wave energy per radian.The phase shifts of the pressure signal are compared with the laboratory and field results of other investigators, and with the theoretical predictions of Miles's (1957) inviscid laminar model of wave growth. Agreement is reasonably good among the experimental results, but observed phase shifts are an order of magnitude larger than the theoretically predicted values.Integrals under the momentum flux spectra are compared in all runs with the predictions of the standard empirical formula, and in two cases are compared with the values of the total wind stress as measured with a sonic anemometer; the indication is that a large fraction of the total flux of momentum from the air to the sea goes initially into the wave field.The ζ spectra are compared with the field results of Snyder & Cox (1966) and with the theoretical predictions of Miles's (1957) model; agreement is again good between the field results while the theory underpredicts ζ by factors of between 5 and 8.A simple dimensionless relation is found between ζ and the ratio of wind speed to wave phase speed.