ABSTRACT

A hot-film anemometer was used in a laboratory facility for wind and waves to measure the size and number of water droplets over the air-water interface. The wind speed was controlled in the range 0-15 m/sec over the water surface. Experiments were conducted both with and without mechanically generated waves and each with both fresh and salt water. The effect of wind speed and wave height on the vertical distribution of spray were investigated. The mechanism of spray generation was explored.

The results of this study indicate that the vertical distribution of the total production of droplets can be expressed by a logarithmic distribution. The drop size distribution (drop size spectrum) at a fixed elevation can be described a negative power law of drop diameter as suggested by Junge (1963) in which he used the power law to describe aerosal distribution. The droplet production mechanism by bursting bubbles suggested by Blanchard (1963) is found to account for only a portion of the total production. The effect of spray on the remote sensing of the airsea interface is evaluated. The spray is found to strongly affect the measurements obtained from a radiometer. The contribution of droplets to the evaporation rate is found to be 22% under 15 m/sec. wind speed.

INTRODUCTION

Spray is produced by wind through different mechanisms which are not completely understood yet. Possible mechanisms are the direct shearing of droplets from the wave crests, the bursting of bubbles produced by breaking waves and the aerodynamic suction of droplets from the crests of capillary waves. Spray plays a significant role in increasing the heat and mass transfers at the ocean surface. Spray can affect the microwave radiometer reading. In the infrared range the droplets create a scattering medium for microwaves which have wave lengths of the same order of magnitude as the droplet diameter. Even for longer waves the spray absorbs a portion of the wave energy. The attenuation of the radiometer reading is a function of the total water mass present in the column of air above the ocean surface.

Spray was considered by Valenzuela and Laing (1970) to produce possibly broader spectra of radar sea echo with horizontal polarization compared to those with vertical polarization.

Previous studies on spray by Okuda and Hayami (1959) and Easterbrook (1970) were aimed at estimating the increased evaporation due to spray. Blanchard (1963) investigated spray to determine the salt nuclei over the ocean. Monahan (1968) obtained spray measurements at a constant level above the water surface by a floating raft for different wind speeds. The available information does not adequately cover the details of the spectral distribution of droplet diameters at different elevations, and an investigation under controlled conditions becomes desirable to assess the influence of spray on remote sensing techniques.

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