Estimating sea spray volume with a laser altimeter

Down-looking laser altimeters are commonly used to measure the sea surface elevation. However, because the laser radiation is attenuated by spray droplets suspended along the transmission path, it is presumed that altimeters may also provide an indirect measure of the sea spray volume. Here we discuss this conjecture by means of laboratory experiments, which have been conducted in a wind-wave flume. A large number of wind conditions were considered between equivalent 10-metre wind speeds of 20 and 60 m/s in order to generate different spray volumes above the water surface. The facility was equipped with a laser and side-looking camera system to estimate the spray volume as well as a nearby down-looking laser altimeter. Results confirm that there is a robust degradation of the laser intensity for increasing wind speed and hence amount of spray droplets above the water surface. A simple regression model to extract spray volume from the average intensity of the laser radiation is presented, demonstrating the promise of laser altimeters for making in-situ spray observations. Additional observations will be required to calibrate the altimeters for applications in the open ocean marine environment.