Laboratory measurements of artificial rain impinging on slick-free and slick-covered water surfaces

Abstract

Laboratory measurements with artificial rain were carried out in the wind-wave tank of the University of Hamburg, in order to gain better understanding of the radar backscattering from a slick-free and slick-covered water surface, particularly when it is agitated by strong rain. We used a coherent 9.8 GHz (X band) scatterometer at different polarisations and at an incidence angle of 28 degrees, a resistance-type wire gauge, and a two-dimensional laser slope gauge. A water surface area of 2.3 m² was agitated by strong artificial rain with a rain rate of 160 mm h⁻¹ and rain drops of 2.9 mm diameter. The wind-speed range used in the present investigation was between 2 and 10 m s⁻¹. A monomolecular surface film was produced by deploying oleyl alcohol on the water surface. The results of the analyses of the measured radar Doppler spectra and wave amplitude and slope spectra are presented. We show that while the wind-induced surface roughness is strongly reduced in the presence of the slick, at a high rain rate (of 160 mm h⁻¹) the surface slick less strongly affects the rain-induced increase of the surface roughness (i.e., the generation of crowns, cavities, stalks, ring waves, and secondary drops).