A Ku-band laboratory experiment on the electromagnetic bias

A uniformly valid algorithm relating the normalized (or nondimensional) sea-surface electromagnetic (EM) bias to physical variables has not yet been established. Laboratory experiments are conducted to guide model development. Simultaneous and collocated measurements of surface topography and altimeter backscattered power are made in a wind-wave facility for a wide range of wind and mechanically generated wave conditions. A small microwave footprint on the water surface is produced by a focused-beam 13.5-GHz radar system that has a high signal-to-noise ratio. Specular facets are easily identifiable, and the data show that troughs are on average better reflectors than crests. The laboratory coefficients are considerably greater than those of in situ algorithms, and when the normalized EM bias is displayed as a function of wave height skewness or wave age, laboratory and field data converge into consistent trends. A two-parameter model is proposed using a nondimensional wave height, which is computed for local winds, and a significant slope, which is computed for nonlocally generated waves. Analysis of the laboratory data shows that the normalized EM bias for mixed conditions is well modeled as a product of these two parameters