Dependence of radar backscatter on the energetics of the air-sea interface
Colton, Marie C.
Thornton, Edward B.
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The Normalized Radar Cross-Section (NRCS), the fundamental measurement made by radar scatterometers, was obtained as part of the Water-Air Vertical Exchanges 1987 (WAVES87) experiment. The experiment was designed to evaluate the effects of environmental parameters on the NRCS and was performed from a research tower located in Lake Ontario, on which two microwave scatterometers operating at 14.0 and5.0 GHz were installed for six weeks in the autumn of 1987. The novel aspect of this experiment was that the 14.0 GHz radar automatically rotated through 300 degrees in azimuth angle at six different incidence angles to the water surface, accompanied by simultaneous measurements of wind stress and high resolution directional wave spectra. Therefore, the incident and azimuthal angle behavior of the NRCS was examined as a function of wind speed, friction velocity, wind direction, and atmospheric stability. The dependence of the NRCS on wind speed for various incidence angles is similar to previous results. However, the slope exponents of the NRCS vs. 19.5 wind speed curves at intermediate incidence angles are higher than the corresponding open ocean measurements. Scaling the lake neutral wind speed data by the ratio of lake to ocean drag coefficients reduces the slope of the curves and suggests the drag coefficient has a sea state dependence. The correlation between NRCS and neutral wind speed at 1 m is higher (0.91)than between the NRCS and friction velocity (.073 at 40 degrees). The minima in the sinusoidal modulation of the NRCS as a function of relative wind angle (the angle between the wind and antenna directions) are often shifted (by as much as 45 degrees) such that the minima do not always occur at cross-wind angles. Instead, the angular distance between the NRCS minima in the case of a wind-wave sea appears to approximate the directional spread of the waves about the upwind direction, generally less than 180 degrees. The degree of the sinusoidal modulation of the NRCS with relative wind angle is highly correlated with significant slope and inverse wave age at 20 degree incidence angle (0.90) and moderately correlated at 40 degrees (0.75); i.e., increased azimuthal modulation at 20 degrees is associated with a steeper wave field. The dependence of the NRCS on atmospheric stability shows the NRCS to decrease by about 5 dB between air-water temperature difference of -16 and +10 degrees centigrade. This stability effect is removed by parameterization of the NRCS in terms of either the friction velocity or neutral wind speed at 1m, with the neutral wind speed providing the best normalization of the data. The result show that radar scatterometers are an especially sensitive means by which to study the air-sea interface; the magnitudes of the 5.- GHz and 14 GHz NRCS respond nearly instantaneously to changes in the near-surface neutral wind speed, but the directionality of the (Ku-band) NRCS is the result of complicated interrelationships among the influencing environmental variables.
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