Dependence of radar backscatter on the energetics of the air-sea interface

M.C. Colton
10th Annual International Symposium on Geoscience and Remote Sensing  
Approved for public release; distribution Is unlimited. Naval Oceanographic and Atmosph -Ic Resoarch L aboratory, Stennis Space Centor, Mississippi 39529-5004. Foreword These are times of global change. Politicians are challenged by the reversal of communist dogma, while scientists are challenged to reverse dangerous climatic trends. By necessity, the strategy of the U.S. military will evolve to include more passive defense measures to assure peaceful transitions and maintain global stability.
more » ... he role of remote sensing satellites cannot be underestimated at this early stage of a "whole Earth" philosophy. Satellites will provide the surveillance data needed to verify arms treaties and the synoptic geophysical data required for either real-time monitoring of environmental conditions or data assimilation into prognostic models. However, the quality of these remotely sensed data will be limited by our understanding of the physical mechanisms which produce the measured signals. Surface wind over the oceans is an example of a physical phenomenon which can be sensed from remote platforms and which is paramount in monitoring the types of global changes descr!, ed. The wind affects both strategic and tactical naval operations, drives ocean currents and determines air-sea fluxes important for climate modeling. In 1978 a microwave radar scatterometer onboard SEASAT, the first oceanographic satellite, used an algorithm known as SASSI to infer wind velocity from the measured radar returns. Twelve years later, the scatterometer data from this mission are still being analyzed, not only to confirm the instrument as a breakthrough in maritime meteorology and oceanography, but also to reveal both the shortcomings of the SASS1 algorithm and the possibility that the radar return is sensitive to environmental parameters other than wind. This study evaluates environmental effects on the functional form, which relates radar echoes from the sea surface to the sea surface wind vector. Normalized Radar Cross-Section (NRCS), the fundamental measuremfnt 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. WAVES87 was performed from a research tower located in Lake Ontario, on which two n.,orowave scatterometers operating at 14.0 GHz and 5.0 GHz were installed for six weeks in the autumn of 1987. The novel aspect of this experiment was that the 14.0-CHz radar automatically rotated through 300' in azimuth angle at six different incidence angles to the water surface. Simultaneous measurements of wind stress and high resolution directional wave spectra were made. Therefore, the incidence and azimuthal angle behavior of the NRCS was examined as a function of wind speed, friction velocity, wind direction, wave 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 versus the 19.5-m wind speed curves at intermediate 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 slopes 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 (0.73). The minima in the sinusoidal modulation of the NRCS as a function of the relative angle between the wind and the antenna are often shifted such that the minima do not always occur in the cross-wind direction. 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 rather less than 1800. The dependence of the NRCS on atmospheric stability shows that the NRCS decreases by about 5 dB between air-water temperature differences of about -16 0 C to +10 0 C. This stability effect is removed by parameterization of the NRCS in terms of either friction velocity or neutral wind at I m, with the neutral wind speed providing the best normalization of the data. Acknowledgments Funding for this effort was provided by the Office of Naval Technology through the Atmospnerc Remote aensing program (Program Element 62435N), previously managed by Mr. Glenn Spalding and currently by CDR Lee Bounds.
doi:10.1109/igarss.1990.688599 fatcat:xiy52rrccvfvzkwnrkiyx6pkra