The Wavenumber Spectra of Scatterometer-derived Winds
نویسندگان
چکیده
Spaceborne scatterometers are the only proven method for global all-weather measurement of vector winds at the ocean’s surface. Such measurements are critical inputs in studies of oceanic circulation and air/seainteraction where the time variability of the surface wind field and the wind stress curl drive the ocean; hence, our interest in the spectrum of the surface wind field. In this paper we address the accuracy of scatterometer-derived winds using a frequency domain analysis and simulation. While using simulation of SASS measurements to evaluate new methods of wind estimation from scatterometer measurements, the wavenumber spectra of the SASS-derived wind fields were observed to be accentuated relative to the input wind field. If similar accentuation occurs in winds estimated from actual scatterometer measurements, the wind spectrum will be overestimated, which may lead to erroneous inputs to ocean forcing models. This paper reports the results of extensive simulations designed to t.est this observation. In these experiments actual SASS measurements of the normalized radar backscatter (o”) over an orbit (rev) were used as a template to generate simulated oo measurements. Poilitwise wind estimation (traditional wind retrieval) of winds from the simulated U’ measurements was then done with the ambiguity closest to the true wind selected as the unique wind vector estimate. For comparison, winds were also estimated using a new model-based approach. After wind retrieval, the spectra of the estimated wind fields were computed and compared to the input wind field. The high wavenumber portion of the spectra of the point-wise estimated winds was higher than the spectra of the true winds by an amount which depended on the wind speed variance. The model-based spectra exhibited a high degree of agreement with the true input spectra. We discuss the implications these results have for actual SASS data.
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تاریخ انتشار 2004