Satellite altimeter measurements of surface wind

Abstract

Recent analyses of wind speed measurements by the Geosat altimeter showed that the radar cross section is affected by oceanographic factors, particularly by the degree of sea development, which are not directly accounted for in the geophysical model functions (GMF). In the present work, two new GMFs which account for the effects of the actual degree of sea development are proposed. Along with the radar cross section, these models use significant wave height information. One particular version is recommended for applications in oceanographic and climate studies where wind speed (or wind stress) data have to be binned (i.e., averaged over time and/or space intervals). The accuracy of this GMF (overall bias of 0.1 m/s and rms error of about 1.6 m/s) is higher than the accuracy of commonly employed GMFs, while the wave-age-related trend is reduced to a geophysically insignificant level. Finally, the wind speed histograms for the collocated data set are derived and compared with the ground truth data as well as with the histograms yielded by presently known GMFs. It is also shown that the accuracy of altimeter measurements could be increased even further if some additional information on the wave field were available from independent sources (e.g., the dominant wavelength from synthetic aperture radar images).

Relationships Between Ku-Band Radar Backscatter and Integrated Wind and Wave Parameters at Low Incidence Angles

“…The use of H s to characterize sea state was justified by the availability of collocated H s data obtained directly from the altimeter, which were previously shown to offer improvements in the precision of wind speed retrieval [4], [5], [26], [27]. Here, we show in Fig.…”

“…Due to an absence of dominant wave period and water depth data, the pseudo wave age has been introduced to characterize sea state development in the same manner as real wave age in other papers but is derived purely from wind speed and H s . One expression for pseudo wave age is given by Glazman and Greysukh [27] …”

Influence of Regional Features on Accuracy in Determining wind Speed Over the Oceans

“…Applicable to altimeter measurements, this matter was for the first time seriously examined in [1,2], and nevertheless the regional features even now are not taken into account when processing data. It is possible to avoid the error associated with the difference in wave climate in different regions if more detailed information is available concerning waves, such as information on surface slope dispersion.…”

Impact of fetch conditions on altimeter wind and wave measurements

“…The operational wind speed algorithm [3] is semi empirical, fitted through comparisons with buoy data, corresponding to open ocean wind and wave situations. Some impact of fetch and wave age has been observed [4,5,6] but one difficulty is to estimate accuratly these secondary parameters, particularly under the narrow footprint of the altimeter. Here we analyze TOPEX altimeter and in situ data in the Black Sea and in the Mediterranean Sea.…”