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).
Investigations of the sea state bias (SSB) in altimeter measurements of sea surface height (SSH) have been reported by many authors based on aircraft, sea tower, and satellite-borne observations. These investigations have resulted in several proposed algorithms of the form SSB = ell, where H is the significant wave height (SWH) and e is a nondimensional function of wind speed U and SWH available from altimeter measurements. In the present work, on the basis of the full set of Geosat and an 8month set of TOPEX altimeter measurements, all known algorithms are examined and a conclusion is reached that the altimeter-based U and SWH are insufficient to estimate the SSB correction with uniformly high accuracy. As a criterion of model performance we employ the value (called here the accuracy gain) by which the total variance of temporal changes in surface elevation is reduced owing to an SSB correction. This quantity is estimated for global data as well as for several selected regions of sufficiently large size. The linear geophysical model function (GMF) of the form e = a0 + a l U is shown to yield an improvement over the simplest GMF with a constant e. A three-parameter linear form e = ao + a l U + a2H produces somewhat better results. A two-parameter, physically based GMF relating e to the pseudo wave age • (where • is estimated using altimeter wind and SWH) yields even higher accuracy, while the three-parameter GMF of form e = ao + a l U + a2 U2 yields the highest accuracy gain for global data sets. However, in terms of the SSB values, the difference between different GMFs is marginal, and the accuracy gain (as a measure of the SSB models performance) is shown to have serious deficiencies. We find that for all the SSB models, globally tuned empirical parameters often yield unacceptably poor results for certain regions in which local physical conditions differ from the global average: when the globally tuned GMFs are applied to such regions, the SSB-related error in SSH may well exceed 5 cm. 1. contemporary models [e.g., Choy et al., 1984;Melville et al., 1991; Ray and Koblinsky, 1991; Walsh et al., 1991] assume Paper number 94JC00478. 0148-0227/94/94JC-00478 $05.00 SSB to be linearly proportional to significant wave height (SWH) with the proportionality coefficient e (introduced by equation (1) below) related to the local wind. An exception is given by a physically based model [Glazman and Srokosz, 1991; Fu and Glazman, 1991] which indicates that for global data, the SSB dependence on SWH is weaker. The theory suggests that in the absence of swell and ocean currents, SSB is controlled primarily by the degree of the sea development which can be crudely quantified by two (essentially nonlocal) nondimensional parameters: the wave age and the ratio of the wind fetch to the intrinsic inner scale of the gravity wave turbulence. Practical implementation of this model requires expressing these factors in terms of the satellite-reported quantities, which is not always possible. Thus the central issues addressed by the presen...
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).
Abstract. Using a recently proposed technique for statistical analysis of non-gridded satellite altimeter data, regime of long equatorially-trapped baroclinic Rossby waves is studied. One-dimensional spatial and spatiotemporal autocorrelation functions of sea surface height (SSH) variations yield a broad spectrum of baroclinic Rossby waves and permit determination of their propagation speed. The 1-d wavenumber spectrum of zonal variations is given by a power-law k-2 on scales from about 103 km to 104 km. We demonstrate that the observed wave regime exhibits features of soliton turbulence developing in the long baroclinic Rossby waves. However, being limited to second statistical moments, the present analysis does not allow us to rule out a possibility of weak wave turbulence.
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