Effects of sea maturity on satellite altimeter measurements

Abstract

For equilibrium arid near‐equilibrium sea states, the wave slope variance is a function of wind speed U and of the sea maturity. The influence of both factors on the altimeter measurements of wind speed, wave height, and radar cross section σ0 is studied experimentally. The necessary theoretical background, both on wave statistics and on the near‐nadir radar backscatter, is outlined. It is based on a recently proposed view that the effective fractal dimension for a surface patch corresponding to the equilibrium range of wave spectra increases as the wave field matures. The experimental part is based on 1 year's worth of Geosat altimeter observations colocated with in situ wind and wave measurements by 20 NOAA buoys. Errors and biases in altimeter wind speed and wave height measurements are investigated. A geophysically significant error trend correlated with the sea maturity (i.e., with C0/U, where C0 is the phase velocity of waves corresponding to the spectral peak and U is the mean wind speed well above the surface) is found in wind speed measurements. This trend is explained by examining the effect of the generalized wind fetch, , on the curves of the observed dependence σ0 = f(U, X). We conclude, in particular, that unambiguous measurements of wind speed by altimeter, in a wide range of sea states, are impossible without accounting for the actual degree of wave development. An approach to a more accurate retrieval of wind speeds is suggested as based on accounting for the actual degree of wave development, which can be achieved through processing the total radar cross section simultaneously with the significant wave height information contained in altimeter waveforms. The potential capability of a satellite altimeter to estimate wave age and similar measures of wind‐wave coupling is discussed. This capability expands the use of satellite altimeters for air‐sea interaction studies.