Directional spectrum of the sea surface and wind scatterometry

Abstract

This article is concerned with the spectral description of the ocean surface, as required for microwave remote sensing applications. Users are interested in the availability of efTicient inversion algorithms for the determination of geophysical parameters such as the wind speed or the wave heights. An accurate statistical description of the surface is required as an input for electromagnetic scattering and emissivity calculations. The basic question of the azimuthal symmetry of the surface spectrum is discussed in detail, with some emphasis on the interpretation of scatterometric measurements. Two definitions of the spectrum can be found in the literature: the classical statistical spectrum, here called the roughness spectrum, and the wave spectrum as used by oceano-graphers. The first one is symmetric in the (k, ω) space, the second one is not. Conditions of validity of the random plane wave representation, on which the wave spectrum is based, are discussed and the links with the roughness spectrum are stated explicitly. Finally measuring methods of the wave spectrum are briefly summarized.