The Sar Measurement of Ocean Waves: Wave Session Whitepaper

Remote sensing techniques enable the measurement of ocean wave fields with both high resolution and large coverage. As the acquisition of active radar data is independent of daylight and cloud conditions, these data are therefore believed to be most suited for operational use at weather centers and governmental agencies, as well as for many ongoing scientific investigations. In the present overview paper the determination of sea state parameters from radar images together with some results from the operational use at weather centers are given. Synthetic aperture radar (SAR) yields high resolution two dimensional images of the radar backscatter properties of the sea surface and can thus be used to measure wind fields and sea state from space. The determination of ocean wave spectra from SAR image spectra is sensitive to various imaging effects due to sea surface features, spatial variation of wind speed, rain, current and motion of the sea surface. Thus, measuring ocean wave parameters is related to most of the other measurements of marine parameters described in this volume. In this paper an overview of the SAR satellite missions is given, as well as a description of the basic techniques to measure ocean waves by SAR and validation against wave model results and other space borne and in situ measurements. The full two dimensional SAR image spectrum provides information on the directional ocean wave spectrum. This can be used to determine expectation values of, e.g. , significant wave height, mean wavelength and direction in order to improve the wave model prediction. In addition information can be gathered from the radar images directly such as individual wave height, crest length and groupiness of the ocean waves. Thus, the distribution of maximum wave height can be investigated globally, or wave refraction and diffraction in coastal areas can be studied in detail. Fig. 1: 10 x 15 km cutout from a SEASAT SAR image acquired on 20 Aug 1978 at the Portuguese coast showing swell refraction, wave breaking and internal waves. In the following an overview of ocean wave measurements from space are given, the papers presented in the wave session are put into the general context and applications for weather prediction, ship routing, oil and shipbuilding industry and ship classifying societies are discussed. A general overview on theory and measurement of directional wave spectra, including a special chapter on remote sensing of directional ocean wave spectra together with an extensive literature list is given in the