Local climatology of wind and sea state by means of satellite radar altimeter measurements

Abstract

The wind speed and significant wave height (SWH) climatology at a given location of the ocean is a matter of concern in marine technology for planning offshore structures or ship routing as well as in meteorology and oceanography. The purpose of this paper is to present a feasibility study of a new approach to determine the local sea state and wind speed climatology using satellite‐borne radar altimeter. The Geosat data for the first two years (cycles 1 to 43) of the Exact Repeat Mission, and the in situ data from the Quarter Platform (QP) of the Frigg oil field (59°53′N, 2°3′E) in the North Sea have been used for this study. Because of the satellite sampling scheme, a given location of the ocean might never be overflown by the Geosat radar altimeter. Thus, altimetric measurements have to be considered for an area surrounding the location where the climatology is to be estimated. Thus the main question for the determination of the local climatology of wind and SWH is this: given the time and space sampling of Geosat, is it possible to estimate the statistical characteristics of a random process ? Analysis of the Geosat altimeter wind speed and SWH data set over concentric zones surrounding QP Frigg shows that up to 200 km, the SWH and wind speed data set belong to the same population, and that the estimated probability laws are very stable. Comparison of the results of this analysis with the data set of in situ measurements available from QP Frigg during the same period shows that the SWH climatology computed from altimetric measurements in a zone of 100 km radius surrounding the platform is very close to the one computed from in situ data. As compared to the in situ measurements, the wind speed variability is not well represented by the altimetric measurements and the variance estimated from the altimetric data set is lower by more than 20%. This can be attributed to poor signal dynamics with respect to the characteristic noise level in the altimeter wind measurements.