Global Statistics of Extreme Windspeed and Sea State from SAR

Abstract

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 field and sea state from space independent of daylight and cloud coverage. This makes SAR particularly suited to investigate severe weather events over the oceans. ERS SAR wave mode is acquired over the ocean every 200 km along the satellite track as a 5 km x 10 km image, whenever the 100 km x 100 km ERS image mode is not requested. Thus ERS wave mode yields about 1000 small SAR images daily over the global oceans. Derived image spectra are used at weather centers to improve ocean wave forecast by model validation and assimilation. In this paper we use the information available in the SAR images in addition to the spectral information in order to generate global maps of meteo-marine parameters. In the scope of the project WAVEATLAS, ESA provided a two year wave-mode dataset of ERS-2 SAR raw data, mainly collected during 1999 and 2000, which was reprocessed to single-look-complex images at DLR using the BSAR processor [1]. New algorithms to determine individual and spectral sea state parameters together with the wind speed over the ocean from SAR images were developed at the German Aerospace Center (DLR), see [2], [3] and [4]. The derived parameters like wind speed u10 (in 10m height), significant wave height Hs, mean period Tmean and individual maximum wave height Hmax and their respective regional and temporal variability are compiled into global ocean maps of 3° x 3° resolution for the seasons of the year 1999 and 2000.