Statistical Analysis of a Two Year Global Dataset of Re-Processed ERS-2 Imagettes over the Global Oceans
Susanne Lehner(1) , Thomas König(1) , Johannes Schulz-Stellenfleth(1) , and Birgit Schättler(1)
Deutsches Zentrum für Luft- und Raumfahrt e.V.,
In the scope of the ESA AO WaveAtlas, ESA provided a two years wave-mode dataset of ERS-2 raw data, mainly collected in 1999 and 2000, which was reprocessed to single-look-complex imagettes at DLR using the BSAR processor. ERS-2 was and is operated in wave-mode over the global oceans whenever no image-mode acquisition is requested. So, the dataset contains typically between 1300 and 1500 images of 10 km by 5 km size from all global oceans daily. Recently it has been shown that wind speed and wave spectra as well as information on individual waves can be derived from these data.
In this study the data were used for a statistical evaluation providing global ocean maps of different basic image and oceanic parameters like, e.g., mean radar cross section and its standard deviation, the modulation depth, the image inhomogeneity, the significant wave height, the mean period and their respective regional and temporal variability during all seasons of the year for the first time. The analysis of these parameters form a basis for the retrieval of different more complex marine parameters, especially the estimation of two-dimensional sea surface elevation fields leading to higher level products such as inividual wave heights and freak wave statistics. The paper demonstrates the consequences of these results for different oceanographic applications and its high practical relevance for ship design or offshore operations. Products derived from SAR wave mode data are well suited for the assimilation of numerical wave models thus helping to improve wave forecast.
The reprocessing of wave mode data is planned to be extended to the full lifetime of ERS-1 and ERS-2, which spans at least the time peroid of 1991-2005. This will allow the derivation of wave climatologies on a decadal basis in particular enabling the analysis of climate trends.
As wave mode data are also available from the ENVISAT mission a consistent update of these climatologies is possible.