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Analysis of a global ERS-2 wave mode data set acquired over the ocean taking a synergetic approach

Guiting Song(1), Lehner Susanne(1), Johannes Schulz-Stellenfleth(1) and Hartmut Grassl(2)

(1) German Aerospace Center(DLR), Muenchener Str. 20, 82234 Wessling, Germany
(2) University of Hamburg, Bundesstr. 55, 20146 Hamburg, Germany

Abstract

The goal of this study is to develop and validate algorithms that can be used to measure ocean wave height and wind speed by synthetic aperture radar (SAR) without using a first guess from a numerical model. The work was performed in the framework of the virtual institute EXTROP. Significant wave height (Hs) and wind speed at ten meter height (U10) are derived from SAR imagettes using the new empirical algorithms CWAVE and CWIND developed at DLR. It is shown that the SAR wave height measurements are of the accuracy of altimeter data.

The wave mode SAR imagettes applied in this study are acquired by the ERS-2 satellite every 200 km along the satellite track. Between 1300 and 1500 imagettes were collected each day by the ERS satellites. About 1 million imagettes from September, 1998 to December, 2000 were processed at DLR and used for this study.

The results are validated by spatially and temporally collocated in situ data of NOAA buoys and observations undertaken on the German research vessel Polarstern. They are globally compared to other satellite measurements; wind speed is compared to ERS scatterometer and HOAPS passive microwave SSM/I data and Hs to ERS altimeter measurements. The HOAPS-II (Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Data set) contains global fields of different oceanic and atmospheric parameters (e.g. precipation, turbulent heat fluxes, evaporation minus precipitation, wind) derived from SSM/I data over the ice free ocean. In addition, the scatterometer data used in this study were provided by CERSAT and have 50km x 50km resolution. The ERS-2 altimeter provides Hs measurements each second along the orbit.

The validation between SAR results and in situ data shows rms order of less than 0.5 m in Hs and less than 2 m/s in U10. It is shown that SAR measurements of Hs are comparable in accuracy to altimeter measurements.

A statistical analysis of SAR derived measurements in comparison to ECMWF ERA 40 data is given. ERA-40 datasets contain reanalyzed values Hs and U10 interpolated to a 2.5° x 2.5° regular latitude/longitude grid at the four synoptical hours 00:00, 06:00, 12:00 and 18:00 UTC each day. Cases of difference between SAR measurements and model are discussed. In particular the role of inhomogeneous structures found on some of the wave mode imagettes is discussed.

Using SAR results and ERA-40 data the inter-annual and seasonal variability of Hs and U10 is investigated. Case of high wind speed and high Hs are analyzed and areas of high storm activity are mapped.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry