Long Term Assessment of Ers-1 and Ers-2 Wind and Wave Products

The wind and wave products from the ERS mission provide an invaluable data set. This paper provides an overview of the performance of both ERS-1 and ERS-2 products on the basis of surface winds from the consistent ECMWF 40-Year Reanalysis (ERA-40), and wave fields from a long term stand-alone wave model hindcast run forced by ERA-40 winds. Furthermore, buoy and other in-situ measurements are used in the assessment. The products involved are the fast delivery (FD) AMI scatterometer wind (UWI), the FD SAR Wave Mode (UWA), and the off-line OPR (Ocean Product) altimeter wind and wave products. The results show that ERS wind and wave products are generally of high quality. However, several abrupt changes in some ERS products are noticed. 1. THE ERS-1 AND ERS-2 SATELLITES The European Space Agency (ESA) started the European Remote Sensing (ERS) mission with the launch of ERS1 on 17 July 1991. ERS-1 has provided data until December 1999. During this lifetime, its orbit configuration was changed several times. Fig.1 summarises the major changes and the purpose of each configuration (c.f. Abdalla and Hersbach, 2006). On 21 April 1995, ERS-2 was launched and still operational. ERS-2 was unfortunate to suffer several problems as outlined in Fig. 2. In January 2000 some of its gyroscopes (gyros) stopped working. This forced the piloting of the spacecraft in what is called the ”mono-gyro mode”. A year later, more gyros were lost to force operational piloting without any gyros in what is called the ”Extra Back-up Mode (EBM)” leaving the only available gyro being operated during the ERS-2 manoeuvring. A procedure to aid the piloting of ERS-2 without the need of gyros known as the ”ZeroGyro Mode (ZGM)” was fully implemented in June 2001. There are three instruments providing wind and wave data. The Active Microwave Instrument (AMI) Synthetic Aperture Radar (SAR), while in the wave mode, images the ocean surface and provides SAR spectra which can be converted into ocean wave spectra (for the long waves). Radar Altimeter (RA) is able to measure the significant wave height (SWH) and the ocean surface wind speed. The AMI scatterometer is able to measure the surface wind speed and direction over the ocean. 2. ECMWF ERA-40 WIND AND WAVE FIELDS As both operational atmospheric and wave models at the European Centre for Medium-Range Weather Forecasts (ECMWF) are changing frequently, the operational ECMWF data archive does not represent a consistent data set to be used for the evaluation of the ERS products. The quality of both wind and wave fields are improving with time. The ECMWF 40-Year Reanalysis, widely known as ERA-40, Uppala [1], however, is an appropriate candidate since the same model was used for the entire period (September 1957 to August 2002). The horizontal resolution of the atmospheric component (about 125 km) is sufficient for a fair assessment for wind products. The resolution of the wave model component, however, was of a rather coarse resolution (about 150 km). This is a major concern for wave fields in the inner seas and near the coasts. Therefore, a long-term stand-alone wave model hindcast run with a resolution of 55 km and without data assimilation was forced by ERA-40 winds. The wave fields produced from this hindcast were used for the evaluation of ERS RA OPR wave products. 3. THE SYNTHETIC APERTURE RADAR (SAR) UWA PRODUCT SAR UWA product is inverted using an iterative inversion scheme based on the MPI Scheme which is a forward closed integral transformation Hasselmann and Hasselmann [2] to obtain the corresponding ocean wave spectra. Long-term verification is based on integrated parameters such as SWH. Only operational ECMWF wave model products are used for the verification of the UWI _____________________________________________________ Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007) ! " # # #$ #$ # %& %& #$ '( ) * + , $° ) * . + ) . / & 0 -