MERIS CASE 2 WATER VALIDATION EARLY RESULTS NORTH SEA / HELGOLAND /GERMAN BIGHT

Two MERIS scenes of case 2 water of the German Bight have been compared with measurements from ship. These first very preliminary tests have indicated that the atmospheric correction in the blue part of the MERIS spectrum is not sufficient. An alternative atmospheric correction, using a neural network, produces water leaving radiance reflectances, which are much closer to ship measurements. However, although the MERIS L2 processor indicates that the reflectance spectra are out of range, the derived concentrations show relative profiles along ship transects, which are still in reasonable agreement with observations. Most critical for further work is the atmospheric correction, including investigations on the impact by Rayleigh scattering, cirrus clouds and sun glitter. Furthermore, due to the high patchiness of substances in the German Bight, full resolution data are necessary to get a better comparison between water samples and MERIS data and their variances. OVERVIEW ABOUT VALIDATION ACTIVITIES AND AVAILABLE DATA The GKSS MERIS validation team has carried out a number of cruises during the spring-summer period of 2002 for the validation of MERIS level 2 data of coastal waters. They are listed along with the MERIS scenes, which were available until end of 2002, in Table 1. The weather of this period was mainly cloudy so that the number of blue sky days with a MERIS overpass was small. Only between mid August and end of September we had some stable periods, where validation experiments could be planned. Another limitation was that only two corrected MERIS scenes for which match up data exists were provided by ESA, shortly before the validation workshop. In addition to the level 2 scenes, which were produced with the MERIS processor, we have evaluated the scene of July 29, level 1b, with an experimental breadboard processor. It uses the same neural network for the retrieval of the optical properties of water constituents but has an alternative procedure for atmospheric correction, which is also based on a neural network. Our main validation cruise was carried out with the research vessel "Heincke" from April 23 to May 2. This period was chosen to cover the main phytoplankton bloom in the North Sea. When the application for this ship has to be made early 2001, the launch date for ENVISAT was still 2001. Unfortunately, MERIS data are now available only from April 29 on. A map of the stations is given in Fig. 1. Due to the cloudy weather during the summer 2002 we decided to change the strategy and do the validation measurements from a ship of opportunity, i.e. the ferry boat "Wappen von Hamburg", which operates daily between Cuxhaven and the island of Helgoland (s. Fig. 2). Advantage was that the decision for a cruise could be made on very short notice, disadvantage was that the ship cannot stop during the cruise for measuring vertical profiles. The water types, which are covered by this transect, ranges from turbid estuarine water at Cuxhaven to clear blue-green water around Helgoland so that a variety of different water constituents are covered by one match-up scene. Problem is that the distribution is very patchy. Further data for the validation exist from measurements with the GKSS ferry box. It is installed on the ferry boat, which sails between Cuxhaven and Harwich. This box consists of an autonomous package of instruments, which measure continuously variables such as temperature, salinity, turbidity, fluorescence of phytoplankton and nutrients so that transects of these variables exist for nearly every day. Beside the ship cruises we have operated a CIMEL sun photometer on the island of Helgoland for the validation of the atmospheric correction. It is part of AERONET, the global network of sun photometers for aerosol research. The validation was focussed on the water leaving radiance reflectance product, which is produced by the atmospheric correction procedure and which is critical for the success of the NN algorithm. In a further step we have compared the derived concentrations with in situ measurements.