Validation of MERIS water products and bio‐optical relationships in the Skagerrak

The MERIS Level 2 Reduced Resolution products for Case 2 water available in September 2003, and conversion functions used in the reference model, have been validated against in situ data from the Skagerrak, collected during the summers of 2002 and 2003. The MERIS water‐leaving reflectance deviated less than 20% from the measured reflectances in the blue‐green part of the spectrum, but it had a tendency of being overestimated by up to 40% in the blue part of the spectrum and underestimated by up to 20% in the red part. The average relative deviation between the MERIS product for total suspended material (dry weight) and the in situ values was 30%. The MERIS values for chlorophyll a were on an average a factor 2 higher than the in situ values, and a new conversion factor should be used for the Skagerrak area. The absorption coefficients for the sum of yellow substance and bleached particles at 442 nm were underestimated by a factor of up to 10 by the MERIS product. The mean values of the spectral slopes of particle scattering and bleached particle absorption were close to the values of the reference model, while the observed slope of yellow substance was slightly lower than the model slope.