Inter-comparison of satellite ocean colour data products during algal blooms in the North Sea region in 2004

Several satellite-borne ocean colour Earth observation (EO) sensors are presently collecting data on an operational basis. This study aims at quantifying differences in ocean colour EO sensor performances for ocean monitoring and in particular the study of algal bloom situations. The motivation for the present study is to explore how data from different sensors can be utilized in one system for HAB detection and monitoring. Ocean products from the MODIS/Terra, MODIS/Aqua, MERIS, and SeaWiFS sensors have been processed and intercompared for data acquired during the development of an early spring algal bloom in 2004 in the North Sea region. The study assesses the comparability of these OC sensors in the cases of bloom and non-bloom situations. Particular focus has been on the assessment fo the quality of the MODIS/Terra data provided by Kongsberg Spacetec AS.
The conclusions are that the MODIS/Terra products processed and delivered by KSAT are inconsistent with equivalent products from derived from the other EO ocean colour sensors. We suspect the discrepancies to be mostly caused by the radiometric performance and the calibration procedure of the MODIS/Terra sensor used by Kongsberg Spacetec. Due to the discrepancies observed between the MODIS/Terra and the other sensors’ products, the quality of the present product is not satisfactory for use of the data in an operational system for algae bloom monitoring in Norwegian coastal waters. A further harmonisation with the processing tools and algorithms used in Tromsø should be done using the similar tools being developed and validated by NASA. We suggests that Spacetec undertake parallel processing of MODIS-Terra and -Aqua data as well as SeaWiFS data using their implementation of the software. It should also be considered to use the NASA SeaDAS OC4 software or other operational software implemented and tested by NASA.