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Open Ocean Monitoring with MERIS

Andre Morel(1)

(1) Observatoire oceanologique de Villefranche, B.P. 08, Villefranche-sur-Mer, France


The MERIS mission has been conceived with a multidisciplinary character, even if the sensor specifications were mostly driven by open ocean observation requirements, which, actually, are the most stringent in terms of radiometric accuracy, sensitivity, and reliability. From this viewpoint, the instrument has remarkably met the goals. Despite the weakness of the marine signals drowned into the atmospheric signal, the sensor is able to resolve all the situations encountered in the open ocean.

The initial algorithms for the retrieval of the chlorophyll concentration (over 3 orders of magnitude, with an expected accuracy of 10 classes per decade) have proven to be valid. They have been slightly refined, as well as tested thanks to measurements carried out during recent dedicated cruises (BENCAL and BIOSOPE in particular). Also, "matchups" (temporal and spatial coincidences between satellite and field measurements) have been achieved in several locations in the open ocean, at the buoys BOUSSOLE and MOBY, and along cruise transects. These matchups nicely confirm both the radiometric quality of the remote measurements and the skill of the atmospheric correction scheme, which was specifically developed for MERIS and its near-infra-red channels. The comparison between MERIS products and those delivered by other ocean color sensors (SeaWiFS and MODIS) demonstrates that close agreements are reached when dealing with Case 1 waters. For instance, it has even been possible to successfully merge data from MERIS and MODIS into a unique level 3 product (ACRI Coastwatch product).

Regarding the mission of a global observation of the whole ocean, the compromise with other missions embarked on the same ENVISAT platform (not surprisingly) revealed to be somewhat detrimental. Indeed, the reduced field of view of the instrument (from 6 to 5 cameras) leading to a rather narrow swath (about 1150 Km) does not allow an efficient earth coverage, particularly in the inter-tropical zone. In principle (i.e. geometrically), a complete coverage is obtained within 3 days. The cloud coverage (inevitable), and the sun glint contamination, which affects extended portions (eastward) of the scenes considerably diminish the nominal capabilities. Also, the early over flight of the platform (Equator crossing time at about 10h, solar time) affects negatively the observation of the ocean, because frequent morning hazes over the sea are not yet dissipated by solar heating, and extended shadows from the clouds entail the rejection of a considerable number of pixels. Obviously, a solution for future instruments is a crossing time around noon, and a tilt capability allowing the sun glint to be avoided. An optimized use of a MERIS-like instrument, only by simply changing these orbital and mechanical characteristics, would double the amount of valid sampled pixels.

The biogeochemical objectives (photosynthetic activity, Carbon fluxes, modeling, assimilation...) require long term monitoring and adapted tools, not represented by sequences of individual images, but by large scale (basin, whole ocean) maps, averaged over adequate periods (weeks, months...), and documented about the significance and variance of the data used in the merging operations. Such level 3 data are not yet available (demonstration products have been elaborated, however). They can be meaningfully produced only if consolidated and homogeneous data at the Level 2 are regularly distributed. This disappointingly was not the case up to now, and explains that significant oceanographic applications of MERIS data have not yet emerged, after 3 years and half of operation in orbit. This situation, according to the latest news, is going to improve. The reconstruction of the past archive, and then its warranted maintenance are also important considerations for future biogeochemical and climate change oriented studies.


Workshop presentation