Operational MERIS (MEdium Resolution Imaging Spectrometer) level-2 processing uses auxiliary data generated by two radiative transfer tools. These two codes simulate upwelling radiances within a coupled 'Atmosphere-Ocean' system, using different approaches based on the matrix-operator method (MOMO) and the successive orders (SO) technique. Intervalidation of these two radiative transfer codes was performed in order to implement them in the MERIS level-2 processing. MOMO and SO simulations were then conducted on a set of representative test cases. Results stressed both for all test cases good agreements were observed. The scattering processes are retrieved within a few tenths of a percent. Nevertheless, some substantial discrepancies occurred if the polarization is not taken into account mainly in the Rayleigh scattering computations. A preliminary study indicates that the impact of the code inaccuracy in the water leaving radiances retrieval (a level-2 MERIS product) is large, up to 50% in relative difference. Applying the OC2 algorithm, the effect on the retrieval chlorophyll concentration is less than 10%.
Remote sensed chlorophyll α (chlα) maps are useful to proceed to phytoplankton dynamics study and primary production modelization in coastal waters. However, chlα remote sensing depend on the bio-optical characteristics of waters masses which are site-specific and highly variable according to space and time in coastal waters. In order to study the seasonal variability of phytoplankton and bio-optic parameters as well as the factors controlling their spatio-temporal distributions, five mesoscale cruises (BIOPTEL) were carried out in the eastern English Channel during the spring 2000 (between February and October). Phytoplankton absorption spectrum showed a high variability according to space and time due to the algal community composition and the physiological state. Considering these scales of variation, a local model was defined. The variability of the two other components of marine absorption: yellow substances and non algal particles are also presented and related to exogenous parameters such as river flow, temperature and hydrodynamism. Furthermore, in order to appreciate how important are the local IOPs (Inherent Optical Properties) variability on AOPs (Apparent Optical Properties) restitution a sensitivity analysis was realized. In the frame of biomass estimation, marine reflectance were simulated considering the local absorption coefficients rather than the standard ones.
Rayleigh scattering targets over clear oceans under large solar and viewing angles have proved their efficiency to calibrate remote sensing instruments in the low wavelength channels (AVHRR C1, SPOT-HRV XS1). To obtain a good accuracy, this method needs an evaluation of the different contributors participating in the total TOA signal: aerosol, atmospheric conditions, foam, water reflectance. For instruments such as SPOT or VEGETATION, the near infra red band is used to estimate the aerosol content which is after transferred in the band to be calibrated depending on the considered aerosol type. Two methods are described for the spectral transfer of the aerosol optical depth: one using the interband calibration coefficient and a new one using the absolute calibration coefficient of the near infra red band. Simulations are done to evaluate the performances of these two methods using different extreme cases of solar and viewing zenith angles, wind speed, water vapor content and ozone amount. The results show the method using the absolute calibration coefficient is more accurate.
The successive orders of scattering method is used to solve the transfer equation in the ocean-atmosphere system including the polarization. The code is made fast by the use of the Fourier series expansion of the radiance. Through runs for conservative cases, we can assess that the relative accuracy of the code is of the order of few percent. We first applied to satellite radiance simulations the classical algorithm to determine for ocean case 1 waters the chlorophyll concentration showing that we can decouple the ocean and the atmosphere, first to apply atmospheric corrections assuming the water body as a Lambertian reflector, second to normalize the water leaving radiance Then, we emphasized the need to include the polarization to achieve correct estimates of the water leaving radiances; otherwise, the chlorophyll determinations are strongly affected.
Rayleigh scattering targets over clear oceans under large solar and viewing angles have proved their efficiency to calibrate remote sensing instruments in the blue channels. To obtain a good accuracy, this method needs an evaluation of the different contributors participant in the total TOA signal: aerosol, atmospheric conditions, foam, water reflectance. The near infrared is used to estimate the aerosol content which is after transfered in the band to be calibrated depending on the considered aerosol type. Atmospheric conditions and foam are evaluated with ECMWF data: water vapor content, surface pressure, wind speed. Two methods are described for the spectral transfer of the aerosol optical thickness and compared using different SPOT acquisitions: one using the absolute calibration coefficient and the other interband calibration coefficient. The absolute calibration coefficients are computed in the blue and green channels of SPOT3.