Meris Land Products Validation

Over land, the aerosol remote sensing is based on the observation of Dense Dark Vegetation (DDV) and this concept is applied on MERIS with a spectral index (ARVI, Atmospherically Resistant Vegetation Index) to detect the DDV and the use of the bands at 412, 443 and 670 nm to characterize the aerosols. The aerosol size distribution is assumed to follow the Junge law while the aerosol refractive index is set to 1.44. The aerosol product consists on the aerosol optical thickness (AOT) at 865 nm and on the spectral dependence of the aerosol path radiance (Epsilon coefficient ε which is the ratio of the aerosol reflectance at 765 nm to that at 865 nm). The validation exercise is mostly based on the use of ground based optical measurements from the AERONET network. A classical validation of the aerosol product is conducted using the extinction measurements. A deeper validation is done in order to investigate the different assumptions used in the aerosol remote sensing module by: (i) using the ground based measurements to validate the DDV reflectance model. Atmospheric correction will be done, including the aerosols, to derive DDV reflectances for comparison to standard values. (ii) using the ground based measurements to validate the choice of the Junge size distribution by comparing the simulated radiances with this model to the measurements in the principal plane. The AOT at 865 nm is badly retrieved because of the inaccuracy of the DDV reflectance model in the red whereas the AOT at 443 nm is in good agreement with AERONET data and accuracy is comparable to what is achieved by MODIS over comparable sites. The Junge size distribution is well adapted for the representation of aerosols optical properties. The main algorithm improvement we recommend consists in introducing a dynamical DDV reflectance model that is a reflectance which varies with the ARVI of the target. Under clear sky conditions, the surface pressure is a level-2 MERIS product based on a two band ratio between 761.75 nm and 753 nm. This ratio is first corrected from the coupling between scattering and absorption to get the oxygen transmittance TO2. TO2 is pre computed versus the air mass m time P to simply derive P from look up tables. The validation of the product itself is done by comparison to the barometric pressure collected from ECMWF and Digital Elevation Model attached to the MERIS product.