The surface pressure retrieval in the MERIS O/sub 2/ absorption: validation and potential improvements

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 T/sub o2/. T/sub o2/ 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. Above very bright surfaces, the two band ratio directly corresponds to the oxygen transmittance and we can validate the spectral characterization of the instrument as well the as the line by line computations done to simulate T/sub o2/. Above less bright targets, we have to correct from the coupling between scattering and absorption and this correction will be evaluate on different types of surfaces. We will mainly investigate the variation of this correction with the smile effect and air mass. It is also shown how to use the retrieved pressure for flagging cirrus clouds.