LAND-SURFACE PRESSURE ESTIMATE FROM MEASUREMENTS IN THE OXYGEN-A ABSORPTION-BAND

The POLDER (polarization and directionality of the earth reflectances) instrument to be launched in 1996 carries two channels that cover the oxygen A absorption band(near IR). The authors investigate the possib ility of using these measurements to achieve cloud detection: An estim ate of the surface pressure is made from the two measurement ratios. T his apparent pressure can then be compared to what is expected in clea r conditions. For this objective, the authors analyze here the uncerta inty on the apparent pressure in clear conditions through radiative tr ansfer simulations. It is found that the radiometric noise yields an u ncertainty on the order of 20 hPa. The variability in the temperature profile has a negligible influence on the apparent pressure. On the ot her hand, the spectral shape of the surface reflectance yields a varia bility of 160 hPa in the apparent pressure. The aerosol effect on the apparent profile depends on their altitude. A decrease in apparent pre ssure of 14 hPa is found for a typical stratospheric aerosol layer of optical thickness 0.05 and altitude 20 km. A boundary layer aerosol ha s a smaller influence. In clear conditions, the uncertainty on the app arent pressure is on the order of 100 hPa. Therefore, a cloud-detectio n test based on the oxygen absorption will only be able to detect medi um and high clouds.