Land Surface Pressure Estimate from Measurements in the Oxygen A Absorption Band

Abstract

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 possibility of using these measurements to achieve cloud detection: An estimate of the surface pressure is made from the two measurement ratios. This apparent pressure can then be compared to what is expected in clear conditions. For this objective, the authors analyze here the uncertainty on the apparent pressure in clear conditions through radiative transfer simulations. It is found that the radiometric noise yields an uncertainty on the order of 20 hPa. The variability in the temperature profile has a negligible influence on the apparent pressure. On the other hand, the spectral shape of the surface reflectance yields a variability of 160 hPa in the apparent pressure. The aerosol effect on the apparent profile depends on their altitude. A decrease in apparent pressure of 14 hPa is found for a typical stratosphere aerosol layer of optical thickness 0.05 and altitude 20 km. A boundary layer aerosol has a smaller influence. In clear conditions, the uncertainty on the apparent pressure is on the order of 100 hPa. Therefore, a cloud-detection test based on the oxygen absorption will only be able to detect medium and high clouds.