Apparent pressure derived from ADEOS-POLDER observations in the oxygen A-band over ocean

The POLDER radiometer was on board the ADEOS satellite from August 1996 to June 1997. This instrument measures radiances in eight narrow spectral bands of the visible and near infrared spectrum. Two of them are centered on the O-2 A-band in order to infer cloud pressure. By assuming the atmosphere behaves as a pure absorbing medium overlying a perfect reflector, an "apparent" pressure P-app is derived from POLDER data. For validation purposes, P-app is first compared to the sea-surface pressure P-s for clear-sky conditions; P-app is found to be close to P-s (within similar to 30 hPa) for measurements in the sunglint region. For overcast conditions, P-app differs from the cloud-top pressure mainly because of multiple scattering inside the cloud. When P-app is compared to the cloud pressure determined from brightness temperature measurements, large differences are observed (typically 180 hPa).