Surface pressure retrieval from SCIAMACHY measurements in the O2 A Band: validation of the measurements and sensitivity on aerosols

摘要

We perform surface pressure retrievals from cloud-free Oxygen A bandmeasurements of SCIAMACHY. These retrievals can be well validatedbecause surface pressure is a quantity that is, in general, accuratelyknown from meteorological models. Therefore, surface pressureretrievals and their validation provide important insight into thequality of the instrument calibration. Furthermore, they can provideinsight into retrievals which are affected by similar radiationtransport processes, for example the retrieval of total columns ofHO, CO, CO and CH. In our retrieval aerosols areneglected. Using synthetic measurements, it is shown that for low tomoderate surface albedos this leads to an underestimation of theretrieved surface pressures. For high surface albedos this generallyleads to an overestimation of the retrieved surface pressures. Thesurface pressures retrieved from the SCIAMACHY measurements indeedshow this dependence on surface albedo, when compared to thecorresponding pressures from a meteorological database. However, anoffset of about 20 hPa was found, which can not be caused byneglecting aerosols in the retrieval. The same offset was found whencomparing the retrieved surface pressures to those retrieved fromco-located GOME Oxygen A band measurements. This implies a calibrationerror in the SCIAMACHY measurements. By adding an offset of 0.86% ofthe continuum reflectance at 756 nm to the SCIAMACHY reflectancemeasurements, this systematic bias vanishes.