Total column water vapour measurements from GOME-2 MetOp-A and MetOp-B

摘要

Knowledge of the total column water vapour (TCWV) global distribution isfundamental for climate analysis and weather monitoring. In this work, wepresent the retrieval algorithm used to derive the operational TCWV from theGOME-2 sensors aboard EUMETSAT's MetOp-A and MetOp-B satellites and performan extensive inter-comparison in order to evaluate their consistency andtemporal stability. For the analysis, the GOME-2 data sets are generated byDLR in the framework of the EUMETSAT O3M-SAF project using the GOME DataProcessor (GDP) version 4.7. The retrieval algorithm is based on a classicalDifferential Optical Absorption Spectroscopy (DOAS) method and combines aH₂O and O₂ retrieval for the computation of the trace gas verticalcolumn density. We introduce a further enhancement in the quality of theH₂O total column by optimizing the cloud screening and developing anempirical correction in order to eliminate the instrument scan angledependencies. The overall consistency between measurements from the newerGOME-2 instrument on board of the MetOp-B platform and the GOME-2/MetOp-Adata is evaluated in the overlap period (December 2012–June 2014).Furthermore, we compare GOME-2 results with independent TCWV data from theECMWF ERA-Interim reanalysis, with SSMIS satellite measurements during thefull period January 2007–June 2014 and against the combined SSM/I + MERISsatellite data set developed in the framework of the ESA DUE GlobVapourproject (January 2007–December 2008). Global mean biases as small as ±0.035 g cm?2 are found between GOME-2A and all other data sets. Thecombined SSM/I-MERIS sample and the ECMWF ERA-Interim data set are typicallydrier than the GOME-2 retrievals, while on average GOME-2 data overestimatethe SSMIS measurements by only 0.006 g cm?2. However, the size of thesebiases is seasonally dependent. Monthly average differences can be as largeas 0.1 g cm?2, based on the analysis against SSMIS measurements, whichinclude only data over ocean. The seasonal behaviour is not as evident whencomparing GOME-2 TCWV to the ECMWF ERA-Interim and the SSM/I+MERIS data sets,since the different biases over land and ocean surfaces partly compensateeach other. Studying two exemplary months, we estimate regional differencesand identify a very good agreement between GOME-2 total columns and all threedata sets, especially for land areas, although some discrepancies (biaslarger than ±0.5 g cm?2) over ocean and over land areas with highhumidity or a relatively large surface albedo are observed.