Quantification of uncertainties of water vapour column retrievals using future instruments

[摘要] This study presents a quantification of uncertainties of total column watervapour retrievals based on simulated near-infrared measurements of upcominginstruments. The concepts of three scheduled spectrometers were taken intoaccount: OLCI (Ocean and Land Color Instrument) on Sentinel-3, METimage on anEPS-SG (EUMETSAT Polar System – Second Generation) satellite and FCI(Flexible Combined Imager) on MTG (Meteosat Third Generation). Optimalestimation theory was used to estimate the error of a hypothetical totalwater vapour column retrieval for 27 different atmospheric cases. The errorsrange from 100% in very dry cases to 2% in humid cases with a veryhigh surface albedo. Generally, the absolute uncertainties increase withhigher water vapour column content due to H₂O-saturation and decrease witha brighter surface albedo. Uncertainties increase with higher aerosol opticalthickness, apart from very dark cases. Overall, the METimage channel settingenables the most accurate retrievals. The retrieval using the MTG-FCIbuild-up has the highest uncertainties apart from very bright cases.

On the one hand, a retrieval using two absorption channels increases theaccuracy, in some cases by one order of magnitude, in comparison to aretrieval using just one absorption channel. On the other hand, a retrievalusing three absorption channels has no significant advantage over atwo-absorption channel retrieval.

Furthermore, the optimal position of the absorption channels was determinedusing the concept of the "information content". For a single channelretrieval, a channel at 900 or 915 nm has the highest mean informationcontent over all cases. The second absorption channel is ideally weaklycorrelated with the first one, and therefore positioned at 935 nm, in aregion with stronger water vapour absorption.

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