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. Total column water vapour has been retrieved from TROPOMI measurementsin the visible blue spectral range and compared to a variety of differentreference data sets for clear-sky conditions during boreal summer andwinter. The retrieval consists of the common two-step DOAS approach:first the spectral analysis is performed within a linearized schemeand then the retrieved slant column densities are converted to verticalcolumns using an iterative scheme for the water vapour a priori profileshape, which is based on an empirical parameterization of the watervapour scale height. Moreover, a modified albedo map was usedcombining the OMI LER albedo and scaled MODIS albedo map. The use ofthe alternative albedo is especially important over regions with verylow albedo and high probability of clouds like the Amazon region. The errors of the total column water vapour (TCWV) retrieval have been theoretically estimated considering the contribution of a variety of different uncertaintysources. For observations during clear-sky conditions, over ocean surface, and at low solar zenith angles the error typically is around values of 10 %–20 %, and during cloudy-sky conditions, over land surface, andat high solar zenith angles it reaches values around 20 %–50 %. In the framework of a validation study the retrieval demonstratesthat it can well capture the global water vapour distribution:the retrieved H2O vertical columndensities (VCDs) show very good agreement with the reference data sets over ocean for boreal summer and winter whereby the modified albedo map substantially improves the retrieval's consistency to thereference data sets, in particular over tropical land masses. However, over land the retrieval underestimates the VCD by about 10 %, particularlyduring summertime. Our investigations show that this underestimationis likely caused by uncertainties within the surface albedo andthe cloud input data: low-level clouds cause an underestimation, but for mid- to high-level clouds good agreement is found.In addition, our investigations indicate that thesebiases can probably be further reduced by the use of improvedcloud input data. For the general purpose we recommend only using VCDs with cloud fraction and AMF >0.1 , whichrepresents a good compromise between spatial coverage andretrieval accuracy. The TCWV retrieval can be easily applied to further satellitesensors (e.g. GOME-2 or OMI) for creating uniform, long-term measurement data sets, which is particularly interesting for climate and trend studiesof water vapour.

Total column water vapour retrieval from S-5P/TROPOMI in the visible blue spectral range