Abstract
Abstract. Total column water vapour has been retrieved from TROPOMI measurements in the visible blue spectral range and compared to a variety of different reference data sets for clear-sky conditions during boreal summer and winter. The retrieval consists of the common two-step DOAS approach: first the spectral analysis is performed within a linearized scheme and then the retrieved slant column densities are converted to vertical columns using an iterative scheme for the water vapour a priori profile shape, which is based on an empirical parameterization of the water vapour scale height. Moreover, a modified albedo map was used combining the OMI LER albedo and scaled MODIS albedo map. The use of the alternative albedo is especially important over regions with very low 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 uncertainty sources. 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, and at high solar zenith angles it reaches values around 20 %–50 %. In the framework of a validation study the retrieval demonstrates that it can well capture the global water vapour distribution: the retrieved H2O vertical column densities (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 the reference data sets, in particular over tropical land masses. However, over land the retrieval underestimates the VCD by about 10 %, particularly during summertime. Our investigations show that this underestimation is likely caused by uncertainties within the surface albedo and the 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 these biases can probably be further reduced by the use of improved cloud input data. For the general purpose we recommend only using VCDs with cloud fraction <20 % and AMF >0.1, which represents a good compromise between spatial coverage and retrieval accuracy. The TCWV retrieval can be easily applied to further satellite sensors (e.g. GOME-2 or OMI) for creating uniform, long-term measurement data sets, which is particularly interesting for climate and trend studies of water vapour.
Highlights
Water vapour is the most important natural greenhouse gas in the atmosphere and plays a key role in the atmospheric energy balance via radiative effects and latent heat transport (Held and Soden, 2000)
The parameters of the linear regression and the coefficient of determination are given in the box in each panel
We introduce a total column water vapour retrieval from TROPOMI spectra in the visible blue spectral range using an iterative vertical column conversion scheme and provide a detailed characterization of our retrieved H2O vertical column densities (VCDs) by performing a detailed uncertainty analysis and intercomparisons to reference data sets from the microwave sensor Sensor Microwave Imager/Sounder (SSMIS), from the reanalysis model ERA-5, and from the ground-based GPS network SuomiNet
Summary
Water vapour is the most important natural greenhouse gas in the atmosphere and plays a key role in the atmospheric energy balance via radiative effects and latent heat transport (Held and Soden, 2000). The particular absorption properties of water vapour allow for the retrieval of the water vapour content via satellites for several different spectral ranges from the radio (Kursinski et al, 1997), microwave, e.g. AMSU (Rosenkranz, 2001), thermal infrared, e.g. AIRS (Susskind et al, 2003), near. The visible spectral range is interesting for the retrieval of total column water vapour (TCWV): in contrast to the microwave range it has a similar sensitivity for the ocean and land surface, allowing for global coverage. TCWV has been retrieved mostly in the visible “red” spectral range because the absorption is strongest there For this spectral range the ocean surface albedo is relatively low, leading to a low sensitivity for the lowermost troposphere, where the highest water vapour concentrations occur.
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