Abstract
Abstract. Accurate knowledge of cloud properties is essential to the measurement of atmospheric composition from space. In this work we assess the quality of the cloud data from three Copernicus Sentinel-5 Precursor (S5P) TROPOMI cloud products: (i) S5P OCRA/ROCINN_CAL (Optical Cloud Recognition Algorithm/Retrieval of Cloud Information using Neural Networks;Clouds-As-Layers), (ii) S5P OCRA/ROCINN_CRB (Clouds-as-Reflecting Boundaries), and (iii) S5P FRESCO-S (Fast Retrieval Scheme for Clouds from Oxygen absorption bands – Sentinel). Target properties of this work are cloud-top height and cloud optical thickness (OCRA/ROCINN_CAL), cloud height (OCRA/ROCINN_CRB and FRESCO-S), and radiometric cloud fraction (all three algorithms). The analysis combines (i) the examination of cloud maps for artificial geographical patterns, (ii) the comparison to other satellite cloud data (MODIS, NPP-VIIRS, and OMI O2–O2), and (iii) ground-based validation with respect to correlative observations (30 April 2018 to 27 February 2020) from the Cloudnet network of ceilometers, lidars, and radars. Zonal mean latitudinal variation of S5P cloud properties is similar to that of other satellite data. S5P OCRA/ROCINN_CAL agrees well with NPP VIIRS cloud-top height and cloud optical thickness and with Cloudnet cloud-top height, especially for the low (mostly liquid) clouds. For the high clouds, S5P OCRA/ROCINN_CAL cloud-top height is below the cloud-top height of VIIRS and of Cloudnet, while its cloud optical thickness is higher than that of VIIRS. S5P OCRA/ROCINN_CRB and S5P FRESCO cloud height are well below the Cloudnet cloud mean height for the low clouds but match on average better with the Cloudnet cloud mean height for the higher clouds. As opposed to S5P OCRA/ROCINN_CRB and S5P FRESCO, S5P OCRA/ROCINN_CAL is well able to match the lowest CTH mode of the Cloudnet observations. Peculiar geographical patterns are identified in the cloud products and will be mitigated in future releases of the cloud data products.
Highlights
For decades the global distribution of atmospheric constituents has been monitored by ultraviolet/visible/nearinfrared (UV/VIS/NIR) spectrometers measuring at the nadir of a satellite the radiance scattered by the Earth’s atmosphere and reflected by its surface
Sentinel-5 Precursor (S5P) OCRA/ROCINN_CAL radiometric cloud fraction1 (RCF) and Visible/Infrared Imager/Radiometer Suite (VIIRS) GCF show a similar latitudinal variation, but, as expected, the geometrical cloud fraction is higher than the S5P OCRA radiometric cloud fraction (Loyola et al, 2010)
While similar cloud features can be discerned in the S5P OCRA/ROCINN_CAL and the National Polar-orbiting Partnership (NPP) VIIRS plots, there are quantitative differences
Summary
For decades the global distribution of atmospheric constituents has been monitored by ultraviolet/visible/nearinfrared (UV/VIS/NIR) spectrometers measuring at the nadir of a satellite the radiance scattered by the Earth’s atmosphere and reflected by its surface. Since GOME, all UV/VIS nadir sounders with the exception of OMI have included measurements of the oxygen A-band around 760 nm, from which two independent cloud parameters can be retrieved (Schuessler et al, 2014) – in addition to cloud height, either cloud fraction (Stammes et al, 2008) or cloud optical thickness (Loyola et al, 2010). Operational for TROPOMI on Sentinel-5 Precursor (Loyola et al, 2018), the OCRA/ROCINN cloud retrieval scheme will be used operationally for the upcoming UVN instrument on Sentinel-4, the first mission for a geostationary view of air quality over Europe. Cloud height of the S5P products is compared with ground-based Cloudnet data in Sect.
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