Operational Cloud Products In The UV-VIS-NIR: From Sentinel-5 Precursor Towards Sentinel-4 And Sentinel-5

Kurzfassung

The Copernicus missions focused on atmospheric composition and trace gas retrieval operate in the UV/VIS/NIR/SWIR spectral region. For an accurate trace gas retrieval, also a precise knowledge of the cloud properties in this wavelength region at each given scene is required. In this work we present the algorithms for retrieving the operational cloud products from the low earth orbit (LEO) Sentinel-5 Precursor and Sentinel-5 missions and the geostationary (GEO) Sentinel-4 mission. The cloud retrieval algorithms OCRA (Optical Cloud Recognition Algorithm) and ROCINN (Retrieval of Cloud Information using Neural Networks) have their heritage with GOME/ERS-2 and GOME-2 MetOp-A/B, where they have already been successfully implemented in an operational environment. The ROCINN algorithm retrieves cloud height, cloud optical thickness and cloud albedo from NIR measurements in and around the oxygen A-band, taking as input the cloud fraction computed with the OCRA algorithm that is based on a broad-band UV/VIS/NIR color space approach. This OCRA color space approach makes use of the assumption that clouds usually have a higher reflectivity than the surrounding surface and that the cloud reflectivity is almost wavelength independent across the UV/VIS/NIR region. Assigning R, G and B colors to respective broad-band reflectances in the NIR, VIS and UV leads to clouds appearing white in the normalized RGB color space. The scene furthest away from the white point is the one where we expect the least possible amount of cloud contamination. Joining all these scenes on a global grid allows to generate a cloud-free reflectance background composite map which, together with the measured reflectance, can be used to calculate a radiometric cloud fraction for any given measurement. The cloud height, optical thickness and albedo retrieved by ROCINN are provided for two different cloud models. One which treats clouds more realistically as layers of scattering water droplets (clouds-as-layers, CAL) and one which treats clouds as simple Lambertian reflectors (clouds-as-reflecting boundaries, CRB). Initial comparisons and validation results are shown for Sentinel-5P and perspectives are outlined for Sentinel-4.