Cloud Products for the Atmospheric Copernicus Missions Sentinel-5 Precursor and Sentinel-4

Abstract

The atmospheric Copernicus missions Sentinel-5 Precursor (S5P) and Sentinel-4 (S4) are focused on trace gas, aerosol and cloud properties retrieval in the UV/VIS/NIR/SWIR spectral region with applications in air quality and climate monitoring. With S5P being in orbit since October 2017, global daily data covering all seasons are already available. S4, anticipated to be launched around 2022, will focus on hourly measurements over Europe and give more insights in diurnal variations. While clouds themselves play a crucial role for the Earth radiation budget, they are also an important input to the trace gas retrievals for air quality and climate applications. The cloud properties need to be well characterized in the UV/VIS/NIR spectral region where most of the trace gases are retrieved. Here we present the operational cloud products from the S5P mission and the prospects for S4. 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. OCRA employs a broad band color space approach to determine a radiometric cloud fraction and assumes that the cloud reflectance is almost wavelength independent across the UV/VIS wavelength region. The ROCINN algorithm retrieves cloud top height, cloud optical thickness and cloud albedo from NIR measurements in and around the oxygen A-band 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). The current implementation status and setup of the algorithms is presented along with results for two years of S5P data and synthetic S4 data.