Quality assessment of ozone column and profile Climate Data Records procured to the Copernicus Climate Change Service
Résumé
Atmospheric ozone is an Essential Climate Variable (ECV) monitored in the framework of the Global Climate Observing System (GCOS), among others due to its impact on the radiation budget of the Earth, its chemical influence on other radiatively active species, and its role in atmospheric dynamics and climate. Its corresponding importance in the context of climate change has led ECMWF to set up a dedicated procurement of state-of-the-art ozone Climate Data Records (CDRs) to the Climate Data Store of the Copernicus Climate Change Service (C3S), mainly in the form of level-3/4 gridded data products. In support, ESA ensures further improvements of the underlying level-2 ozone data products and the development of new ozone CDRs and valued added data through its Climate Change Initiative on ozone (Ozone_cci+ project). In order to confirm the fitness-for-purpose of the datasets available to the public through the Copernicus CDS, a process has been established to monitor the data health, check compliance with GCOS requirements, and regularly report key performance indicators. This presentation summarizes the ground-based quality assessment and geophysical validation of the C3S ozone CDRs procured to the CDS in 2021. These CDRs consist of ozone column and vertical profile data at level-3 (monthly gridded) and level-4 (assimilated), from several nadir and limb/occultation satellite sounders, retrieval systems, and merging schemes (see details on C3S Climate Data Store at https://cds.climate.copernicus.eu/). The validation of these climate-oriented data records is based on multi-decade time series of correlative measurements collected from monitoring networks contributing to WMO’s Global Atmosphere Watch, such as GO3OS, NDACC, and SHADOZ. The correlative measurements are quality controlled, harmonized, and compared to the various satellite CDRs using BIRA-IASB’s Multi-TASTE versatile validation system, following the latest state-of-the-art protocols and tools. Validation studies focus in particular on the long-term stability of the satellite data series, which may exhibit cyclic errors, drifts, and other long-term patterns reflecting, e.g., instrumental drift and degradation, residual biases between different instruments, and changes in sampling of atmospheric variability and patterns. Comparison results document the achieved quality of the CDRs, in particular in terms of temporal stability: The total ozone column products, covering up to four decades, are found to be stable with respect to the reference measurements at the 0.1 % per decade level. Similarly, most nadir and limb profile CDRs achieve a level of stability that is consistent with what is expected from instrument specifications. Compliance is verified with respect to user requirements identified for GCOS, from WMO rolling review of requirements (RRR), and from the climate research community through the CCI Climate Modelling User Group, whose requirements can be more stringent and more specific.