Description of algorithms for co-locating and comparing gridded model data with remote-sensing observations

Abstract. MACC-II,III, Monitoring Atmospheric Composition and Climate, is the current pre-operational Copernicus Atmosphere Monitoring Service (CAMS). It provides data records on atmospheric composition for recent years, present conditions and forecasts (a few days ahead). To support the quality assessment of the CAMS products, the EU FP7 project Network Of ground-based Remote-Sensing Observations (NORS) created a server to validate the gridded MACC-II,III/CAMS model data against remote-sensing observations from the Network for the Detection of Atmospheric Composition Change (NDACC), for a selected set of target species and pilot stations. This paper describes in detail the algorithms used in this validation server. Amongst others, the algorithms take into account the horizontal displacement of the measured profiles from the location of the instrument, the vertical averaging and uncertainty propagation.