Both the CAMS and ECMWF’s NWP forecasting systems are based on the IFS (Integrated Forecasting System); CAMS runs the IFS in a slightly different configuration. The CAMS configuration makes use of additional detailed chemistry and aerosol modules, but the meteorological processes are in principle the same. However, because the chemistry is computationally costly, the CAMS forecasts are at lower resolution than the ECMWF NWP forecasts (currently ~40km versus ~9km).
The IFS is upgraded once or twice per year resulting in the various cycle versions. The operational CAMS forecasting system is upgraded as close as possible to the upgrade of the NWP forecasting system, so we run on the same IFS cycle, which makes it easier for our operators to detect and fix any issues in both forecasting systems.
However, there are exceptions to the rule, and this is what happened with the cycles (41R2 and 41R1). CAMS implemented CY41R1 in July 2015, a few months after CY41R1 was introduced for NWP (12 May 2015). When a new cycle was then implemented for NWP (CY41R2) in March 2016, CAMS continued to use CY41R1 and only changed the horizontal resolution on 21 June 2016 (when a new cycle number was not used). During this period, different cycles were therefore used for the NWP and the CAMS configurations (CY41R2 versus CY41R1, respectively). The reason for this was that the upgrade in resolution for CAMS was so significant for the results, and it was not desirable to include scientific changes (meteorological and chemical) at the same time.
With CY43R1, there was convergence as the same IFS cycle was used for both CAMS and NWP operational services (although still running at different horizontal resolutions).