Extension and evaluation of the Integrated Forecast System (IFS) cycle 49R1 to stratospheric aerosols and chemistry for the global Copernicus Atmospheric Monitoring Service (CAMS)

The ECMWF’s Integrated Forecast System (IFS) is the global atmospheric model used by the Copernicus Atmospheric Monitoring Service (CAMS) to provide analyses and forecasts on atmospheric composition. Currently, the CAMS global model includes the aerosol model of the IFS, the aerosol module IFS-AER making use of a sectional-bulk scheme, and the chemistry scheme based on a CB05-based carbon-bond mechanism, with the option to couple this to stratospheric chemistry module BASCOE. The combined BASCOE will be used operationally in the CAMS global system starting from the upgrade to cycle 48R1 planned in June 2023. This abstract focuses on further developments related to stratospheric chemistry and aerosols that are to be implemented in the future operational cycle 49R1, as well as on a first evaluation of IFS’ performances in representing stratospheric aerosols and chemistry against different datasets.

Initially focussing on the troposphere, IFS-AER has been extended to include and represent stratospheric sulfate aerosol processes, keeping the existing tracers. The extended IFS-AER(strato) has been coupled to IFS(BASCOE) through the gaseous sulphuric acid tracer, to the IFS radiation scheme, and to the 4Dvar assimilation scheme. The evaluation of aerosol aspects makes use of aerosol datasets (aerosol extinction, AOD, …) from the Global Ozone Monitoring by Occultation of Stars (GOMOS, onboard Envisat), and the Global Space-based Stratospheric Aerosol Climatology (GloSSAC), based on different cases studies including quiescent and (highly) volcanic periods. It has also been tested against reference simulations from WACCM-CARMA. These intercomparisons show a reasonable agreement against retrieval datasets such as GloSSAC and reference simulations from WACCM-CARMA. In quiescent conditions, the new system showed a decreasing trend with respect to the reference datasets.

BASCOE includes a simple PSC parameterization, which has been updated and tuned in cycle 49R1. In order to assess the impact of this upgrade, we evaluate the composition of the polar lower stratosphere during the winter-spring seasons ("ozone hole" events) of 2008, 2009 and 2020 above the Antarctic and 2009, 2011, 2012 and 2020 above the Arctic, with a focus on 5 key species observed by Aura-MLS. This evaluation demonstrates the capacity of IFS(BASCOE) to forecast the chemical composition of the polar lower stratosphere above both the Arctic and the Antarctic for several years with very different evolution of the polar vortex. While further improvements are desirable and will require an overhaul of the PSC parameterization, the current performance allows us to study the interannual variability of ozone hole episodes.