Modelling stratospheric composition for the Copernicus Atmosphere Monitoring Service Cy49R1: polar ozone depletion and sulfate aerosols

The daily analyses and forecasts of atmospheric composition delivered by the Copernicus Atmosphere Monitoring Service (CAMS) are produced by the  ECMWF Integrated Forecasting System configured for COMPOsition (IFS-COMPO). On 27 June 2023, this system was upgraded to Cy48R1 which solves explicitly for stratospheric chemistry through a module extracted from the  Belgian Assimilation System for Chemical ObsErvations (BASCOE). On 12 November 2024, the system was further upgraded to Cy49R1 which improves the representation of stratospheric composition with an adjusted parameterization of Polar Stratospheric Clouds (PSC), updated chemical rates for heterogeneous chemistry, and the implementation of missing processes to model the distribution of sulfate aerosols in the stratosphere.

We report on these improvements and evaluate the resulting stratospheric composition in forecast mode, i.e. with no assimilation of composition observations. These evaluations focus on aerosol extinction in the global stratosphere and on ozone depletion processes in the polar lower stratosphere. For ozone depletion events we compare forecasts of ozone, water vapor, N2O, HNO3, HCl and ClO with a reanalysis of observations by the Aura Microwave Limb Sounder (MLS) for three Antarctic events (2008, 2009, 2020) and four Arctic events (ending in 2009, 2011, 2012 and 2020). We show that the model configuration currently used by CAMS (IFS-COMPO Cy49R1) simulates successfully these processes and events. This enables the assimilation of multiple satellite observations of stratospheric composition in an operational Data Assimilation System developed primarily for Numerical Weather Forecasting and provides a useful tool for further studies of the couplings between stratospheric aerosols and gas-phase chemistry.