CCMI Phase Two

We consider it a very high priority to provide updated guidance on the future evolution of stratospheric ozone to the Parties to the Montreal Protocol and we believe this guidance is highly valued by the Parties. To support the Assessment, we plan to ask for simulations in two steps; a first request for a hindcast simulation that will be used to assess the models against observations (REF-D1, using the CMIP6 historical forcings), and a second request for a baseline scenario (REF-D2, likely defined very similarly to SSP2-4.5) with an additional, but very limited, number of scenarios. The initial request for the hindcast simulation will allow for model assessment to be undertaken and provide feedback to modelling groups before they launch the scenario simulations.

There was some criticism during the writing of the 2018 Ozone Assessment that it was not possible to assess the validity of the CCMs being used for the projections. It has also been 10 years since the last community-wide assessment was undertaken in CCMVal-2 and many new datasets have been produced (ERA-Interim, MERRA, GOZCARDS, etc.) and assessed (SPARC-DI, LOTUS, etc.) since then. And while more recent comparisons of models against observations have been performed (the analysis of ozone trends in the CCMI-1 simulations in the LOTUS report, for example), we feel there is tremendous value in having a more holistic and process-based analysis of CCMs to more fully understand the strengths and weaknesses of our current generation of models. The time seems right to update our assessment of the representation of stratospheric chemical and dynamical processes in stratosphere-resolving CCMs.