General description for senD2-fix

[mollymacrae]

The senD2-fix simulation is designed to investigate the effects of differing background stratospheric aerosols across models in the refD2 simulation and to help in the interpretation of the stratospheric aerosol injection (senD2-sai) experiment. The simulation is set up similarly to the senD2-sai simulation, but prescribes a repeating annual cycle of stratospheric aerosol calculated as the average over 2020 - 2024 from the WACCM simulation that provided the full timeseries of stratospheric aerosol for the senD2-sai simulation. The 2020 - 2024 period represents the background stratospheric aerosol from WACCM before stratospheric aerosol injection was started.