The Control Run External Forcings

This discussion was initiated by Alan Iwi over email. It adresses the question of which external forcing to use for the control run(s), pre-industrial or present-day greenhouse gases?

So far the conclusion has been that if we do a control run including William Connoley's ice scheme then it might as well be present-day GHGs as there is no other run to compare with. If we do a "standard" HadCM3 run, the issue is not as clear. The advantage of using pre-industrial GHGs is that our run can be checked for acuracy against an existing verified control run. The advantage of present-day GHGs is that our experiments will most likely use present-day GHGs, so we need a control run to compare the experiments to.

Rowan and I are of the opinion that the both control runs should be done with present-day GHGs (if indeed a "standard" run is necessary). -- LeonHermanson - 11 May 2006

• Doug has pointed out that the real present-day climate is not in equilibrium with current forcings, so if you spin up a control with steady present-day forcings it still won't be directly comparable against present-day obs. So better to spin up a pre-industrial control, and then after that bring it up to present-day using a time-dependent forcings run. This now seems to be agreed (e.g. by Rowan). -- AlanIwi - 15 May 2006