Chapter 1 of AR4 has some surprisingly interesting comments about models that, to the extent that the points are disclosed in the body chapters, are disclosed so opaquely that they would be undecipherable to anyone other than a few. Here are some interesting comments about flux adjustment – an issue that must surely raise civilian eyebrows. A “flux adjustment” in a GCM is defined below as an “empirical correction that could not be justified on physical principles” i.e. a fudge factor, and one of the accomplishments of recent GCMs has been to apparently get past that. AR4:
The strong emphasis placed on the realism of the simulated base state provided a rationale for introducing flux adjustments or flux corrections (Manabe and Stouffer, 1988; Sausen et al., 1988) in early simulations. These were essentially empirical corrections that could not be justified on physical principles, and that consisted of arbitrary additions of surface fluxes of heat and salinity in order to prevent the drift of the simulated climate away from a realistic state. The National Center for Atmospheric Research model may have been the first to realise non-flux-corrected coupled simulations systematically, and it was able to achieve simulations of climate change into the 21st century, in spite of a persistent drift that still affected many of its early simulations. Both the FAR and the SAR pointed out the apparent need for flux adjustments as a problematic feature of climate modelling (Cubasch et al., 1990; Gates et al., 1996).
By the time of the TAR, however, the situation had evolved, and about half the coupled GCMs assessed in the TAR did not employ flux adjustments. That report noted that some non-flux adjusted models are now able to maintain stable climatologies of comparable quality to flux-adjusted models (McAvaney et al., 2001). Since that time, evolution away from flux correction (or flux adjustment) has continued at some modelling centres, although a number of state-of-the-art models continue to rely on it.
This raises an obvious question: which “state-of-the-art models” continue to rely on flux adjustments? One of the annoying aspects of IPCC WG1 reports is their refusal to make such identifications, which might put one of the group in hot water with his funders, I suppose. I’d like to know which models make flux adjustments so that I can keep an eye out when the “ensemble” results are reported.
They go on to make the following interesting comment that I;ve not seen in print elsewhere:
(1.5.3) The design of the coupled model simulations is also strongly linked with the methods chosen for model initialisation. In flux adjusted models, the initial ocean state is necessarily the result of preliminary and typically thousand-year-long simulations to bring the ocean model into equilibrium. Non-flux-adjusted models often employ a simpler procedure based on ocean observations, such as those compiled by Levitus et al. (1994), although some spin-up phase is even then necessary. One argument brought forward is that non-adjusted models made use of ad hoc tuning of radiative parameters (i.e., an implicit flux adjustment).
No reference is given for this powerful statement. This is exactly what Gavin Schmidt denies and yet here’s IPCC WG1 worrying about “ad hoc tuning”. Does anyone know anything more about this?