The IPCC model simulation archive

In the lead up to the 4th Assessment Report, all the main climate modelling groups (17 of them at last count) made a series of coordinated simulations for the 20th Century and various scenarios for the future. All of this output is publicly available in the PCMDI IPCC AR4 archive (now officially called the CMIP3 archive, in recognition of the two previous, though less comprehensive, collections). We’ve mentioned this archive before in passing, but we’ve never really discussed what it is, how it came to be, how it is being used and how it is (or should be) radically transforming the comparisons of model output and observational data.

First off, it’s important to note that this effort was not organised by IPCC itself. Instead, it was coordinated by the Working Group on Coupled Modelling (WGCM), an unpaid committee that is part of an alphabet soup of committees, nominally run by the WMO, that try to coordinate all aspects of climate-related research. In the lead up to AR4, WGCM took up the task of deciding what the key experiments would be, what would be requested from the modelling groups and how the data archive would be organised. This was highly non-trivial, and adjustments to the data requirements were still being made right up until the last minute. While this may seem arcane, or even boring, the point I’d like to leave is that just ‘making data available’ is the least of the problems in making data useful. There was a good summary of the process in Bulletin of the American Meteorological Society last month.

Previous efforts to coordinate model simulations had come up against two main barriers: getting the modelling groups to participate and making sure enough data was saved that useful work could be done.

Modelling groups tend to work in cycles. That is, there will be a period of a few years of development of a new model then a year or two of analysis and use of that model, until there is enough momentum and new ideas to upgrade the model and starting a new round of development. These cycles can be driven by purchasing policies for new computers, staff turnover, general enthusiasm, developmental delays etc. and until recently were unique to each modelling group. When new initiatives are announced (and they come roughly once every six months), the decision of the modelling group to participate depends on where they are in their cycle. If they are in the middle of the development phase, they will likely not want to use their last model (because the new one will almost certainly be better), but they might not be able to use the new one either because it just isn’t ready. These phasing issues definitely impacted earlier attempts to produce model output archives.

What was different this time round is that the IPCC timetable has, after almost 20 years, managed to synchronise development cycles such that, with only a couple of notable exceptions, most groups were ready with their new models early in 2004 – which is when these simulations needed to start if the analysis was going to be available for the AR4 report being written in 2005/6. (It’s interesting to compare this with nonlinear phase synchronisation in, for instance, fireflies).

The other big change this time around was the amount of data requested. The diagnostics in previous archives had been relatively sparse – the main atmospheric variables (temperature, precipitation, winds etc.) but not huge amounts extra, and generally only at monthly resolution. This had limited the usefulness of the previous archives because if something interesting was seen, it was almost impossible to diagnose why it had happened without having access to more information. This time, the diagnostic requests for the atmospheric, ocean, land and ice were much more extensive and a significant amount of high-frequency data was asked for as well (i.e. 6 hourly fields). For the first time, this meant that outsiders could really look at the ‘weather’ regimes of the climate models.

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