It is not always possible to incorporate every physical process that operates in the climate system into a climate model. This may be because the processes involved operate at a spatial resolution that is too fine to be incorporated into a model or due to computational constraints. Consequently, parameterisation schemes are developed to partially address this issue. A fundamental requirement in a parameterisation scheme is the specification of likely values (or a range of likely values) that a particular parameter might take. These parameterisation schemes relate the larger scale processes that are represented in the climate model to the smaller scale (or sub-grid scale) processes through mathematical equations. Within these equations values need to be assigned to the individual parameters. This process often involves a mixture of theory, observations, experimentation and expert judgement, and as a result the definition of parameter values is seldom precise enough to be able to assign accurate values. This is the parameter error component of modelling uncertainty.
In UKCP09, this source of uncertainty is addressed by generating a large ensemble of model variants. Different parameter values are assigned, within plausible limits, for each model variant. As a result, each model variant produces a different climate projection. It is this part of the UKCP09 method that serves as the basis for the perturbed physics ensemble, and represents a major step forward in the quantification of this source of uncertainty.
- An overview of the uncertainties associated with climate modelling is described in Section 3.1 of the UKCP09 Briefing report.
- More details about uncertainties are provided in Chapter 2 of the UKCP09 Climate change projections report. A discussion of sources of uncertainty in the UKCP09 probabilistic projections is available in Annex 2.4.
- An overview of how probabilistic projections are created and presented in UKCP09 is described in Box 3 of the UKCP09 Briefing report. More details are provided in Chapter 3 of the UKCP09 Climate change projections report, including an overview of the methodology in Section 3.2.12, and discussion about the interpretation of UKCP09 probabilistic projections in Section 3.3.
- Parameter error was evaluated in multiple, and in some cases less rigorous, ways in the preparation of marine and coastal projections for UKCP09. Section 1.2 of the UKCP09 Marine & coastal projections report summarises how uncertainty is considered and illustrated in the various marine and coastal projections.