Model variant

A model variant is a run of a climate model where the parameters have been changed from that used in the standard model. Hence it is possible (as in UKCP09) to build many different variants of a climate model in which parameter values are different but within plausible limits.

In detail

For a given model configuration there can be many model variants. This is because, irrespective of model configuration, it is necessary to parameterise the model. The process of parameterising a model involves specifying parameter values for relationships between large scale and small scale processes. These values may not be accurately known and thus may have a wide range of plausible values. This is one source of modelling uncertainty known as parameter error.

• More details about the climate models used in UKCP09 are given in Box 2.1 in the UKCP09 Climate change projections report. Table 2.1 provides a list of the processes and feedbacks included, and not included, in the UKCP09 probabilistic projections.

• 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.

• A comparison of the UKCP09 methodology with an alternative method is described in Annex 2.3, and summarised in Box 4 of the UKCP09 Briefing report.