The UKCP09 Weather Generator & how it works

1.1 The need for a Weather Generator

Impact and adaptation assessments of climate change often require more detailed information than is available from the UKCP09 PDFs described in the UK Climate Projections science report: Climate change projections (Murphy et al. 2009). Extra detail may be needed in terms of higher resolution in space and/or time. For example, projections may be needed at a specific location (a town or small river catchment) rather than an average for a 25 by 25 km grid box, or the intensity of rainfall may be needed on a time scale of an hour or day, rather than the monthly or seasonal value or the long-term average. This type of information may be further analysed in terms of exceedances of thresholds, or accumulations/deficits: these cannot be derived from the UKCP09 probabilistic projections directly. As well as more resolution, some impact assessments are carried out using models which require time series inputs, as they are simulating processes which are sensitive to the history or sequence of events, rather than simply an aggregated average. Examples of such impact models are to be found in numerous applications such as agricultural and ecological studies, or water resource and flood risk assessments.

To be useful, these generated series must be internally consistent between weather variables (e.g. so that temperatures are usually higher on dry days, compared to wet days in the summer). Such data are also needed for both the current climate and a range of possible future periods chosen by the user, so they must also be consistent with a range of observed and projected statistics of the variables (from the UKCP09 probabilistic projections). A further desirable property is that they should adequately represent extreme events such as prolonged rainfall, droughts and heatwaves.

Such high spatial and temporal resolution series are not available from the UK Climate Projections (or indeed from any other climate modelling programme), so a complementary approach has been developed in UKCP09 using a weather generator to provide high resolution time series of weather variables at a 5 by 5 km grid square resolution for user-specified future periods.

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