How do you work out frequency of extreme events over
decadal timescales?

Estimating the frequency of extreme events over decadal timescales, although possible using the UKCP09 Weather Generator output, is not recommended due to the influence of natural climatic variability over decadal timescales. The Weather Generator should be seen as a tool for analysing the the 25 km probabilistic climate projections rather than a standalone climate projection. It is possible to do this type of analysis with the Weather Generator but the results should be treated with the caution that they deserve.

In detail

The UKCP09 weather generator produces long sequences (30 years or more) sequences of synthetic daily climate that are representative of a 30-year time period (either baseline or future). Return period analysis can be used on the Weather Generator outputs to estimate the magnitude and/or frequency of decadal timescale events (i.e. the 10 year return period event, which is that which is expected to occur, on average, once every 10 years).