Is the 30-year baseline climate in the Weather Generator stationary or transient? Are the future projections stationary or transient?

The 30-year time periods used in UKCP09 assume statistical stationarity across 30 years, for both the baseline and future.

In detail

Stationarity means that they will contain realistic day-to-day and year-to-year weather variability, but there will be little variation in the statistical description of that variability over the long term.

A stochastic rainfall model is trained on recent daily observed rainfall data so that its output should have similar statistical properties to the observed data. The statistics of the baseline climate are used to generate synthetic daily rainfall appropriate to the future 30-year time period, using change factors from the PDFs. The other climate variables (temperature, sunshine, vapour pressure etc.) are derived from an empirical relationship between daily rainfall and the other variables. So in essence, the change factors from the PDF are taken for rainfall and the other variables are derived from a statistical relationship between the variables.

This reinforces the point that there is no climate change information in the Weather Generator outputs beyond that in the 25 km probabilistic projections and the uncertainties of using a weather generator should be well understood before it is used.