How are the UKCP09 results validated?

Each of the model variant runs are weighted based on observed climate.

In detail

An important step in the UKCP09 method is that different model variant runs are validated on how well they respresent the past climate. This validation takes the form of a weighting. Runs that give a better representation of past climate receive a higher weighting. The weighting is defined on a global scale and involves measures of both observed climate trends through the twentieth century and the observed climate of the baseline period (1961–1990).