Analysing changes in short-duration extreme rainfall events

Two approaches are presented for using the UKCP09 weather generator (WG) to assess changes in extreme short-duration rainfall. The first provides change factors for design storms of varying duration and frequency, building on established hydrological techniques and with uncertainty estimation. The second approach characterises and facilitates the selection of WG timeseries for use in impact models. The methods have been tested at ten case study sites around the UK. The results show an increase in the intensity of short-duration (1 h and 12 h) events in winter but a reduction in summer, although the latter is subject to high levels of uncertainty. The number of intense events increases slightly and more significantly for some locations, especially in winter. Generally, it is hard to establish any conclusions geographically, although increases in rainfall intensity and events in the Lake District site stand out. The methods provide practical techniques for utilising the UKCP09 WG to assess the impacts of climate change, in particular for urban drainage and sewer modelling.