Estimation of climate factors for future extreme rainfall: Comparing observations and RCM simulations

The application of climate factors has become more common in urban drainage design. The climate factor accounts for the expected increase in the magnitude of the extreme rainfall events during the technical lifetime of the drainage system. The present practice in Denmark is the application of climate factors of 1.2, 1.3 and 1.4 for 2-, 10- and 100-year events, respectively. These estimates are based on a comparison between the ‘control’ and ‘scenario’ period simulated by the regional climate model, HIRHAM4. The present paper presents new estimates of the climate factors on the basis of two new sources of data, focusing on a rainfall duration of 60 minutes. Temporal development of observed extreme rainfall is described by a parametric model and subsequently extrapolated, resulting in estimates of climate factors between 2.3 and 2.5. A recent transient model simulation by the regional climate model RACMO is divided into five time slices whereby their independent extreme rainfall characteristics