Assessment of Extreme Rainfall in the UK Marie Ekström

Sustainable Water: Uncertainty, Risk and Vulnerability in Europe (SWURVE) The project aims to study the impacts of climate variability and change on the sustainable use of water and its related activities in Europe. One objective for CRU is to look at changes in extreme rainfall over the UK, updating a study made by P. Jones and P. Ried (2001): “Assessing future changes in extreme precipitation over Britain using regional climate model integrations. Int. J. Climatol. 21: 1337  1356.”

Return period magnitudes The return period can be seen as the average time between events of a given magnitude. The quantile of a return period (T) is an event magnitude so extreme that it has probability 1/T of being exceeded by any single event. Example: the magnitude associated with a 10 and 50 year return period has a chance of 10 and 2 percent chance to occur within any one year.

Methodology Annual maxima were fitted to a extreme value type I distribution: Gumbel distribution. The Gumbel distribution parameters (location and scale) were estimated using probability weighted moments according to Palutikof et al. (1999): “A review of methods to calculate extreme wind speeds. Met. Appl. 6: 119  132.” Return period magnitudes (mm) were estimated for 5, 10, 20 and 50 year periods.

How good is the estimation? Two tests were applied on all data sets: Kolmogorov-Smirnov (matlab) Anderson-Darling (assumed to be more sensitive to deviations in the tail of the distribution than the older K-S test.) Stephens M.A. 1977: Goodness of Fit for the Extreme Value Distribution. Biometrika, 64:

HadRM3 grid used for the Gumbel analysis Control and Perturbed runs Scenario A2 (three ensamble members) 3* 31 years

Stations used for the Gumbel analysis 105 stations Time period 1960  1990 For each station and year: less than 20 % chance that the annual maxima is missing

… and some more UK Climate Impacts Programme (UKCIP): 5-day rainfall totals for a 5 by 5 km grid over the UK

Common grid 0.5º latitude by 0.5º longitude Regular grid (21,20).

Results Annual maxima: all return periods are displayed Seasonal maxima: only 10-year return period is shown Multiday maxima: only 10-year return period is shown.

Annual return period magnitudes (mm)

Relative difference (%) between obs. and HadRM3

Seasonal return period magnitudes HadRM3) (mm)

Relative difference (%) between HadRM3 and obs.

Return period magnitudes for multiday totals (mm)

Relative difference (%) between HadRM3 and obs.

Summary (10-year return period) On average the HadRM3 values are about +/- 20 % different to those of the observed values. Large overestimation is found for western Scotland.The main part of the overestimation is probably due to the sparseness of data in this region. The largest spatial difference is found during the summer season.