Changes in average and extreme precipitation in two regional climate model experiments

Two regional climate model experiments for northern and central Europe are studied focussing on greenhouse gas‐induced changes in heavy precipitation. The average yearly maximum one‐day precipitation P _max shows a general increase in the whole model domain in both experiments, although the mean precipitation P _mean decreases in the southern part of the area, especially in one of the experiments. The average yearly maximum six‐hour precipitation increases even more than the one‐day P _max, suggesting a decrease in the timescale of heavy precipitation. The contrast between the P _mean and P _max changes in the southern part of the domain and the lack of such a contrast further north are affected by changes in wet‐day frequency that stem, at least in part, from changes in atmospheric circulation. However, the yearly extremes of precipitation exhibit a larger percentage increase than the average wet‐day precipitation. The signal‐to‐noise aspects of the model results are also studied in some detail. The 44 km grid‐box‐scale changes in P _max are very heavily affected by inter‐annual variability, with an estimated standard error of about 20% for the 10‐year mean changes. However, the noise in P _max decreases sharply toward larger horizontal scales, and large‐area mean changes in P _max can be estimated with similar accuracy to those in P _mean. Although a horizontal averaging of model results smooths out the small‐scale details in the true climate change signal as well, this disadvantage is, in the case of P _max changes, much smaller than the advantage of reduced noise.