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

Two regional climate model experiments for northern and central Europe are studied focussingon greenhouse gas-induced changes in heavy precipitation. The average yearly maximum onedayprecipitation Pmax shows a general increase in the whole model domain in both experiments, although the mean precipitation Pmean decreases in the southern part of the area, especially inone of the experiments. The average yearly maximum six-hour precipitation increases evenmore than the one-day Pmax, suggesting a decrease in the timescale of heavy precipitation. Thecontrast between the Pmean and Pmax changes in the southern part of the domain and the lackof such a contrast further north are affected by changes in wet-day frequency that stem, at leastin part, from changes in atmospheric circulation. However, the yearly extremes of precipitationexhibit a larger percentage increase than the average wet-day precipitation. The signal-to-noiseaspects of the model results are also studied in some detail. The 44 km grid-box-scale changesin Pmax are very heavily affected by inter-annual variability, with an estimated standard errorof about 20% for the 10-year mean changes. However, the noise in Pmax decreases sharplytoward larger horizontal scales, and large-area mean changes in Pmax can be estimated withsimilar accuracy to those in Pmean. Although a horizontal averaging of model results smoothsout the small-scale details in the true climate change signal as well, this disadvantage is, in thecase of Pmax changes, much smaller than the advantage of reduced noise.