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 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 inone of the experiments. The average yearly maximum six-hour precipitation increases evenmore than the one-day P_max, suggesting a decrease in the timescale of heavy precipitation. Thecontrast between the P_mean and P_max 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 P_max 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 P_max decreases sharplytoward larger horizontal scales, and large-area mean changes in P_max can be estimated withsimilar accuracy to those in P_mean. 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 P_max changes, much smaller than the advantage of reduced noise.