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 A hole model domain in both experiments, although the mean precipitation P-mcan 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-max, 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-scaie 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-mcan 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.