©Nick Scheerbart

Abstract

Changes of precipitation over the European Alps are investigated with the regional climate model MAR applied with a 7-km resolution over the period 1903–2010 using the reanalysis ERA-20C as forcing. A comparison with several observational datasets demonstrates that the model is able to reproduce the climatology as well as both the inter-annual variability and the seasonal cycle of precipitation over the European Alps. The relatively high resolution allows to estimate precipitation at high elevations. The vertical gradient of precipitation simulated by MAR over the European Alps reaches 33?%?km?1 (1.21?mm.day?1.km?1) in summer and 38?%?km?1 (1.15?mm.day?1.km?1) in winter, on average over 1971–2008 and shows a large spatial variability. A significant (p-value?<?0.05) increase in mean winter precipitation is simulated in the North Western Alps over 1903–2010, with changes typically reaching 20 to 40?% per century. This increase is mainly explained by a stronger simple daily intensity index (SDII) and is associated with less frequent but longer wet spells. A general drying is found in summer over the same period, exceeding 20 to 30?% per century in the Western plains and 40 to 50?% per century in the Southern plains surrounding the Alps, but remaining much smaller (<?10?%) and not significant above 1500?m.asl. Below this level, the summer drying is explained by a reduction of the number of wet days, reaching 20?% per century over the Northwestern part of the Alps and 30–50?% per century in the Southern part of the Alps. It is associated to shorter although more frequent wet spells. Maximum daily precipitation index (Rx1day) takes its highest values in autumn in both the Western and the Eastern parts of the Southern Alps, locally reaching 50 to 70?mm.day?1 on average over 1903–2010. Centennial maxima up to 250 to 300?mm.day?1 are simulated in the Southern Alps, in France and Italy, as well as in the Ticino valley in Switzerland. Over 1903–2010, seasonal Rx1day shows a general and significant increase at the annual timescale and also during the four seasons, reaching local values between 20?% and 40?% per century over large parts of the Alps and the Apennines. Trends of Rx1day are significant (p-value?<?0.05) only when considering long time series, typically 50 to 80 years depending on the area considered. Some of these trends are nonetheless significant when computed over 1970–2010, suggesting a recent acceleration of the increase in extreme precipitation, whereas earlier periods with strong precipitation also occurred, in particular during the 1950s/1960s.