Contrasting seasonal changes in total and intense precipitation in the European Alps from 1903 to 2010

Abstract

Abstract. Changes in precipitation over the European Alps are investigated with the regional climate model MAR (Modèle Atmosphérique Régional) 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 interannual variability and the seasonal cycle of precipitation over the European Alps. The relatively high resolution allows us 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 d−1 km−1) in summer and 38 % km−1 (1.15 mm d−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 northwestern 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 a.s.l. Below this level, the summer drying is explained by a reduction in the number of wet days, reaching 20 % per century over the northwestern part of the Alps and 30 % to 50 % per century in the southern part of the Alps. It is associated with shorter but more-frequent wet spells. The centennial trends are modulated over the last decades, with the drying occurring in the plains in winter also affecting high-altitude areas during this season and with a positive trend of autumn precipitation occurring only over the last decades all over the Alps. 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 d−1 on average over 1903–2010. Centennial maxima up to 250 to 300 mm d−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 and 1960s.

Extreme Ground Snow Loads in Europe from 1951 to 2100

“…The temperature rise, on the one hand, facilitates the melting of snow and increases the fraction of precipitation falling as rain, on the other hand, it can increase the intensity of the single snow events, leading to higher ground snow loads [39]. For example, increasing trends in intensity and frequency of daily heavy precipitation have been already observed, even over the Alps [40,41], confirming theory and early models [4,42].…”

Twentieth century temperature and snow cover changes in the French Alps

“…Nevertheless, in order to asses the dependency of the trends simulated with MAR to the atmospheric reanalysis used as boundary conditions, we drove MAR with the more recent 35-km horizontal resolution ERA5 reanalysis (Hersbach et al 2020) to produce a second experiment over the 1981-2018 period (MAR-ERA5 hereafter). A first MAR-ERA-20C simulation has been recently used to investigate precipitation trends over the Alps (Ménégoz et al 2020). Here, we performed this experiment with identical horizontal (7 km) and vertical (24 levels) resolution using a more recent version of the model (MARv3.9.10, http://mar.cnrs.fr/index.php) while saving additional variables needed to study the snow cover such as snow water equivalent (SWE) and surface albedo.…”

Elevation-dependent trends in extreme snowfall in the French Alps from 1959 to 2019

“…For instance, we observe that trends in daily maxima of winter precipitation, which may generate heavy snowfall, are stronger in the south (+20-40% per century) compared to the north (from -10% to +20% per century) of the French Alps (Fig. 7 of Ménégoz et al 2020). This observation might be due to a stronger increasing trend in extreme precipitation for the Mediterranean circulation than for the Atlantic circulation.…”

“…Furthermore, past trends in daily maxima of precipitation largely depend on the season (Fig. 7 of Ménégoz et al 2020). In winter, daily maxima precipitation (which may generate extreme snowfall) have trends that vary between -40% to +40% per century depending on the location.…”