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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 18, issue 7 | Copyright
Nat. Hazards Earth Syst. Sci., 18, 2047-2056, 2018
https://doi.org/10.5194/nhess-18-2047-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 27 Jul 2018

Research article | 27 Jul 2018

Changing seasonality of moderate and extreme precipitation events in the Alps

Stefan Brönnimann1,2, Jan Rajczak3,a, Erich M. Fischer3, Christoph C. Raible1,4, Marco Rohrer1,2, and Christoph Schär3 Stefan Brönnimann et al.
  • 1Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 2Institute of Geography, University of Bern, Bern, Switzerland
  • 3Institute for Atmospheric and Climate Science, ETH Zurich, Zürich, Switzerland
  • 4Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland
  • anow at: Federal Office of Meteorology and Climatology MeteoSwiss, 8058 Zurich Airport, Zurich, Switzerland

Abstract. The intensity of precipitation events is expected to increase in the future. The rate of increase depends on the strength or rarity of the events; very strong and rare events tend to follow the Clausius–Clapeyron relation, whereas weaker events or precipitation averages increase at a smaller rate than expected from the Clausius–Clapeyron relation. An often overlooked aspect is seasonal occurrence of such events, which might change in the future. To address the impact of seasonality, we use a large ensemble of regional and global climate model simulations, comprising tens of thousands of model years of daily temperature and precipitation for the past, present, and future. In order to make the data comparable, they are quantile mapped to observation-based time series representative of the Aare catchment in Switzerland. Model simulations show no increase in annual maximum 1-day precipitation events (Rx1day) over the last 400 years and an increase of 10%–20% until the end of the century for a strong (RCP8.5) forcing scenario. This fits with a Clausius–Clapeyron scaling of temperature at the event day, which increases less than annual mean temperature. An important reason for this is a shift in seasonality. Rx1day events become less frequent in late summer and more frequent in early summer and early autumn, when it is cooler. The seasonality shift is shown to be related to summer drying. Models with decreasing annual mean or summer mean precipitation show this behaviour more strongly. The highest Rx1day per decade, in contrast, shows no change in seasonality in the future. This discrepancy implies that decadal-scale extremes are thermodynamically limited; conditions conducive to strong events still occur during the hottest time of the year on a decadal scale. In contrast, Rx1day events are also limited by other factors. Conducive conditions are not reached every summer in the present, and even less so in the future. Results suggest that changes in the seasonal cycle need to be accounted for when preparing for moderately extreme precipitation events and assessing their socio-economic impacts.

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Heavy precipitation events in Switzerland are expected to become more intense, but the seasonality also changes. Analysing a large set of model simulations, we find that annual maximum rainfall events become less frequent in late summer and more frequent in early summer and early autumn. The seasonality shift is arguably related to summer drying. Results suggest that changes in the seasonal cycle need to be accounted for when preparing for moderately extreme precipitation events.
Heavy precipitation events in Switzerland are expected to become more intense, but the...
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