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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.

To make sound decisions in the face of climate change, government agencies, policymakers and private stakeholders require suitable climate information on local to regional scales. In Switzerland, the development of climate change scenarios is strongly linked to the climate adaptation strategy of the Confederation. The current climate scenarios for Switzerland CH2018 - released in form of six user-oriented products - were the result of an intensive collaboration between academia and administration under the umbrella of the National Centre for Climate Services (NCCS), accounting for user needs and stakeholder dialogues from the beginning. A rigorous scientific concept ensured consistency throughout the various analysis steps of the EURO-CORDEX projections and a common procedure on how to extract robust results and deal with associated uncertainties. The main results show that Switzerland's climate will face dry summers, heavy precipitation, more hot days and snow-scarce winters. Approximately half of these changes could be alleviated by mid-century through strong global mitigation efforts. A comprehensive communication concept ensured that the results were rolled out and distilled in specific user-oriented communication measures to increase their uptake and to make them actionable. A narrative approach with four fictitious persons was used to communicate the key messages to the general public. Three years after the release, the climate scenarios have proven to be an indispensable information basis for users in climate adaptation and for downstream applications. Potential for extensions and updates has been identified since then and will shape the concept and planning of the next scenario generation in Switzerland. ; Climate Scenarios for Switzerland CH2018 – Approach and Implications ; publishedVersion