Jan Rajczak

21st century climate change in the European Alps—A review☆

Reliable estimates of future climate change in the Alps are relevant for large parts of the European society. At the same time, the complex Alpine region poses considerable challenges to climate models, which translate to uncertainties in the climate projections. Against this background, the present study reviews the state-of-knowledge about 21st century climate change in the Alps based on existing literature and additional analyses. In particular, it explicitly considers the reliability and uncertainty of climate projections. Results show that besides Alpine temperatures, also precipitation, global radiation, relative humidity, and closely related impacts like floods, droughts, snow cover, and natural hazards will be affected by global warming. Under the A1B emission scenario, about 0.25 °C warming per decade until the mid of the 21st century and accelerated 0.36 °C warming per decade in the second half of the century is expected. Warming will probably be associated with changes in the seasonality of precipitation, global radiation, and relative humidity, and more intense precipitation extremes and flooding potential in the colder part of the year. The conditions of currently record breaking warm or hot winter or summer seasons, respectively, may become normal at the end of the 21st century, and there is indication for droughts to become more severe in the future. Snow cover is expected to drastically decrease below 1500-2000 m and natural hazards related to glacier and permafrost retreat are expected to become more frequent. Such changes in climatic parameters and related quantities will have considerable impact on ecosystems and society and will challenge their adaptive capabilities.

Does Quantile Mapping of Simulated Precipitation Correct for Biases in Transition Probabilities and Spell Lengths

AbstractClimate impact studies constitute the basis for the formulation of adaptation strategies. Usually such assessments apply statistically postprocessed output of climate model projections to force impact models. Increasingly, time series with daily resolution are used, which require high consistency, for instance with respect to transition probabilities (TPs) between wet and dry days and spell durations. However, both climate models and commonly applied statistical tools have considerable uncertainties and drawbacks. This paper compares the ability of 1) raw regional climate model (RCM) output, 2) bias-corrected RCM output, and 3) a conventional weather generator (WG) that has been calibrated to match observed TPs to simulate the sequence of dry, wet, and very wet days at a set of long-term weather stations across Switzerland. The study finds systematic biases in TPs and spell lengths for raw RCM output, but a substantial improvement after bias correction using the deterministic quantile mapping tech...

Changing seasonality of moderate and extreme precipitation events in the Alps

The intensity of precipitation events is expected to increase in the future. The rate of 10 increase depends on the strength or rarity of the events; very strong and rare events tend to follow the 11 Clausius-Clapeyron relation, whereas weaker events or precipitation averages do not. An often 12 overlooked aspect is seasonal occurrence of such events, which might change in the future. To address 13 the impact of seasonality, we use a large ensemble of regional and global climate model simulations, 14 comprising tens of thousands of model years of daily temperature and precipitation for the past, 15 present and future. In order to make the data comparable, they are quantile-mapped to observation16 based time series representative of the Aare catchment in Switzerland. Model simulations show no 17 increase in annual maximum 1-day precipitation events (Rx1day) over the last 400 yrs and an increase 18 of 10-20% until the end of the century for a strong (RCP8.5) forcing scenario. This fits with a 19 Clausius-Clapeyron scaling of temperature at the event day, which increases less than annual mean 20 temperature. An important reason for this is a shift in seasonality. Rx1day events become less frequent 21 in late summer and more frequent in early summer and early fall, when it is cooler. The seasonality 22 shift is shown to be related to summer drying. Models with decreasing annual mean or summer mean 23 precipitation show this behavior more strongly. The highest Rx1day per decade, in contrast, shows no 24 change in seasonality in the future. This discrepancy implies that decadal-scale extremes are 25 thermodynamically limited; conditions conducive to strong events still occur during hottest time of the 26 year on a decadal scale. In contrast, Rx1day events are also limited by other factors. Conducive 27 conditions are not reached every summer in the present, and even less so in the future. Results suggest 28 that changes in the seasonal cycle need to be accounted for when preparing for moderately extreme 29 precipitation events and assessing their socio-economic impacts. 30 Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-55 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 5 March 2018 c