Extreme Events

Climate extremes, such as heat waves and heavy precipitation events, can induce major ecological, societal and economical impact. Along with anticipated climate change there are concerns that the frequency and intensity of extreme events may increase in response to atmospheric warming.

In this motivation, a major interest of our research group is (1) the future projected occurrence of extreme events and (2) the proper representation of extreme events and associated processes in state-of-the-art climate models.

Analyses on future projections are typically based on the most recently available regional climate model ensemble datasets. The regional focus of our work is on Europe and the Alpine region in particular. The Alps in specific are highly sensitive to climate variability and extremes and prone to be a hotspot where heavy precipitation events occur more frequently than elsewhere. In the recent past, heavy precipitation events in the Alps have caused tremendous damage and impact, and affected large parts of central Europe (e.g. Aug. 2005, Aug. 2013).

Past analyses on extremes and their projected change have been based on coordinated European multi-model ensemble datasets, such as PRUDENCE (at 50km horizontal resolution) and ENSEMBLES (25km). At the moment, our group is investigating the most recent EURO-CORDEX simulations at 12km and 50km horizontal resolution.

Studies from our group were among the first to find that hot temperature extremes are projected to increase more than the mean temperature (Schär et al, 2004). In this motive, we also intensively study processes and mechanisms associated with the occurrence of heat-waves (e.g. soil-moisture anomalies; Seneviratne 2006, Fischer 2007), the increase in temperature variability (Fischer 2009, Fischer 2012, see Fig. 1) and changes in the European summer-climate in general (Kröner et al., in prep).

Enlarged view: Variability Changes — Fig. 1: Changes in interannual and daily summer temperature variability. Changes in (top) interannual and (bottom) daily summer temperature variability (2070-2099 with respect to the control period) for (a and d) ensemble mean across 8 PRUDENCE RCMs, (b and e) ensemble mean across all 14 ENSEMBLES GCM-RCM chains, (c and f) ensemble mean for six best performing ENSEMBLES RCMs. The control period is 1970-99 for the ENSEMBLES experiments and 1961-1990 for PRUDENCE experiment. ENSEMBLES models are averaged across driving GCM to give each GCM equal weight for the raw ensemble mean. Figure from Fischer 2012.

We have also found systematic and consistent patterns in the future occurrence of high-impact temperature extremes in Europe. There are indications that densely populated areas in Southern Europe are likely to suffer substantially from the consequences of climate change (Fischer 2010, see Fig. 2).

Enlarged view: Consistent patterns impact indices — Fig. 2: Climate-change scenarios for daily summer temperature statistics. Projected ensemble mean of average number of combined hot summer days (Tmax>35°C) and tropical nights (Tmin>20°C) (top row) and average number of summer days exceeding the apparent temperature (heat index) threshold of 40.6°C (105°F). Columns indicate the regarded period. Figure from Fischer and Schär 2010.

Enlarged view: Maps European scale changes — European projections of the ensemble-median climate change signals in the frequency of wet days (>1mm/d), the 97.5% percentile of all-day precipitation and the 20 year return value of 1-day precipitation events (left to right) for the four climatological seasons (top to bottom). The results are based on 10 ENSEMBLES RCMs. Stippling denotes grid points where 90% of the RCMs agree on the sign of change. Figure from Rajczak 2013

Our group has a long tradition in the analysis of precipitation extremes involving research activities on impacts and observed, simulated and projected trends and patterns from various perspectives.

Projections have been assessed based on high-resolution regional climate model ensembles for Europe (e.g. PRUDENCE: Frei 2006, ENSEMBLES: Rajczak 2013). We find complex changes in the hydrological cycle at the end of the 21^st century, with an increased risk of both drought (i.e. extended dry spells) and heavy events in many European regions (Fig. 4). A study of our group finds a prominent decrease in return periods of high-impact precipitation events (Rajczak 2013, see also Fig. 4). For instance, in the Northeastern Switzerland (i.e. region of Zurich) and in fall, a once in 50-years event under present conditions is projected to occur once every 20 years in the future. Currently we analyze the consistency of patterns found throughout previous datasets in new high-resolution climate change simulations.

Our group has also undertaken investigated methods to assess changes in heavy precipitation (Frei and Schär, Schär 2015).

Enlarged view: Changes in Return Levels CHNE — Fig. 4: Return values as a function of return period for observations (OBS: FS98, 1971-1998, grey), simulated present conditions (CTRL: RCMs, 1970-1999, blue) and future conditions (SCEN: RCMs, 2070-2099, red) at seasonal (columns from left to right) and regional scale for Northeastern Switzerland (Region between Lucerne in the southwest and Lake Constance in the Northeast). Lines indicate the best estimate (median of realizations). Shadings and dotted lines indicate the uncertainty range (range between the 10^th and 90^thpercentile of realizations). Figure based on Rajczak 2013

Studies related to observational datasets have described spatial and temporal patterns and trends in Alpine precipitation (Frei et al., 2006). End-user oriented hydrological applications have focused on the prediction and assessment of floods. Here, a particular focus is on the river Rhine and its sub-catchments. The improvement of simulating precipitation (and its extremes) in state-of-the-art kilometer-scale convection-resolving climate simulations is an additional major research topic in our group.

New paper: Percentile indices for assessing changes in heavy precipitation events

Trends and changes in heavy precipitation are often assessed using percentile indices. In a recent paper, we compare different approaches toward percentile calculations and highlight substantial differences between the different in the context of climate change (Schär et al., 2016). In fact, we highlight the danger of using wet-day percentiles and claim that all-day percentile should be used in climate-change studies. In the paper we elaborate a variety of percentile methods and GEV (see Fig.5) and provide a systematic overview of the different methodologies.

Enlarged view: Changes in different precipitation indices — Fig. 5: Impact of percentile definition on projected changes in heavy precipitation events. Results show projections of summer-mean changes between 2070-2099 and 1970-1999 for 10 RCM simulations from the ENSEMBLES project using a computational resolution of 25 km and the SRES A1B emissions scenario. Upper panels show relative changes of (a) all-day percentiles, (b) wet-day percentiles, and (c) frequency of exceedance of an all-day percentile threshold. Lower panels show relative changes in (d) 20-yr return levels, (e) mean seasonal maximum one-day precipitation, and (f) wet-day frequency (based on a threshold of 1 mm/d). Stippling denotes grid points where 9 out of 10 RCMs agree on the sign of change. Estimates in (d) over the Mediterranean are not very robust, due to small number of precipitation days (grey areas). Figure from Schär et al. (2016).

Schär, C., P.L. Vidale, D. Lüthi, C. Frei, C. Häberli, M. Liniger and C. Appenzeller (2004): The role of increasing temperature variability in European summer heat waves. Nature, 427, external pagedoi: 10.1038/nature02300

Seneviratne, S.I., D. Lüthi, M. Litschi, and C. Schär (2006): Land-atmosphere coupling and climate change in Europe. Nature, 443, external pagedoi: 10.1038/nature05095.

Frei, C., R. Schöll, S. Fukutome, J. Schmidli, and P. L. Vidale (2006), Future change of precipitation extremes in Europe: Intercomparison of scenarios from regional climate models, J. Geophys. Res., doi:external page10.1029/2005JD005965.

Fischer, E.M., S.I. Seneviratne, P.L. Vidale, D. Lüthi and C. Schär (2007): Soil moisture - atmosphere interactions during the 2003 European summer heat wave. J. Climate, 20, 20, 5081-5099, external pagedoi:10.1175/JCLI4288.1 .

Fischer, E.M., S. I. Seneviratne, D. Lüthi, and C. Schär (2007): Contribution of land-atmosphere coupling to recent European summer heat waves, Geophys. Res. Lett., external pagedoi:10.1029/2006GL029068 .

Fischer, E.M., C. Schär, 2009: Future changes in daily summer temperature variability: driving processes and role for temperature extremes, Clim. Dynam., external pagedoi: 10.1007/s00382-008-0473-8 .

Fischer, E.M., C. Schär (2010): Consistent geographical patterns of changes in high-impact European heatwaves, Nature Geoscience, external pagedoi: 10.1038/NGEO866.

Fischer, E.M., J Rajczak, and C. Schär (2012): Changes in European summer temperature variability revisited, Geophys. Res. Lett. Geophys., external pagedoi:10.1029/2012GL052730 .

Rajczak, J., P. Pall and C. Schär (2013): Projections of extreme precipitation events in regional climate simulations for Europe and the Alpine Region, J. Geophys. Res. Atmos., external pagedoi:10.1002/jgrd.50297 .

Schär, C., Ban, N., Fischer, E.M., Rajczak, J., Schmidli, S., Frei, C., Giorgi, F., Karl, T.R., Kendon, E.J., Klein Tank, A.M.G., O`Gorman, P.A., Sillmann, J., Zhang, X. and F.W. Zwiers (2016): Percentile indices for assessing changes in heavy precipitation events, Climatic Change, in revision