Questions are related to how well extremes are captured in global and regional climate models.
How can the representation of extremes by climate models be evaluated?
The nature of extremes, that are rare and rarer the more extreme they are, implies that it is fundamentally difficult to: i) evaluate model performance, and ii) assess future changes in such extreme events.
These issues are more pronounced the longer the recurrence time (i.e. the average time it would take before an event is repeated) of interest is. For example, with recurrence times of several hundred years or more, observations to which the models can be compared are rare. For less extreme events, such as a one in a ten-, or one in a twenty-year event, the situation is better, and EURO-CORDEX models have been extensively evaluated for such aspects. Also, assessing the extent of changes in extremes in a future warmer climate is more challenging the rarer the events are. This can be partly treated with more data to assess, in order to get more robust assessments. A consequence of this is that large ensembles of climate projections are needed.
For extreme precipitation events on sub-daily time scales related to convection we note that standard regional climate models such as the EURO-CORDEX models are not well-suited. For this purpose, very high-resolution convection-permitting models has been suggested as a better option (e.g. Prein et al., 2013; Kendon et al., 2014).
Users of EURO-CORDEX data that have particular interest in extremes should carefully investigate to what extent the models can represent these extremes. A number of scientific papers have addressed extremes in these models, e.g. Vautard et al. (2020) and Coppola et al. (2021).
What are compound events and how can climate models represent them?
Another examples of extreme events that may have strong impacts are so called compound events, when two or more phenomena occur at the same time. This can be a low-pressure system bringing high sea-level fluctuations on a coast where a river at the same time see a strong discharge due to strong rainfall in the preceding week. Even if the compound resulting event is extreme it is not necessary that any of these two events are extreme in itself.
Compound events can also relate to a series of repeated events that may not be extreme one at the time, but, where the long period of persistent conditions may be extreme. The warm summer in northern Europe 2018 was an example of such an extreme when repeated heat waves and high-pressure dominated summer weather without precipitation led to severe drought and the warmest summer on record in Sweden (Wilcke et al., 2021). Such events, where several extremes need to take place more or less in the right time in relation to each other, are rarer compared to the individual extremes one at the time.
Due to the rarity of compound events and limited amount of data it is difficult to evaluate how models can represent such events.
References
Coppola E, Nogherotto R, Ciarlo JM, Giorgi F, Somot S, Nabat P, Corre L, Christensen OB, Boberg F, van Meijgaard E, Aalbers E, Lenderink G, Schwingshackl C, Sandstad M, Sillmann J, Bülow K, Teichmann C, Iles C, Kadygrov N, Vautard R, Levavasseur G, Sørland SL, Demory M-E, Kjellström E and Nikulin G (2021) Assessment of the European climate projections as simulated by the large EURO- CORDEX regional climate model ensemble. J. Geophys. Res.: Atmospheres, 126, e2019JD032356, DOI: 10.1029/2019JD032356
Kendon, EJ, NM Roberts, HJ Fowler, MJ Roberts, SC Chan, and CA Senior (2014) Heavier summer downpours with climate change revealed by weather forecast resolution model, Nature Climate Change, 4, 570–576, doi:10.1038/nclimate2258
Prein AF, Gobiet A, Suklitsch M, Truhetz H, Awan NK, Keuler K and Georgievski G (2013) Added value of convection permitting seasonal simulations. Clim. Dyn., 41, 2655–2677, doi:10.1007/s00382-013-1744-6.
Vautard R, Kadygrov N, Iles C, Boberg F, Buonomo E, Bülow K, Coppola E, Corre L, van Meijgaard E, Nogherotto R, Sandstad M, Schwingshackl C, Somot S, Aalbers E, Christensen OB, Ciarlo JM, Demory M-E, Giorgi F, Jacob D, Jones RG, Keuler K, Kjellström E, Lenderink G, Levavasseur G, Nikulin G, Sillmann J, Solidoro C, Sørland SL, Steger C, Teichmann C, Warrach-Sagi K and Wulfmeyer V (2020) Evaluation of the large EURO-CORDEX regional climate model ensemble. J. Geophys. Res. DOI: 10.1029/2019JD032344
Wilcke R, Kjellström E, Liu C, Matei K and Moberg A (2020) The extreme warm summer 2018 in Sweden - set in a historical context. Earth System Dynamics, 11, 1107-1121. DOI: 10.5194/esd-11-1107-2020.
