Session report: Understanding and modelling compound climate and weather events and their impacts

Chairperson: Bart van den Hurk

Conveners: Nina Nadine Ridder | Co-conveners: Bart van den Hurk, Philip Ward, Seth Westra, Jakob Zscheischler, Samuel Jonson Sutanto, Claudia Vitolo, Henny A.J. Van Lanen

This is a short summary of the oral session at the EGU 2019 where once again this year a session on understanding and modelling compound climate and weather events and their impacts was being held. The selection of oral presentations showed that the research community is actively exploring the monitoring and modelling of compound events. While most presentations (and posters) focused on the documentation of compound event occurrence or their potential impacts on various sectors, a few studies report progress on forecasting and modelling events that are selected for the compound nature of their driver.

Water interactions leading to high-impact compound events: multiple stakeholder perspectives

Georgia Destouni, Samaneh Seifollahi-Aghmiuni, Zahra Kalantari, Carmen Prieto, and Yuanying Chen

Georgia Destouni discussed ongoing research within the EU COASTAL project, in which coastal zone issues are being examined using “multi-actor labs”. The presentation discussed how multiple stakeholder objectives can be including in modelling water interactions that lead to compound events. Questions addressed include: Which event and impacts are most relevant for stakeholders? What is/needs to be understood about interacting variables and interaction strengths/quantification?  They started by creating common mind maps by looking at possible drivers of changes and define key interaction and causal links. These were used as a basis to quantify the interactions and feedbacks in a systems approach using fuzzy cognitive mapping. They found that harbors and agriculture are sectors which can be heavily affected by the strength of these interactions. Eventually, this analysis will be used as a basis for co-developing scenarios and roadmaps towards coastal solutions and mitigation of risks.

Changing compound flood probability at the global scale under anthropogenic climate change

Emanuele Bevacqua, Douglas Maraun, Michalis I. Vousdoukas, Lorenzo Mentaschi, Evangelos Voukouvalas, Giuseppe Zappa, and Mathieu Vrac

Emanuele Bevacqua used model simulations to study compound flooding at the global scale from extreme precipitation and sea-levels. They assessed the dependence of high sea levels and precipitation at the global scale, under both current and future climates. They showed where the change in compound flood return periods in future climate could be attributed due to a change in extreme precipitation,  a change in extreme surge, or a change in the dependence between the two. The research shows that the compound flood potential from concurrent storm surge and extreme precipitation in several regions will have substantial changes beyond sea level rise, and should be considered for adaptation planning.

A centennial catalogue of hydro-geomorphological compound events and corresponding atmospheric forcing

Alexandre M. Ramos, Susana Pereira, Ricardo M. Trigo, and José L. Zêzere

Alexandre Ramos studied the hydro-geomorphological occurrences of flood (urban, flash flood or river floods) and landslides in Portugal, using a database from 1865 to 2015 compiled from newspaper articles. Their research assessed the atmospheric drivers that caused combined floods and landslides events, in this database. During ~60% of the compound events, SW/W/NW weather types were dominant. In addition, besides the extra-tropical cyclones associated with this events, the atmospheric rivers were associated with more than half of these events enhancing the precipitation event. They also found that the effect of conditional rainfall previous to the even was important in explaining the impact of these events, which created favorable conditions for landslides to occur either from previously wet soils or due to extreme amounts of precipitation. More details on these findings are available here.

Predicting compound dry-hot events over global land areas based on large-scale climate indices

Zengchao Hao, Xinying Wu, Sifang Feng, Vijay Singh, Fanghua Hao, and Xuan Zhang

Zengchao Hao looked into compound dry-hot events which can have very large impacts globally. Such event can be due to natural cycles such as ENSO, large-scale circulation patterns, and relatively greater regional sensitivity to global change.  He presented a new method to predict these compound dry-hot events based on ENSO, and demonstrated its first application at the global scale. A predictive model has been developed using a binary variable as predictand based on monthly precipitation and temperature, assessing the probability that both of these exceed various thresholds. The results show several regions where the model shows good predictive skill. More details on these findings are available here.

Joint probability of hot and dry meteorological extremes

Ana Russo, Andreia Ribeiro, and Célia M. Gouveia

The Mediterranean region is a hotspot for extremes, including mega-heatwaves which can lead to large socioeconomic impacts. Andreia Ribeiro presented research looking at the joint probability of hot and dry extremes in the Mediterranean, with the goal of assessing whether hot extremes are preceded by moisture deficits. For moisture, they use SPEI as a proxy, and for hot extremes, the number of hot days per month exceeding the 90th percentile is used. The correlation is assessed between these proxies to identify hotspots. The research shows that in most regions of the Mediterranean (but also in Western Europe), there are significant negative correlations between the number of hot days and SPEI, with the correlations becoming stronger throughout the summer season. The results show potential for early warning. These findings can also be found here.

Vulnerability of hop production due to compound climate events over Europe

Vera Potopová, Martin Možny, Luboš Türkott, Josef Soukup, Javier J Cancela, Paula Paredes, Martin Pavlovič Martin Pavlovič, Deniz Bilge, Francis Heitz, Siniša Srečec, Martin Steinhaus Martin Steinhaus, and Florian Weihrauch

Climate extremes affect most of the core ingredients of beer production (hop, water and barley), and Vera Potopová described research assessing the vulnerability of European hop production to compound climate events, both floods and droughts. The research shows that hop production critically depends on both the duration and coincidence of extreme events. Longer and more severe drought and heatwave occurrences have increased more than shorter, less severe concurrences. Overall, the quality of hops is decreasing whilst demand for hops is increasing. Whilst irrigation can reduce the drought impact, a problem is the decreasing content of alpha acids. Vera states that: “It’s so much easier to be a good manager in a wet year!”

Involvement of Stochastic Weather Generators within the DAMOCLES project

Martin Dubrovsky, Ondrej Lhotka, Petr Stepanek, Jiri Miksovsky, and Jan Meitner

Martin Dubrovsky provided an overview of weather generators, i.e. models to produce synthetic weather data statistically similar to real-world weather data. They presented a promising weather generator, SPAGETTA, that he and colleagues have been developing since 2016. Weather generators are a promising tool for studying compound events (CEs), therefore developing SPAGETTA towards a better representation of some relevant CE characteristics would be relevant. SPAGETTA can already reproduce many relevant features of the observed climate system, such as temporal and spatial structures of hot and dry spells or wet and dry events. An interaction between the SPAGETTA developers and DAMOCLES members would allow for the improvement of the weather generator, which could ultimately be useful for assessing the impact of several CEs.

This summary blog was written by Philip Ward, Anaïs Couasnon and Emanuele Bevacqua.