Developing storylines for unprecedented extreme events using ensemble boosting

Recent extreme temperature and precipitation events such as the dry and heat events in summer 2022 in Europe and China, the New Year’s warm spell 2022/23 across Europe, the 2021 heavy rainfall extremes in northwestern Germany, Belgium and the Netherlands and the 2021 Pacific Northwest heatwave broke previous observed record levels by large margins. The probability of such unprecedented record-shattering extremes increases with the rapid rate of warming. Thus, there is a crucial need for analyzing the underlying processes leading to these events and quantifying potential intensities of events possible in the coming decades.

Here, we evaluate how ensemble boosting (Gessner et al. 2021 and Gessner et al. 2022) can help assess the tail of climate distributions and generate climate model-based storylines more resource-efficiently. In ensemble boosting the most extreme simulated events in an intermediate-size initial condition ensembles are re-initialized in targeted experiments in order to efficiently sample very extreme states of the model climatology. Here, we evaluate different ensemble design choices including lead time, ensemble size and potential iteration choices to most efficiently allocate computational resources to simulate events of very extreme intensity.

The resulting boosted events are analyzed through a storyline approach, thus helping to interpret the underlying mechanisms of each physically consistent unfolding extreme event and its consequences. The Pacific Northwest heatwave in June 2021 will be used as a starting point; but ensemble boosting and storylines can be powerful tools for understanding extremes beyond heat. We further discuss how ensemble boosting can also be applied to compound extremes and future climate scenarios.