Guidelines for compound extremes modelling in current and future climates

The overarching goal of the COMPASS project is to develop a harmonised methodological framework for climate and impact attribution of various complex extremes that includes compound, sequences and cascading hazard events. This report, deliverable D1.1, presents an inventory of datasets and introduces modelling workflows for characterizing compound extremes in current and future climates. It contributes to the COMPASS project’s objective of improving hazard modelling with sufficient accuracy at the local scale and to the Work Package 1 objective of developing a flexible hazard modelling approach for compound extremes. This report is a foundational step in guiding modelling efforts, supporting future work in specific hazard scenarios, and linking to other deliverables, such as D2.1 from Work Package 2, which more specifically addresses climate attribution methods and datasets. Deliverable 1.1 will be updated throughout the project, leading to deliverable D1.2 at the end of the project, which will integrate lessons learned from the hazard modelling process and integrated into the guidelines.

This report focuses on the following compound extreme events: compound pluvial, fluvial and coastal flooding either from tropical cyclones or from winter storms, compound coastal flooding and windstorms from extra-tropical cyclones, hydrological drought and saltwater intrusion and compound heatwaves with droughts. We present the hazard modelling workflows, i.e. the modelling chains, that will be applied to model each compound event types along with possible global or continental datasets that can be used. We also list specific requirements needed for each Use Case (UC), i.e. a selected recent event from each compound event type. The UC cover a wide variety of compound hazards, over many different geographical areas, where local or regional data availability differs in terms of spatial and temporal coverage.

Compound extremes have complex spatiotemporal dynamics that must be accurately captured to model their impacts accurately. We identify specific points of attention of importance and recommendations applicable to all UCs for the compound hazard modelling, mainly regarding the uncertainties introduced by the modelling chain and the spatial and temporal resolution of the input datasets. The quality and resolution of the input datasets can significantly affect the accuracy of modelled hazard and thus the ability to make robust attribution statements. Some recommendations have already been included in the workflows, for example by incorporating statistical downscaling and/or dynamical downscaling of climate drivers. Similarly, validation of the workflow will be performed when possible and uncertainties will be quantified along the modelling chain by testing different input datasets. The updated version of this report will expand and consolidate the guidelines based on the outcomes of the UC hazard modelling.