A storm-relative climatology of compound hazards in Mediterranean cyclones

Abstract

Cyclones are responsible for much of the weather damage in the Mediterranean region, and while their association with individual weather hazards is well understood, their association with multivariate compound hazards remains to be quantified. Since hazard compounding is associated with enhanced risk, this study aims to establish a cyclone-relative climatology of three different multivariate hazards in Mediterranean cyclones, namely, the co-occurrences of rain and wind, rain and waves, and particulate matter and warm spells. The hazards are composited separately for nine cyclone classes associated with nine large-scale environments using a recent potential-vorticity-(PV-)based cyclone classification. This cluster-based compositing of multivariate hazards outlines the role of the large-scale environment in the occurrence of impactful cyclones. The composites are computed relative to cyclone centers and at the time of maximum intensity, when the association with compound hazards is strongest for most of the nine cyclone classes, to illustrate the spatial footprint of the multivariate hazards associated with the cyclones. Finally, datasets of cold fronts, warm conveyor belts and dry intrusions are composited alongside the hazards to provide information on the contribution of smaller-scale features to the occurrence of multivariate hazards. We find that few different large-scale configurations are associated with each specific compound event type. Compound rain and wind events are mostly associated with frontal cyclones and cyclones induced by anticyclonic Rossby wave breaking. These events are most frequent in the winter half of the year. Compound rain and wave events also occur primarily during winter but are associated with cyclonic Rossby wave breaking. Particulate matter and heat compound events are associated with heat lows, daughter cyclones and anticyclonic Rossby wave breaking in the warm season and over north Africa. The probability of compounding associated with a cyclone class does not depend monotonically on the probabilities of the individual contributing hazards but also depends on their temporal and spatial correspondence. Finally, we find that warm conveyor belts and cold fronts frequently co-occur with rain and wind and rain and wave events. The association of compound hazards with warm conveyor belts and cold fronts is similar to previous results from the Atlantic basin but substantially modulated by the local topography and land–sea distribution. Particulate matter and warm spells are not strongly associated with these dynamical features. These results, which systematically associate various large-scale environments and dynamical features to different compound event types, have implications for forecasting and climate risk predictions.