On 25th August the hurricane Harvey hit the south of Texas, causing major floodings and cost of damage which will likely exceed that of hurricane Katrina in 2005. The low-pressure system associated with the hurricane drove storm surges and an extraordinarily large amount of precipitation. Along the flat coastal area, the storm surge exacerbated the precipitation-driven flooding. Specifically, the discharge of the precipitation water into the sea was partially obstructed by the high sea level and waves, causing extreme water levels inland.
This type of event is called compound flooding, and it is part of a relatively new field of study in the climate community. Mostly, compound floodings happen when atmospheric low-pressure system hit coastal areas. Before Harvey, USA had already experienced compound floodings in occasion of other hurricanes, and such floodings will happen again in future. However, compound floodings do not require hurricanes to happen and are a widespread threat around coastal area of the world. For example, Ravenna (Italy) was hit by a compound flooding in 2015, which caused tens of millions € of damages.
Compound flooding belongs to the general category of compound events (CEs). CEs can be defined as extreme events that would not be adequately described if considering only one driver. In the case of compound flooding, the flood would not be properly described if considering only precipitation or surge levels. Both the variables have to be considered for a proper description – and therefore prediction – of these events. Improving the predictions of compound events is important as in turn this would reduce the impacts of compound events to society. Indeed, a part of the scientific community is trying to achieve better understanding and predictions of compound events. Moreover, a better understanding of these phenomena is relevant for long-term future risk assessment of these events, which is crucial given the climate change we are experiencing.
The category of compound events is actually very wide, and this makes their study very relevant for society. Natural wildfires, whose probability of occurrence is amplified when particular combinations of temperature, humidity and wind occur, are compound events. Also, heat waves amplified by droughts are compound events. Returning to Harvey, its impacts to society were amplified by the extraordinary stationarity (see animation from NASA) of the weather system over Texas (due to a combination of “blocking” high-pressure systems around the storm). Also from this perspective, Harvey can be seen as a compound event, as the impacts were caused both by the intensity and the (long-)duration of rain and storm surges.
Given the direct impact of compound events on society, together with the other researchers of the international project CE:LLO, we have developed a new statistical approach for modelling compound events. We have used this methodology for studying compound floodings in Ravenna (Italy). Also, we are currently working on the application of the model for studying droughts in Europe.
Compound events represent a challenge in the climate science community. A better understanding of these phenomena will help improve their predictions, and in turn to be better prepared to face such calamities with a direct and positive effect on society.