ImpactETC1.0: Impact-oriented tracking of extratropical cyclones with global optimisation and track reconciliation

Abstract. Extratropical cyclones (ETCs) play a critical role in shaping extreme weather events in the Nordic region, often driving storm surges, heavy precipitation, and high winds that can lead to significant socio-economic and environmental impacts. However, traditional cyclone tracking methods focus primarily on large-scale atmospheric dynamics without explicitly linking cyclone characteristics to their regional impacts. To address this gap, we introduce ImpactETC1.0, a novel framework designed to identify and track ETCs with a specific focus on their impacts, here illustrated for the case of storm surges. The framework includes several novel algorithmic features, including global optimisation for the correspondence problem, BLOB analysis techniques for track fragmentation issues arising from surface-level tracking over complex terrain, and automated calibration of post-processing parameters. Applied to the CERRA reanalysis dataset and with a focus on the Nordic region, ImpactETC1.0 successfully reconstructed ETC tracks across complex terrain and during periods of rapid storm evolution, while keeping computational costs low. Compared with a standard nearest-neighbour heuristics, the global optimisation reduced suboptimal connections by up to one-third, at negligible additional runtime. The track reconciliation step was essential in preventing track fragmentation and premature termination of tracks, producing storm tracks that were, on average, twice as long over complex land-ocean boundaries and mountain ranges. As the post-processing step is extremely quick to perform, a sensitivity analysis could be done, and a score named the Single Storm Score for automated calibration of filtering parameters was developed. A key strength of ImpactETC1.0 is the use of global optimisation and track reconciliation, as the need for these in storm tracking will only grow with the increasing resolution of data sets. Together, these results demonstrate that ImpactETC1.0 enables accurate and impact-relevant ETC tracking.