The numerous approaches to tracking extratropical cyclones and the challenges they present

Accurately determining extratropical cyclone paths can help one to anticipate and understand the regional impacts associated with heavy precipitation and strong winds. Storm tracking algorithms aim to objectively identify these paths in both reanalysis data and climate model simulations. There are numerous methodologies used in locating and tracking extratropical cyclones. However, there is no universally agreed objective definition of what an extratropical cyclone is and how to measure its precise location or movement. Consequently, climatological extratropical cyclone counts can differ greatly between methods, for instance, as we discuss here, these differ by more than 100% when considering all basins in the Northern Hemisphere. Here we give an overview of commonly used methods and show that differences between them arise due to several factors, including the identification variable(s) (mean sea level pressure, vorticity, etc), numerics behind the tracking code, and choice of physical or track duration thresholds. We propose that this method uncertainty could be a major source of total uncertainty in projections of storm tracks, intensities, and associated impacts. We suggest that studies should consider employing more than one tracking method to better estimate the robustness of their findings.