Abstract
Abstract. A new free modular software package is described for tracking tropopause polar vortices (TPVs) natively on structured or unstructured grids. Motivated by limitations in spatial characterization and time tracking within existing approaches, TPVTrack mimics the expected dynamics of TPVs to represent their (1) spatial structure, with variable shapes and intensities, and (2) time evolution, with mergers and splits. TPVs are segmented from the gridded flow field into spatial objects as restricted regional watershed basins on the tropopause, described by geometric metrics, associated over time by overlap similarity into major and minor correspondences, and tracked along major correspondences. Simplified segmentation and correspondence test cases illustrate some of the appeal, sensitivities, and limitations of TPVTrack, including effective representation of spatial shape and reduced false positive associations in time. Tracked TPVs in more realistic historical conditions are consistent in bulk with expectations of life cycle and mean structure. Individual tracks are less reliable when discriminating among multiple overlaps. Modifications to track other physical features are possible, with each application requiring evaluation.
Highlights
Among the disturbances on the extratropical tropopause, tropopause polar vortices (TPVs) are common coherent upper-level potential vorticity anomalies with typical radii of 100 to 1000 km and lifetimes of days to months (Hakim, 2000; Hakim and Canavan, 2005)
Within a framework of Ertel’s potential vorticity (Rossby, 1939; Hoskins et al, 1985; Pedlosky, 2013) in a composite sense (Cavallo and Hakim, 2010), dynamics of TPVs are dominated by quasi-horizontal advection subject to generally smaller diabatic and frictional forcings
Does TPVTrack accurately represent and track TPVs? any validation (Roache, 1998) depends on a reference truth. We explore this question through both simplified component test cases with reference solutions (Sect. 3.1) and historical cases with more realistic conditions but less clear expectations (Sect. 3.2)
Summary
Among the disturbances on the extratropical tropopause, tropopause polar vortices (TPVs) are common coherent upper-level potential vorticity anomalies with typical radii of 100 to 1000 km and lifetimes of days to months (Hakim, 2000; Hakim and Canavan, 2005). Because our primary interest is in the physical TPVs rather than the tracked objects, clarity in the characteristics of each algorithm may lend confidence and rationale for informing whether results derived from automated tracks are artifacts or physical, especially if an ensemble with complementary approaches can be constructed by opportunity (e.g., Tebaldi and Knutti, 2007) or design. Towards such a usable description of TPVTrack, Sect.
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