Our current understanding of tropical cyclone intensity is based on the idea that the storm's secondary circulation constitutes a sort of heat engine that converts heat into kinetic energy. In an axisymmetric framework, the secondary circulation can be depicted, under the further assumption of stationarity, as streamlines in a height-radius cross-section. In the presence of pronounced asymmetries, however, a depiction of the secondary circulation as streamlines in a height-radius cross-section is prohibited by the full 3-dimensionality of the flow. However, for an improved understanding of the intensity of tropical cyclones in vertical shear - or more generally: highly asymmetric cyclones - it seems important to better understand the distinct air stream that comprise the storm's secondary circulation.
In this work we explore the utility of trajectory clustering to identify the distinct air streams through tropical cyclones in vertical wind shear. Extensive sets of trajectories are calculated using data from previously-published, convection-permitting idealized numerical experiments. The clustering algorithm employs similarity measures adopted from the field of time series data mining. The trajectory clusters illustrate low-level inflow pathways with preferred orientation to the shear vector, a significant amount of downdraft' trajectories that descent into the frictional inflow layer before rising in the eyewall, and increased variability in the upper-level outflow that warrants further investigation. Comparison with trajectory clusters of an idealized tropical cyclone in quiescent environment emphasizes the distinct impact of vertical shear on the structure of the secondary circulation.
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