Dynamical properties of developing tropical cyclones using Lagrangian flow topology
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The goal of this study is to characterize the dynamics and structure of tropical cyclone formation from global model analyses to determine thresholds marking the various stages of development which can be computed from the analysis data. We introduce here a new methodology for identifying disturbances that show high likelihood of becoming a tropical depression or tropical storm. We use Lagrangian frame-independent quantities to define intrinsic coordinates for candidate disturbances prior to and post tropical depression declaration in the best-track data (HURDAT2). We use these Lagrangian quantities also as metrics for quantifying the strength of these systems within global model analyses as they are declared depressions, storms or hurricanes in HURDAT2. The criteria proposed are more precise than similar Eulerian criteria since the minimum thresholds for development in the best-track dataset are very close to the threshold at which no false alarms are produced in the global model analyses. Since only very loose thermodynamic thresholds are required, these criteria can be considered dynamically based and require no statistical analysis to compensate for uncertainties in moisture or convection. We describe further the structure of the developing systems using a set of objective profiles where level contours of the Lagrangian averaged rotation rate are mapped to an equivalent radius. Composites of these profiles reveal that the transition to tropical storm strength vortices is marked by the existence of a notable shear sheath outside a region of enhanced solid-body rotation.
The article of record as published may be found at http://dx.doi.org/10.1002/qj.3196
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