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Blake Rutherford We examine which pre-genesis disturbances will eventually become tropical storms or hurricanes using Lagrangian methods. Recent theoretical and observational evidence suggests that a favorable kinematic environment from African easterly wave critical-layer flows is a necessary precursor to a favorable dynamic and thermodynamic environment that actually serve to spin-up the proto-vortex within. Though a protected kinematic environment is necessary for development, predictability of cyclogenesis remains elusive as only about 20% of these waves (including those without a critical layer) actually produce hurricanes.
One reason for this is that the wave-relative boundaries estimated from instantaneous Eulerian data may be permeable to mass and vorticity transport. The inward stirring of environmental dry air, moisture, and vorticity across closed streamlines of the steady flow alters the dynamics inside the otherwise-protected region. Lagrangian flow boundaries offer a way to describe this transport and its implications on development. Illustrations are offered demonstrating that the subtleties of time-dependent Lagrangian transport may either decrease, or increase, the probability of genesis, depending on whether dry or moist air is entrained laterally.
We find that there are two primary places where Lagrangian boundaries improve existing steady-state methods. First, Lagrangian boundaries may be used to identify and quantify transport of moisture into developing and nondeveloping disturbances. In addition, the vorticity transport can be quantified through Stoke’s theorem applied to invariant regions bounded by Lagrangian manifolds. Second, Lagrangian methods identify impenetrable vortex cores and the shear sheaths that protect them while eliminating spurious vortices that are short-lived. Examples are given from ECMWF operational analysis data used in our real-time products over the past few seasons.
Our primary goals are (i) to improve understanding of lateral Lagrangian transport and exchange of air masses interior and exterior to the wave pouch, (ii) to describe the transition from tropical depression to tropical storm, including diagnosis of the kinematic protection due to shear-sheltering of the vortex core, and (iii) studying the predictability of genesis on an idealized vortex strip.
Applying these methods to an idealized barotropic developing vortex will answer how the predictability of genesis is affected by small perturbations.
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