The problem of tropical cyclone formation requires among otherthings an improved understanding of recirculating flow regions onsub-synoptic scales in a time evolving flow with typically sparse real-timedata. This recirculation problem has previously been approached assuming as afirst approximation both a layer-wise two-dimensional and nearly steady flowin a co-moving frame with the parent tropical wave or disturbance. This paperprovides an introduction of Lagrangian techniques for locating flowboundaries that encompass regions of recirculation in time-dependent flowsthat relax the steady flow approximation.Lagrangian methods detect recirculating regions from time-dependent data andoffer a more complete methodology than the approximate steady framework. TheLagrangian reference frame follows particle trajectories so that flowboundaries which constrain particle transport can be viewed in aframe-independent setting. Finite-time Lagrangian scalar field methods fromdynamical systems theory offer a way to compute boundaries from grids ofparticles seeded in and near a disturbance.The methods are applied to both a developing and non-developing disturbanceobserved during the recent pre-depression investigation of cloud systems inthe tropics (PREDICT) experiment. The data for this analysis is derived fromglobal forecast model output that assimilated the dropsonde observations asthey were being collected by research aircraft. Since Lagrangian methodsrequire trajectory integrations, we address some practical issues of usingLagrangian methods in the tropical cyclogenesis problem. Lagrangiandiagnostics are used to evaluate the previously hypothesized import of dryair into ex-Gaston, which did not re-develop into a tropical cyclone, and theexclusion of dry air from pre-Karl, which did become a tropical cyclone andlater a major hurricane.
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