Lagrangian coherent structures in developing and nondeveloping disturbances

The Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) experiment conducted during the 2010 Atlantic hurricane season provided a vast amount of data which was used to study the Lagrangian nature of tropical cyclogenesis. The pouch theory of the marsupial paradigm suggests that the amplification of vorticity in regions of cyclonic recirculation is necessary for cyclogenesis to occur. The amplification process is aided by flow boundaries in the Lagrangian frame of reference which are important for protecting regions of cyclonic recirculation from the intrusion of dry air. The protected region, called the pouch, is centered at the intersection of the wave trough axis of an African easterly wave and the wave critical layer. In the co-moving reference frame moving with the pouch at its mean translation speed, the flow is nearly steady. In this frame, the flow boundaries that protect the pouch can be simply described as the stable and unstable manifolds of a hyperbolic fixed point.

To generalize the flow boundaries to time-dependent flows, we drop the assumption that the flow is steady in the co-moving frame, and instead consider the reference frame along particle trajectories. Lagrangian coherent structures are computed directly from particle trajectories and provide a generalization of the manifolds of a fixed point that are continuous through the time-varying flow, and better represent the boundaries that protect the pouch.

The Lagrangian coherent structures and trajectory analysis are applied to both developing and non-developing pre-genesis disturbances during the PREDICT experiment. In the developing cases, the Lagrangian coherent structures were more persistent and showed more vertical coherence than in non-developing cases. Some practical aspects of computing trajectories and Lagrangian coherent structures from sparse real-time data sets are also addressed.