A Lagrangian Generalization of the Marsupial Paradigm for Tropical Cyclogenesis

The 'marsupial paradigm' provides a dynamical explanation and a prediction of the location of cyclogenesis based on dynamical criteria.  However, it is limited by the fact that the kinematic precursor to cyclogenesis is frame-dependent, and requires the existence and knowledge of the translation speed of an African easterly wave (AEW).  Even when the AEW, does exist, it is in no way guaranteed to provide any distinguished frame of reference in a time-dependent flow.   Nonetheless, the concept of a pouch is understood to mean a protective barrier that is present during cyclogenesis or even during intensification and precedes vorticity concentration driven by convergence.  To allow for more general applicability of the pouch concept requires that we adopt a frame-independent viewpoint.  We consider the pouch to be bounded by a set of Lagrangian coherent structures (LCSs) that are frame-invariant, but that still act as kinematic barriers that provide protection while the developing vortex is able to amplify through convectively induced stretching.  These LCSs are guaranteed to be impermeable over their lifetimes, and are thus more important as transport barriers than those that are located by Eulerian methods.  Thus, the existence of these LCSs shows that a pouch will be protected from environmental dry air.

We find that these LCSs are present not only in all of the AEW development cases, but also in other cases where there is no AEW precursor.  These additional cases that are included under the LCS generalization include downstream development from existing storms, storms originating from ITCZ convection, and storms originating from other wave types including mixtures of waves.  This Lagrangian paradigm for tropical cyclogenesis therefore completes the marsupial paradigm by accounting for the 'pouch' in all disturbance types, in all stages of the tropical cyclone lifecycle, and in time-dependent flows.  This generalization can also be extended to other development regions.

While the location of LCSs is required to assess the permeability of the pouch, methods for the location of such structures has been difficult.  We show that a relatively simple computation of a scalar field called the Lagrangian OW field reveals all LCSs in a single global or basin-wide computation.  This allows for the location of developing vortices far easier than tracking easterly waves and computing translation speeds, as is required under the frame-dependent marsupial paradigm.  The results from these methods are very robust since all developing storms are easily located, and many non-developing pouches are easy dismissed as being incoherent in the Lagrangian flow.  Since there are very clear thresholds fro development, these methods may be used for cyclone detection in global forecast models or climate models.