Examining the Roles of the Easterly Wave Critical Layer and Vorticity Accretion during the Tropical Cyclogenesis of Hurricane Sandy*

Abstract

The tropical cyclogenesis sequence of Hurricane Sandy is examined. It is shown that genesis occurs within a recirculating Kelvin cat's-eye flow of a westward-propagating tropical wave. The cat's-eye flow is able to provide a protective environment for the mesoscale vortex to grow and is characterized by gradual column moistening and increased areal coverage of deep cumulus convection. These findings are generally consistent with a recently proposed tropical cyclogenesis sequence referred to as the ''marsupial paradigm.'' Sandy's cyclogenesis provides a useful illustration of the marsupial paradigm within a partially open recirculating region, with the opening located south of the pouch center. It is suggested that the opening acts to enhance the genesis process because it is adjacent to an environment characterized by warm, moist air, conditions favorable for tropical cyclogenesis. From a dynamical perspective, accretion of low-level cyclonic vorticity filaments into the developing vortex from several sources (the South American convergence zone, an easterly wave located west of the pre-Sandy wave, and cyclonic vorticity generated along Hispaniola) is documented. Organization and growth of the nascent storm is enhanced by this accretion of cyclonic vorticity. A Lagrangian trajectory analysis is used to assess potential contributions to Sandy's spinup from a Caribbean gyre and the easterly wave that formed Hurricane Tony. This analysis indicates that these features are outside of the Lagrangian flow boundaries that define the pre-Sandy wave and do not directly contribute to spinup of the vortex. Finally, the effectiveness of forecasts from the U.S. and European operational numerical weather prediction models is discussed for this case.

Why Do Model Tropical Cyclones Grow Progressively in Size and Decay in Intensity after Reaching Maturity?

“…Examples of such extreme sizes include at the large end supertyphoon Tip (1979), which occurred in the western North Pacific (Dunnavan and Diercks, 1980), and hurricane Sandy (2012) which caused widespread damage along the eastern seaboard of the United States (e.g. Lussier et al, 2015). An example at the small end is tropical cyclone Tracy (1974), which devastated the Australian city of Darwin on Christmas Day (Davidson 2010 and refs).…”

Lagrangian vortices in developing tropical cyclones

“…Including divergence introduces an additional pathway of entrainment through non-intersecting manifolds which allow an open pathway of material to enter in (Riemer and Montgomery, 2011). Lussier et al (2015) gives examples of how these manifold boundaries can be used to identify the boundaries in pre-genesis disturbances.…”

“…These Lagrangian manifolds have been used to show that dry air entering a pouch lacking a fully developed unstable manifold on one side may be detrimental to genesis (Rutherford and Montgomery, 2012), while interactions limited by the lobe dynamics mechanism may still allow genesis (in progress). Interaction with the outer environment may even help genesis, as in the case of hurricane Sandy where moist air with high vorticity was imported through a large pouch opening to the south (Lussier et al, 2015). § Among these methods are finite-time Lyapunov exponents (FTLEs), finitesize Lyapunov exponents (FSLEs), Lagrangian descriptors, and strain lines.…”

“…The outer boundaries in the time-dependent flow can be seen by stable and unstable manifolds of hyperbolic trajectories (Lussier et al, 2015) or by their representation as Lagrangian coherent structures (LCSs; Rutherford and Montgomery, 2012), and in some cases differs drastically from the translating Eulerian boundaries.…”

Why did the storm ex-Gaston (2010) fail to redevelop during the PREDICT experiment?

“…1 The new cyclogenesis model and accompanying scientific hypotheses were established observationally in the Atlantic and eastern Pacific sectors by DMW09. The new models find additional support in idealized numerical modeling simulations (Wang et al, 2010a, b;Montgomery et al, 2010a;Nicholls and Montgomery, 2013), recent case studies in the field in the western North Pacific during the Tropical Cyclone Structure Experiment 2008 (TCS08, Montgomery et al, 2010b;Lussier III, 2010;Montgomery et al, 2012;Raymond and López-Carrillo, 2011;Lussier III et al, 2014), in the Atlantic during the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT) campaign in 2010 Smith and Montgomery, 2012;Ahijevych, 2012, 2013), in NASA's ongoing Hurricane and Severe Storm (HS3) missions (2012)(2013)(2014)(2015)(2016) and the case of Hurricane Sandy (Lussier III et al, 2015). The field data afford a resolved view of horizontal and vertical structure in the wave pouch and its immediate surroundings, valuable for system centering, circulation magnitude, vortic-8512 T. M. Freismuth et al: Why did the storm ex-Gaston (2010) fail to redevelop?…”