Articles | Volume 17, issue 17
Research article
04 Sep 2017
Research article |  | 04 Sep 2017

The genesis of Hurricane Nate and its interaction with a nearby environment of very dry air

Blake Rutherford, Timothy Dunkerton, Michael Montgomery, and Scott Braun

Abstract. The interaction of a tropical disturbance with its environment is thought to play an important role in whether a disturbance will develop or not. Most developing disturbances are somewhat protected from the intrusion of environmental dry air at mid-levels. For African easterly wave (AEW) disturbances, the protective boundary is approximated by closed streamlines in the wave-relative frame, and their interior is called the wave pouch. The dynamic and thermodynamic processes of spin-up occur inside the pouch.

In this study, we define the kinematic boundaries for a non-AEW disturbance in the Bay of Campeche that originated along a sharp frontal boundary in a confluent region of low pressure. We examine these boundaries during the genesis of Hurricane Nate (2011) to show how a pouch boundary on isobaric levels in the Lagrangian frame may allow for some transport into the pouch along the frontal boundary while still protecting the innermost development region. This result illustrates a generic property of weakly unsteady flows, including the time-dependent critical layer of AEWs, that lateral exchange of air occurs along a segment of the boundary formed by the instantaneous, closed translating streamlines.

Transport in the Lagrangian frame is simplest when measured with respect to the stable and unstable manifolds of a hyperbolic trajectory, which are topologically invariant. In this framework, an exact analysis of vorticity transport identifies the primary source as the advection of vorticity through the entrainment and expulsion of bounded material regions called lobes. We also show how these Lagrangian boundaries impact the concentration of moisture, influence convection, and contribute to the pouch vertical structure.

Short summary
Whether a tropical disturbance develops into a tropical cyclone is determined by many factors including whether nearby dry air is able to intrude into the disturbance and disrupt key thermodynamic processes. In this research, we explored a way to diagnose this interaction from a time-evolving rather than instantaneous viewpoint, so that the dry air import can be seen more precisely. We expect that this framework, here applicable to Hurricane Nate (2011), will also apply to other disturbances.
Final-revised paper