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Transport and mixing in idealized barotropic hurricane‐like vortices

The effective diffusivity diagnostic is used to obtain basic insight into the two‐dimensional transport and mixing properties of idealized barotropic tropical‐storm and hurricane‐like vortices. Three flow configurations believed to be relevant to hurricane dynamics are examined in a non‐divergent barotropic model: (i) an elliptical vortex, (ii) a Rankine vortex in a turbulent background vorticity field, and (iii) unstable vorticity rings. During the evolution of these vortical flows, effective diffusivity is used as a mixing diagnostic on a passive tracer field that also evolves in the non‐divergent flow. The internal dynamical processes causing mixing, as well as the location and magnitude of both turbulent mixing and partial barrier regions, are identified in the evolving vortices. Breaking vortex Rossby waves (VRWs) are found to create turbulent mixing regions of finite radial extent. For monotonic vortices, which are analogous to tropical storms, the wave breaking and axisymmetrization creates a surf zone outside the radius of maximum wind, while the vortex core remains a partial barrier or containment vessel. For unstable vorticity rings, which are analogous to intensifying hurricanes, two regimes of internal mixing are found. During barotropic instability of thick rings, the inner and outer breaking VRWs create two local mixing regions, separated by a partial barrier region at the location of the tangential jet. For barotropic instability of thin rings, the entire hurricane inner core becomes a turbulent mixing region, allowing passive tracers to be radially mixed between the eye, eyewall and local environment. In either case, the horizontal mixing associated with the inner, breaking VRW would support intensification, provided the passive tracer is equivalent potential temperature with a maximum in the eye. In addition to the insights obtained for internal mixing in hurricanes, effective diffusivity is shown to be a robust diagnostic for two‐dimensional turbulence, complementing its previous use in large‐scale atmospheric dynamics. Copyright © 2009 Royal Meteorological Society