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Internal Dynamic Control of Hurricane Intensity Change: The Dual Nature of Potential Vorticity Mixing James Kossin University of Wisconsin—Madison Space Science and Engineering Center Cooperative Institute for Meteorological Satellite Studies Madison, WI.

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Internal Dynamic Controlof Hurricane Intensity Change: The Dual Nature of Potential Vorticity Mixing

James Kossin

University of Wisconsin—Madison

Space Science and Engineering Center

Cooperative Institute for Meteorological Satellite Studies

Madison, WI

CIMSS Colloquium Series, 13 April 2006


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Collaborators:

Wayne Schubert (Colorado State University)

Pedro Jose Mulero (University of Wisconsin—Madison)

Christopher Rozoff (Colorado State University)

This work is supported under NSF Grant No. ATM-0453694 and ATM-0435644.


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A physical mechanism for PV mixing

Production of potential vorticity

where j is a unit vector perpendicular to θ-surfaces, andkis a unit vector along the absolute vorticity vector ζa

Persistent convection forms a positive PV anomaly in the lower troposphere. Under certain convective conditions, a reversal of PV gradient can emerge and set the stage for dynamic instability.


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Rectilinear case (ITCZ)

Cylindrical case (Hurricanes)


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ITCZ breakdown

Figure from Nieto-Ferreira and Schubert (1997)


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ITCZ breakdown

Figure from Nieto-Ferreira and Schubert (1997)


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Application to hurricanes

As a nascent tropical storm develops, persistent diabatic heating forms a “tower” of PV. When an eye develops, diabatic heating and PV production is confined to an annulus. This results in a “hollow tower” PV structure (Möller and Smith 1994).

Hurricane Hilda (1964)

The radial gradient of PV changes sign and the flow can support barotropic instability (Schubert et al. 1999).


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PV distribution in numerical simulation of Hurricane Andrew

PSU—NCAR MM5 simulation with 2 km grid length.

Figure taken from Yau et al. 2004.


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Numerical integrations:

Framework of maximum simplicity while retaining nonlinearity; unforced nondivergent barotropic model described by


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Numerical

Integration:

Relaxation to a monopole

Hurricane Alberto (2000)


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Modifying the axisymmetric monopole paradigm:

The flow in the hurricane eyewall is frontogenetic and can tend toward a discontinuity (a circular vortex sheet).

Such flows can support barotropic instability at high wavenumbers and fast growth rates.

Flight-level vorticity and tangential wind in Gilbert (1988) and Guillermo (1997).


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The numerical evolution of nearly singular flows can produce persistent or quasi-equilibrated states that are non-ergodic/ asymmetric (non-monopolar).

Wavenumber-8 maximum instability forms 8 mesovortices that merge over the following few hours into 4 mesovortices.


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Depending on the initial geometry of the eyewall PV, a variety of persistent configurations are predicted in the 2D barotropic framework.



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Isabel on 12 September on 12 Sep 2003.

Vorticity and wind vectors from the numerical experiment


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Isabel provided a rare validation of theory based on predictions of idealized equations of motion.


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This pathway for PV rearrangement has been shown to offer explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.

What are the greater ramifications of PV rearrangement?


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Effect on hurricane structure and intensity explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.

add forcing terms


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Axisymmetric Potential Intensity explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.

evolution due only to forcing

solution

asymptotic behavior


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Axisymmetric Potential Intensity explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.


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Numerical integration explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.


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Intensity evolution explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.

mixing events

Palinstrophy evolution


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Concluding remarks explanations for a number of observed features in hurricanes. These include polygonally shaped eyes, vortical convolutions of low-level eye clouds, mesovortices, and “vortex crystals”.

  • The transverse circulation in hurricanes with eyes destabilizes the eyewall flow by producing PV in an annulus, which causes a reversal in the PV gradient.

  • Removal of the instability can involve vigorous PV mixing and rearrangement.

  • The PV mixing produces a variety of structure changes that coincide with a number of observed features in hurricanes.

  • The rearrangement of PV can affect hurricane intensity in two very different ways — as a transient intensification brake and as a longer-timescale amplifier of intensification.

  • The amount of intensity change disruption depends on a number of factors, such as the amplitude and geometry of the forcing. Episodic mixing events seem to require a PV sink in the eye.