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

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1 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 CIMSS Colloquium Series, 13 April 2006

2 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 and ATM

3 A physical mechanism for PV mixing Production of potential vorticity where j is a unit vector perpendicular to θ-surfaces, and k is a unit vector along the absolute vorticity vector ζ a 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.

4 Rectilinear case (ITCZ) Cylindrical case (Hurricanes)

5 ITCZ breakdown Figure from Nieto-Ferreira and Schubert (1997)

6 ITCZ breakdown Figure from Nieto-Ferreira and Schubert (1997)

7 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). The radial gradient of PV changes sign and the flow can support barotropic instability (Schubert et al. 1999). Hurricane Hilda (1964)

8 PV distribution in numerical simulation of Hurricane Andrew PSU NCAR MM5 simulation with 2 km grid length. Figure taken from Yau et al

9 Numerical integrations: Framework of maximum simplicity while retaining nonlinearity; unforced nondivergent barotropic model described by

10 Numerical Integration: Relaxation to a monopole Hurricane Alberto (2000)

11 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).

12 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.

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

14 DMSP satellite image of Hurricane Isabel near local sunrise on 12 Sep 2003.

15 Vorticity and wind vectors from the numerical experiment Isabel on 12 September

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

17 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?

18 Effect on hurricane structure and intensity add forcing terms

19

20 Axisymmetric Potential Intensity evolution due only to forcing solution asymptotic behavior

21 Axisymmetric Potential Intensity

22 Numerical integration

23 Intensity evolution mixing events Palinstrophy evolution

24 Concluding remarks 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 longertimescale 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.