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Tropical cyclone intensification

Tropical cyclone intensification. Roger Smith Ludwig-Maximilians University of Munich Collaborators: Michael Montgomery, Naval Postgraduate School, Monterey, California Ph. D. students: Sang Nguyen, Seoleun Shin (LMU) Postdoc: Hai Hoang, Vietnam National University. Topics.

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Tropical cyclone intensification

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  1. Tropical cyclone intensification Roger Smith Ludwig-Maximilians University of Munich Collaborators: Michael Montgomery, Naval Postgraduate School, Monterey, California Ph. D. students: Sang Nguyen, Seoleun Shin (LMU) Postdoc: Hai Hoang, Vietnam National University

  2. Topics • How do tropical cyclones intensify? • The basic thought experiment for intensification • Important physical principles • Paradigms for intensification • Recent discoveries using idealized model simulations with simple physics • Dynamics of vortex spin up • Is WISHE relevant? • Axisymmetric view of spin up – comparison with the other paradigms • New frontiers

  3. The basic thought experiment for intensification Initial condition Mean sounding Axisymmetric vortex T(z) q(z) V(r,z) r p 28oC sea

  4. The primary circulation Pressure gradient force LO r v sea Centrifugal force and Coriolis force

  5. Frictionally-induced secondary circulation primary circulation Secondary circulation Pressure gradient force r Friction layer v v Centrifugal force and Coriolis force are reduced by friction

  6. “Tea cup” Experiment

  7. Hurricane intensification • Basic principle - Conservation of absolute angular momentum: M = rv + r2f/2 f = Coriolis parameter = 2Wsin(latitude) r v v = M/r - rf/2 If r decreases, v increases! Spin up requires radial convergence

  8. Paradigms for intensification Conventional view articulated by Ooyama (1969, 1982), Willoughby (AMM 1998, WMO, 1995) involving convectively-induced convergence together with absolute angular momentum conservation above the boundary layer. Thermodynamic view(E-theory)articulated by Emanuel (1989, 1994, 1995, 1997) involving the WISHE mechanism. Asymmetric view(M-theory)invoking “Vortical Hot Towers” or VHTs (Hendricks et al. 2004, Montgomery et al. 2006, Nguyen et al. 2008, Shin and Smith 2008, Montgomery et al. 2009, Smith et al. 2009, Hoang et al. 2009).

  9. Conventional view 15 z km 10 5 M conserved 0 50 r km 100 M not conserved,

  10. Thermodynamic view: A steady hurricane model 1986 Journal of the Atmospheric Sciences

  11. Emanuel’s 1986 steady hurricane model z M = M(qe)  qe= qe(p) Boundary layer controls dqe/dM h Region I Region II Region III (RH = 80%) rmax E86 ignores BL dynamics here! re r

  12. Thermodynamic view has problems! 2008 Available on my website

  13. Revised Cannot ignore unbalanced BL dynamics!

  14. Accepted subject to minor revision WISHE = Wind induced surface heat exchange • Basic air-sea interaction feedback loop: • Increase in surface wind speed => • Increase in surface moisture transfer from the sea surface => • Increase in “fuel supply” to the storm => • Increasing wind speed …

  15. Asymmetric view Available: http://www.meteo.physik.uni-muenchen.de/~roger • Idealized numerical model simulations with simple physics (MM5) • 5 km (1.67 km) resolution in the finest nest, 24s-levels

  16. Evolution of Intensity

  17. Vertical velocity\vorticity pattern at 24 h 850 mb ~ 1.5 km 850 mb Vertical velocity Relative vorticity

  18. Vertical vorticity evolution at 850 mb 10 h 12 h

  19. Vertical vorticity pattern at 850 mb 18 h 24 h 300 km 300 km 300 km

  20. Vertical vorticity pattern at 850 mb 36 h 48 h

  21. Interim conclusions • The flow evolution is intrinsically asymmetric. • The asymmetries are associated with rotating convective structures that are essentially stochastic in nature. • We call these structures vortical hot towers (VHTs). • Their convective nature suggests that the structures may be sensitive to the low-level moisture distribution, which is known to possess significant variability on small space scales. • Suggests a need for ensemble experiments with random moisture perturbations.

  22. Evolution of local intensity: 10 ensembles control VTmax From Nguyen et al. 2008

  23. Vertical velocity pattern at 850 mb at 24 h control Ensemble 1 Ensemble 2 Ensemble 3 From Nguyen et al. 2008

  24. Is WISHE relevant? Capped flux experiments

  25. In press

  26. Azimuthal average of the Nguyen et al. control calculation Tangential wind speed

  27. Radial wind speed vertical velocity

  28. Movie Time-height sequence of Absolute Angular Momentum

  29. Revised view of intensification: two mechanisms 15 10 z km 5 M conserved 0 50 r km 100 Mreduced by friction, but strong convergence  small r

  30. Exciting times! • There is much work to do to pursue all the consequences of our recent findings and the new paradigm for intensification. • We need to determine the limits of predictability for intensity and especially for rapid intensification. • We need to much better understand the flow in the inner core, beneath and inside the eyewall, and to determine the utility of conventional (boundary layer) representations of this region in models. • We need to develop a new theory for the potential intensity of tropical cyclones for climate assessments.

  31. Thank you

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