Tropical Cyclone Intensification

Transcription

1 Tropical Cyclone Intensification

2 Theories for tropical cyclone intensification and structure CISK (Charney and Eliassen 1964) Cooperative Intensification Theory (Ooyama 1969). WISHE (Emanuel 1986, Holton and Hakim, 2012) Vortical deep convection paradigm whose mean field view provides an extended Cooperative Intensification Theory To appear in The Bruce Morton Memorial Volume of the Australian Meteorological and Oceanographical Society Journal

3 Schematic cross-section through a hurricane Cirrus Cirrus Eye Eyewall Spiral bands

4 Force balance in a vortex Primary (tangential) circulation Rotation axis Lowest pressure in the centre Pressure gradient force r v Centrifugal force and Coriolis force Gradient wind balance 2 v 1 p r fv r

5 Are the eyewall clouds buoyant? Eye is the warmest place in the storm See Smith, Montgomery and Zhu (2005) Dyn. Atmos & Oceans

6 Have to consider generalized buoyancy! g e ( ) ref 2 v r fv p = constant p ref ref z r g g e g e 2 v fv, g r

7 Boundary layers in nonrotating fluids A simple example: steady two-dimensional boundary layer on a flat plate at normal incidence to an almost parallel stream U(x). U(x) U(x) z boundary layer x u(x,z) < U(x)

8 Frictionally-induced secondary circulation Secondary circulation V(r) 1 p v r r 2 Pressure gradient force fv r V Boundary layer v Centrifugal force and Coriolis force reduced by friction

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10 Tropical cyclone intensification Basic principle: conservation of absolute angular momentum: M rv fr M r M v v M r 1 2 fr If r decreases, v increases! Spin up requires radial convergence

11 Dynamics of vortex spin down Vertical cross section V t w z 0 V w z 0 Boundary layer Level of non-divergence

12 The secondary, or in-up-out, circulation secondary circulation secondary circulation C A ds V C Vds A da

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14 Conventional view of intensification: axisymmetric 15 Thermally-forced secondary circulation leads to spin up z km 10 5 M 1 v fr r 2 M conserved 0 50 r km 100 M not conserved, inflow feeds the clouds with moisture Is that it?

15 The basic thought experiment for TC intensification Initial condition Mean sounding Axisymmetric vortex r V(r,z) p(z) T(z) q(z) sea 27 o C Nguyen, Smith and Montgomery calculation, QJRMS, 2008: Idealized numerical model simulations, simple physics, MM5 5 km (1.67 km) resolution in the finest nest, 24 -levels

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17 Vertical velocity\vorticity pattern at 24 h 850 mb ~ 1.5 km 850 mb Vertical velocity Relative vorticity From Nguyen, Smith and Montgomery, QJ, 2008

18 Movie

19 From Montgomery, Nguyen & Smith (2009): QJRMS

20 From Montgomery, Nguyen & Smith (2009): QJRMS

21 From Montgomery, Nguyen & Smith (2009): QJRMS

22 Movie Time-height sequence of Absolute Angular Momentum 1 M rv fr 2 2 v M 1 r 2 fr

23 Movie

24 Revised view : two spin-up mechanisms 15 z km 10 5 M 1 v fr r 2 M conserved 0 50 r km 100 M reduced by friction, but strong convergence small r large v

25 A surprise The maximum wind speed in the boundary layer can exceed that above the boundary layer. An unusual in feature boundary layers in general. A special feature of the termination boundary layer of intense vortices. Anthes (1972): Inward increase of V due to large radial displacements in the boundary layer with partial conservation of AAM opposes the fictional loss of AAM to the surface.

26 Some steady theories/models Emanuel (JAS 1986) : An air-sea interaction theory for tropical cyclones. Part I: Steady state maintenance. Wirth & Dunkerton (JAS 2006): A unified perspective on the dynamics of axisymmetric hurricanes and monsoons. Hakim (JAS 2011): The mean state of axisymmetric hurricanes in statistical equilibrium. Questions What would be the requirements for a (globally) steady-state hurricane to exist? Do the foregoing theories/formulations satisfy such conditions?

27 Angular momentum considerations Anthes (Rev. Geophys. 1974): The dynamics and energetics of mature tropical cyclones. For a tropical cyclone to achieve a steady state, there would have to be an input of cyclonic relative angular momentum to replenish that lost by friction to the sea surface. Carrier et al. (JFM 1971): A model of the mature hurricane. A hurricane could last about 30 days before it used up its angular momentum.

28 Absolute angular momentum M satisfies Steady flow, no diffusion or eddies:

29 Continuity Flux form

30 Integrate the flux form for the steady state: H 0 0 R Advective flux through r = R Diffusion at z = 0 Eddy and diffusive flux flux through r = R

31 The basic thought experiment for intensification Initial condition Mean sounding Axisymmetric vortex r V(r,z) p(z) T(z) q(z) 26 o C sea Persing, Montgomery, McWilliams, Smith, Atm. Chem. Phys. 2013: Idealized numerical model simulations, simple physics, CM1 3 km horizontal grid spacing, 24 -levels

32 Evolution in intensity

33 Azimuthal mean tangential wind component

34 The overturning circulation

35 Radius-time plots of v and M at 1.5 km v M

36 Radius-time plots of v and M at 12 km v M

37 Evolution in intensity at 12 km cyclone anticyclone

38 M surfaces

39 RAM surfaces

40 RAM surfaces

41 M tendencies

42 M tendencies

43 Integrated RAM / Integrated RAM(t = 0) cyclonic anticyclonic

44 Integrated KE / Integrated KE(t = 0) cyclonic anticyclonic

45 The Emanuel (1986) steady model

46 Summary 1: thermodynamic requirements for a steady state e conserved Subsidence, radiative cooling, dry rain u Moisture fluxes

47 Summary 2: dynamic requirements for a steady state Eddy flux of cyclonic AM Loss of cyclonic AM Gain of cyclonic AM?

48 See: Thank you

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