Tropical cyclones in ver/cal shear: dynamic, kinema/c, and thermodynamic aspects of intensity modification

Transcription

1 Tropical cyclones in ver/cal shear: dynamic, kinema/c, and thermodynamic aspects of intensity modification Michael Riemer 1, Michael T. Montgomery 2,3, Mel E. Nicholls 4 1 Johannes Gutenberg-Universität, Mainz, Germany 2 Naval Postgraduate School, Monterey, CA, USA 3 NOAA Hurricane Research Division, Miami, FL, USA 4 University of Colorado, CIRES, Boulder, CO, USA

2 Vertical shear is a main contributor to intensity change predictors for SHIPS (statistical TC intensity forecast model) Understanding of governing processes is s/ll incomplete. Our goal: Improve understanding by analyzing idealized numerical experiments.

3 Numerical experiment: spin up TC and hit it with shear as pioneered by e.g. Bender 1997, and Frank & Ritchie 1999, 2001 TC vortex height (~10 km) RMW (~30 km) experiment u max no shear 0 m/s 5mps 5 m/s 10mps 10 m/s 15mps 15 m/s 20mps 20 m/s hpa shear shear profile u(z) = 0.5 u max (cos(π z/ 12) - 1), z = height in km in thermal wind balance virtually steady in a novortex experiment

4 Numerical model: the virtue of simplicity RAMS (non-hydrostatic) l surface fluxes: o bulk aerodynamic formula, C k /C D = 1 o Deacon s formula for drag coefficients l parameterizations: o warm rain microphysics o no cumulus convection scheme o no radiative processes o turbulence (based on Smagorinsky) l SST = 28.5 C, f-plane l double, two-way nested domain, 5 km l intense and resilient TCs Focus on structural changes (meso-β scale) and conceptual understanding

5 An unsung pathway to shear-induced weakening averaged over inflow layer θ e top of inflow layer (1km) w θ e (w < 0) shear vector 400 km 400 km 5 h 400 km 1 h - 6 h average 400 km Km/s updrays at 2 km: 0.2 m/s 1 m/s Thermodynamic impact on inflow layer: significant θ e depression: O(15 K) à reduc/on of eyewall θ e by a few Kelvin à (rela/ve) weakening of some 10 m/s

6 Weakening of TC s thermodynamic (Carnot) cycle z quenching the source diabatic processes: ocean fluxes and radiation θ e 3 θ e 2 θ e 1 moist isentropes: θ e 1 < θ e 2 < θ e 3 r distinct, shear-induced thermodynamic impact on inflow layer

7 A distinct structural change averaged over inflow layer θ e top of inflow layer (1km) W θ e (w < 0) shear vector 400 km 400 km 5 h 400 km 1 h - 6 h average 400 km Km/s updrays at 2 km: 0.2 m/s 1 m/s Forma/on of convec/ve asymmetry outside of the eyewall sta/onary band complex (SBC)

8 Downdraft formation and the stationary band complex shear vector 2 km vertical cross section along 75 km radius: 6h average 400 km 6h average 15mps 400 km Km/s low level updrafts N E S W N w: θ e : every 2.5 K, 350 K cm s - 1 downdrafts form underneath the helical updrafts of the SBC precipitation evaporating in unsaturated air below

9 Dynamic response contribution of moist, to stationary baroclinic vortex: band tilt complex direction formation vortex settles into left-of-shear tilt equilibrium (e.g. Reasor, Montgomery and Grasso 2004) height in km 12 7 W S E 15mps tilt direction outer-vortex tilt = standing VRW wave #1 pattern = low-level vorticity anomaly wave # 1 vorticity asymmetry downshear right Tilt evolves consistent with balanced dynamics (not shown here) ver/cal cross sec/ons along /lt axis 15mps vor/city time in [h] radius in km contours = [5,10,20,50] x10-5 s - 1 downshear left x10-5 s - 1 Figure 6a from Jones 1995 (dry PE experiment, note the different aspect ra/o)

10 Forcing of vertical motion by low-level vorticity anomaly w Ekman ~ 1 / 2 H BL ζ #1 vertical motion: wave # 1 asymmetry 10 h 2 km frictional convergence provided by vortex tilt: favorable meso-β scale environment for SBC formation balanced TC vortex dynamics à thermodynamic impact cm s - 1

11 Kinematic contribution to stationary band complex formation moist envelope = local (meso-β scale) region of high- θ e air Streamline in co- moving frame à flow quasi- steady 2 km 6 h average θ e tracer of full 3- D flow θ e distribu/on governed by advec/on and steering of the quasi- steady flow moist envelope confined to TC seminal work by Willoughby et al km 4.5ms - 1 storm- rela/ve environmental flow 700 km θ e K updrays: 1 ms ms - 1) distor/on of moist envelope: favorable meso- β scale, high- θ e environment for SBC forma/on

12 Shear-induced environmental storm-relative flow Streamline in co- moving frame à flow quasi- steady 2 km z v rel v abs v trans o 700 km y v rel x shear profile 4.5ms - 1 storm- rela/ve environmental flow 700 km θ e K updrays: 1 ms ms - 1) Ver/cal wind shear à environmental storm- rela/ve flow

13 Shear-induced deformation of the moist envelope Streamline in co- moving frame à flow quasi- steady 2 km > > > > ψ Vrel + > x > ψ vortex simple superposi/on yields 700 km 4.5ms - 1 storm- rela/ve environmental flow 700 km θ e K updrays: 1 ms ms - 1) Deforma/on of moist envelope is simple kinema/c consequence of ver/cal shear x

14 Downdrafts outside of the moist envelope Streamline in co- moving frame à flow quasi- steady 2 km 700 km 700 km 4.5ms - 1 storm- rela/ve environmental flow 700 km θ e K updrays: 1 ms ms - 1) 700 km m/s forma/on of downdrays outside of moist envelope

15 Robustness of results in our suite of experiments a) c) RMN68 ref b) shear Cat RMN e) shear same general pauern: a) SBC and b) θ e - depression + same general /lt behavior (not shown) à results robust in our suite of experiments CBLAST CBLAST68 shear ICE68 shear

16 Some supporting evidence from the real atmosphere

17 Synthesis z shearvector y tilt equilibrium dynamic (vortex /lt) + kinema1c (moist envelope) consequences of ver/cal shear à favorable meso- β scale environment for SBC forma/on ~200 km x

18 Synthesis z shearvector y tilt equilibrium dynamic (vortex /lt) + kinema1c (moist envelope) consequences of ver/cal shear à favorable meso- β scale environment for SBC forma/on ~200 km x z swirling winds à helical updrays à precip falls into environmental low- θ e air à downdrays form and flush low- θ e into inflow layer y inflow layer x

19 Dynamic Conclusions response of moist, baroclinic vortex: balance assessment shear-induced, thermodynamic impact on the inflow layer downdrafts associated with stationary band complex favorable meso-β scale environment for SCB by vortex tilt (dynamics) and distortion of moist envelope (kinematics) same basic structural evolution with associated weakening is found for weaker TCs, more realistic values of C K and C D, and ice microphysics also Atmospheric Chemistry and Physics 2013: Further examination of the thermodynamic modification of the inflow layer of tropical cyclones by vertical wind shear M. Riemer 1, M. T. Montgomery 2,3, and M. E. Nicholls : 2010: Simple kinematic models for the environmental interaction of tropical cyclones in vertical wind shear M. Riemer 1,* and M. T. Montgomery 1,2 1 A new paradigm for intensity modification of tropical cyclones: thermodynamic impact of vertical wind shear on the inflow layer M. Riemer 1, M. T. Montgomery 1,2, and M. E. Nicholls 3

20 Flow boundaries in idealized numerical experiment θ e tracer of full 3- D flow 2 km 5 km limit cycle 4.5ms - 1 storm- rela/ve environmental flow limit cycle 1.5ms - 1 storm- rela/ve environmental flow θ e K updrays: 1 ms ms - 1) 1) θ e distribu/on limit cycle à distor/on of moist envelope governed by steady, horizontal flow 2) Eyewall well protected from intrusion by steady, horizontal flow

21 Rapid and pronounced weakening with ice microphysics associated with by the far the most pronounced depression of inflow layer θ e θ e ~ 2-3 /mes of warm rain a) 7 h e) 5 h ICE68 c) 23 h ICE68 shear ICE68