impacts of turbulence on hurricanes (onr-baa-09-012) pi: yongsheng chen, york university, toronto,...
TRANSCRIPT
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IMPACTS OF TURBULENCE ON HURRICANES(ONR-BAA-09-012)
PI: Yongsheng Chen,
York University, Toronto, Ontario, Canada
Co-PIs: George H. Bryan and Richard Rotunno,
National Center for Atmospheric Research, Boulder, Colorado, USA
NOPP Progress Report
February 25, 2011
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Agenda
1. Background
1. New work
1. Next steps
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1. Background
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Hurricane Simulation Models
Fast, simple, but all effects of nonaxisymmetric motionsmust be somehow represented
Axisymmetric
Tangential velocity from an axisymmetric numerical model
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Axisymmetric Model Sensitivity of Wind Speed to Mixing Length lh
Bryan and Rotunno (2009 MWR)
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What is ?
There are no observations of radial turbulent fluxes in a hurricane
lh
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Marks et al. (2008, MWR)
Reflectivity (dBZ) at 1726 UTC
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Marks et al. (2008, MWR)
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Davis et al. (2008 MWR)
Δ =1.33kmΔ =4.0km
Radar Reflectivity at z=3kma) WRFb) WRFc) ELDORA
Hurricane Simulation Models Three-Dimensional Mesoscale Forecast Model
Vortex asymmetries computed, but effects of small-scale (< 1000m) turbulent motionsmust be somehow represented
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(Δ =1.67km) (Δ =556m)
(Δ =185m)
2020
20
20
−20−20
−20
−20
00
0
0
x[km] x[km]
y[km
]y[km
]
(Δ =62m)
WRF Model Idealized TC resolution study, 10-m Wind Speed t=9.75d
max=61.5
max=121.7max=86.2
max=86.7
Rotunno et al. (2009 BAMS)
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37km
Turbulent fluxes computed, buthigh resolution (<100m) required
Hurricane Simulation Models
Large Eddy Simulation
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Idealized TC:f-plane zero env windfixed SST
Nested Grids
WRF Model Physics:WSM3 simple iceNo radiationRelax to initial temp.Cd (Donelan)Ce (Carlson-Boland)Ce/Cd ~ 0.65YSU PBLLES PBL
(Δ ≥1.67km)(Δ <1.67km)
Domain6075km
1500km1000km
333km111km
37km
(Δ =15km)
(Δ =5km)(Δ =1.67km)
(Δ =556m)(Δ =185m)
(Δ =62m)
50 vertical levels z=60m~1kmZtop=27km
Rotunno et al. (2009 BAMS)
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10-m Wind Speed
37km 37 km
Max=85.5 Max=82.3 Max=83.7
t =9.75d , Δ =62m
instantaneous 1-min average
max=121.7 max=78.8
Rotunno et al. (2009 BAMS)
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Vorticity Magnitude t =9.75d , Δ =62m
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10-m Tangentially Averaged Wind Speed vs Grid Interval
Δ Rotunno et al. (2009 BAMS)
ms
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2. New Work
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• Stretched structured grid
• In center: dx = dy =1.0km (cloud-resolving)
• 3D version of axisymetric model of
Bryan and Rotunno (2009 MWR)
128 km
128 km
Another approach for high-resolution is grid stretching
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Reflectivity at surface (shaded)
and w at 1 km AGL (contours)
Compare axisymmetric results with 3D solutions at similar resolution (dx=dy=1km) on Cartesian grid
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Axi. vs. 3D Sol’n Sensitivity to lh
(using lv = 200 m)
NOTE:
in ARW: lh = Δx / 4
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<v> (m/s) at 1 km AGL
(at level of maximum <v>)
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Summary of cloud-scale (dx=dy=1 km) 3D simulations
• Although 3D Vmax is systematically lower than that in the axisymmetric model, it is still very sensitive to parameterized horizontal diffusion
• In terms of typical mesoscale-model (e.g. WRF) parameters, hurricane intensity is sensitive to lh
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• Stretched structured grid
• In center: dx = dy =0.62km (Large Eddy Simulation)
• 3D version of axisymetric model of
Bryan and Rotunno (2009 MWR)
49 km
49 km
LES
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w (m/s) at z = 1 km: dx= 1000m
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w (m/s) at z = 1 km: dx= 62.5 m
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Subgrid-scale tke:
Resolved-scale tke:
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<θe> with two different resolutions:
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<θe> with two different resolutions:
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u 's ' , w 's '( ) =−KEDDY
∂ s∂r
,∂ s∂z
⎛⎝⎜
⎞⎠⎟
; KEDDY =l2 Def
s = s + s'
azimuthal averagegrid-scale
TC Vortex
grid-scale waves & turbulence
Estimation of Eddy Viscosity for Mesoscale Models using LES Results
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turbulence length scale (lh):3D, dx= 62.5 m
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sensitivity to horizontal turbulence intensity:
max observed
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New Work Summary• Parameterized turbulence in the eyewall of hurricanes
reduces hurricane intensity even with 3D cloud-resolving (dx=1km) resolution.
• Large Eddy Simulations using a different model and different resolution-enhancement technique produce results consistent with previous ones indicating that very high resolution (dx< 100 m) in three dimensions is required to simulate turbulent processes
• Analysis of the new LES indicates lh ~ 1000 m
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3. Next Steps
Higher-res LES, vary SST, moving hurricane…