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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
Agenda
1. Background
1. New work
1. Next steps
1. Background
Hurricane Simulation Models
Fast, simple, but all effects of nonaxisymmetric motionsmust be somehow represented
Axisymmetric
Tangential velocity from an axisymmetric numerical model
Axisymmetric Model Sensitivity of Wind Speed to Mixing Length lh
Bryan and Rotunno (2009 MWR)
What is ?
There are no observations of radial turbulent fluxes in a hurricane
lh
Marks et al. (2008, MWR)
Reflectivity (dBZ) at 1726 UTC
Marks et al. (2008, MWR)
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
(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)
37km
Turbulent fluxes computed, buthigh resolution (<100m) required
Hurricane Simulation Models
Large Eddy Simulation
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)
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)
Vorticity Magnitude t =9.75d , Δ =62m
10-m Tangentially Averaged Wind Speed vs Grid Interval
Δ Rotunno et al. (2009 BAMS)
ms
2. New Work
• 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
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
Axi. vs. 3D Sol’n Sensitivity to lh
(using lv = 200 m)
NOTE:
in ARW: lh = Δx / 4
<v> (m/s) at 1 km AGL
(at level of maximum <v>)
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
• 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
w (m/s) at z = 1 km: dx= 1000m
w (m/s) at z = 1 km: dx= 62.5 m
Subgrid-scale tke:
Resolved-scale tke:
<θe> with two different resolutions:
<θe> with two different resolutions:
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
turbulence length scale (lh):3D, dx= 62.5 m
sensitivity to horizontal turbulence intensity:
max observed
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
3. Next Steps
Higher-res LES, vary SST, moving hurricane…