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…