IMPACTS OF TURBULENCE ON HURRICANES (ONR-BAA ) 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 motions must be somehow represented Axisymmetric Tangential velocity from an axisymmetric numerical model

Axisymmetric Model Sensitivity of Wind Speed to Mixing Length l h Bryan and Rotunno (2009 MWR)

What is ? There are no observations of radial turbulent fluxes in a hurricane

Marks et al. (2008, MWR) Reflectivity (dBZ) at 1726 UTC

Marks et al. (2008, MWR)

Davis et al. (2008 MWR) Radar Reflectivity at z=3km a)WRF b)WRF c)ELDORA Hurricane Simulation Models Three-Dimensional Mesoscale Forecast Model Vortex asymmetries computed, but effects of small-scale (< 1000m) turbulent motions must be somehow represented

y[ km ] 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)

Regimes of Numerical Modeling* 1/ Δ meso 1/ Δ LES 1/ l Mesoscale limit LES limit the “terra incognita” k F(k) *Wyngaard (2004 JAS)

37km Turbulent fluxes computed, but high resolution (<100m) required Hurricane Simulation Models Large Eddy Simulation

Idealized TC: f-plane zero env wind fixed SST Nested Grids WRF Model Physics: WSM3 simple ice No radiation Relax to initial temp. Cd (Donelan) Ce (Carlson-Boland) Ce/Cd ~ 0.65 YSU PBL LES PBL Domain 6075km 1500km 1000km 333km 111km 37km 50 vertical levels  z=60m~1km Ztop=27km Rotunno et al. (2009 BAMS)

10-m Wind Speed 37km Max=85.5Max=82.3Max=83.7 instantaneous 1-min average max=121.7max=78.8 Rotunno et al. (2009 BAMS)

Vorticity Magnitude

10-m Tangentially Averaged Wind Speed vs Grid Interval Rotunno et al. (2009 BAMS)

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 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 l h (using l v = 200 m) NOTE: in ARW: l h = Δx / 4

(m/s) at 1 km AGL (at level of maximum )

Summary of cloud-scale (dx=dy=1 km) 3D simulations Although 3D V max 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 l h

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 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:

with two different resolutions:

azimuthal averagegrid-scale TC Vortex grid-scale waves & turbulence Estimation of Eddy Viscosity for Mesoscale Models using LES Results

turbulence length scale ( l h ): 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 l h ~ 1000 m

3. Next Steps Higher-res LES, vary SST, moving hurricane…