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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 motions must be somehow represented Axisymmetric Tangential velocity from an axisymmetric numerical model
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Axisymmetric Model Sensitivity of Wind Speed to Mixing Length l h Bryan and Rotunno (2009 MWR)
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What is ? There are no observations of radial turbulent fluxes in a hurricane
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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) 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
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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)
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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)
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37km Turbulent fluxes computed, but high resolution (<100m) required Hurricane Simulation Models Large Eddy Simulation
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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)
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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)
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Vorticity Magnitude
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10-m Tangentially Averaged Wind Speed vs Grid Interval Rotunno et al. (2009 BAMS)
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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 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 l h (using l v = 200 m) NOTE: in ARW: l h = Δx / 4
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(m/s) at 1 km AGL (at level of maximum )
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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
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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 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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with two different resolutions:
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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 ( l h ): 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 l h ~ 1000 m
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3. Next Steps Higher-res LES, vary SST, moving hurricane…
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