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IMPACTS OF TURBULENCE ON HURRICANES (ONR-BAA-09-012)

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. Background

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IMPACTS OF TURBULENCE ON HURRICANES (ONR-BAA-09-012)

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  1. 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

  2. Agenda • Background • New work • Next steps

  3. 1. Background

  4. Tangential velocity from an axisymmetric numerical model Hurricane Simulation Models Axisymmetric Fast, simple, but all effects of nonaxisymmetric motions must be somehow represented

  5. Axisymmetric Model Sensitivity of Wind Speed to Mixing Length lh Bryan and Rotunno (2009 MWR)

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

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

  8. Marks et al. (2008, MWR)

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

  10. WRF Model Idealized TC resolution study , 10-m Wind Speed t=9.75d max=61.5 max=86.7 y[km] max=86.2 max=121.7 y[km] Rotunno et al. (2009 BAMS)

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

  12. 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 Idealized TC: f-plane zero env wind fixed SST Nested Grids 1500km 1000km 111km 333km 37km 50 vertical levels Dz=60m~1km Ztop=27km Rotunno et al. (2009 BAMS)

  13. 10-m Wind Speed instantaneous 1-min average max=121.7 max=78.8 Max=85.5 Max=82.3 Max=83.7 37km 37 km Rotunno et al. (2009 BAMS)

  14. Vorticity Magnitude

  15. 10-m Tangentially Averaged Wind Speed vs Grid Interval Rotunno et al. (2009 BAMS)

  16. 2. New Work

  17. Another approach for high-resolution is grid stretching • 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

  18. Compare axisymmetric results with 3D solutions at similar resolution (dx=dy=1km) on Cartesian grid Reflectivity at surface (shaded) and w at 1 km AGL (contours)

  19. Axi. vs. 3D Sol’n Sensitivity to lh (using lv= 200 m) NOTE: in ARW: lh = Δx / 4

  20. <v> (m/s) at 1 km AGL (at level of maximum <v>)

  21. 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

  22. LES • 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

  23. w (m/s) at z = 1 km: dx= 1000m

  24. w (m/s) at z = 1 km: dx= 62.5 m

  25. Subgrid-scale tke: Resolved-scale tke:

  26. <θe> with two different resolutions:

  27. <θe> with two different resolutions:

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

  29. turbulence length scale (lh): 3D, dx= 62.5 m

  30. sensitivity to horizontal turbulence intensity: max observed

  31. 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

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

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