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Sensitivity of High-Resolution Simulations of Hurricane Bob (1991) to PBL Parameterizations

This research investigates the sensitivity of high-resolution simulations of Hurricane Bob (1991) to different planetary boundary layer (PBL) parameterizations. Utilizing the MM5 model, simulations were conducted over various grids, resulting in substantial variations in minimum sea level pressure and maximum wind speeds. The study confirms that PBL schemes significantly influence hurricane intensity, surface fluxes, and vertical mixing, highlighting the need for accurate parameterization in predictive models. The findings underscore the importance of improving observational methods to enhance future forecasts and simulations.

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Sensitivity of High-Resolution Simulations of Hurricane Bob (1991) to PBL Parameterizations

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  1. Sensitivity of High-Resolution Simulations of Hurricane Bob (1991) to Planetary Boundary Layer Parametrizations Scott A. Braun & Wei-Kuo Tao Chris Birchfield Atmospheric Sciences, Spanish minor

  2. What is a Hurricane? • Center of low pressure • Warm Core • No fronts attached

  3. How do they form? • Warm tropical oceans between 8° and 20° latitude • Typically 80°F< • Tropical disturbance • Convection - Thunderstorms form • Rotation due to Coriolis Force • Depression  Tropical Storm  Hurricane

  4. Anatomy of a Hurricane

  5. Saffir-Simpson Hurricane Scale

  6. Purpose of this research • Inner-core observations of hurricanes limited to dropsondes or buoys • Modelers forced to use high-sensitivity PBL parameterizations • Importance of surface fluxes & vertical mixing • Equivalent potential temperature (Θe)

  7. Hurricane Bob (1991)

  8. 9 total simulations conducted • Burk-Thompson PBL scheme • Bulk-Aerodynamic PBL scheme • Blackadar PBL scheme • MRF PBL scheme • BL/BT • BL/BU1 • BL/BU2 • BU/BT • MRF/BT

  9. Map of domains • Domain A – 36-km grid • Domain B – 12-km grid • C1 & C2 – moving 4-km grids

  10. Simulation of MM5 Model • Used to conduct 72-hr simulations • Uses course grid of 193 X 163 grid points • Uses x,y spacing of 36 km (fig.1) • Grid centered at 33°N, 84°W • Conditions were obtained from ECMWF • 9 high-resolution simulations

  11. Min Pressure & Max Winds

  12. Surface Wind Analysis • Shows max winds in excess of 50 m/s E • Shaded region = winds of 35< m/s

  13. Winds at lowest (42m) level • BT yields winds of 55< m/s • BA has wavenumber 2 pattern. 100 km area of 35 m/s winds • BL and MRF display weaker winds

  14. NEXRAD radar (Cape Hatteras, NC) • Indicates partial eyewall • Radius of ~25km • Northern convective bands showed high reflectivities

  15. Simulated Radar Reflectivity • Each case shows defined eyewall • Precipitation less defined – more scattered

  16. Vertical Cross Sections (1) Vertical Velocity Tangential Velocity • BT • BA • BL • MRF

  17. Vertical Cross Sections (2) Radial Velocity Equivalent Potential Energy (Θe) • BT • BA • BL • MRF

  18. Vertical Cross Sections (3) Avg. Tangential Velocity Temp. Tendency • BT • BA • BL • MRF

  19. Vertical Cross Sections (4) Water Vapor Tendencies Eddy Diffusivity Coefficient • BT • BA • BL • MRF

  20. PBL Moisture Tendency Equation • qv = Vapor mixing ratio • K = Eddy diffusivity • Lv = Latent heat of vaporization • ρ = Density of air • Δz1 = Vertical grid increment of lowest layer

  21. Surface Fluxes and Exchange coefficients • Es & Cq = Moisture • Hs & C0 = Heat • τ s & CD = Momentum • Ck= Enthalpy

  22. Drag Coefficients • Blackadar agrees with Hawkins and Imbembo up to 45 m/s • BT values parallel, but lower due to weaker dependence of zo. • BA is uniform due to lack of wind speed dependence zo

  23. Min SLP & Max Wind Speed • Original Blackadar produces weakest storm (Ck/CD) is smallest • BL/BU1 neglecting zoproduces the strongest storm • BL/BU2 including zo does not produce strongest

  24. Conclusion • The MM5 model simulated Hurricane Bob in high resolution • Results exhibited high sensitivity to PBL processes. • Min SLP and max winds varied by 16mb and 15 m/s • BT & BA produced the strongest storms • Vertical structures similar in BT, BL and BA • Each PBL scheme differed in vertical mixing

  25. Conclusion Cont’d • Intensity increases as Ck/CD increases • Surface fluxes & vertical mixing differ  difficult to ascertain individual roles. • Simulated intensity varies depending on wind speed dependence of the surface roughness parameter z0 • Precipitation forecasts very sensitive • Obtaining measurements for heat, moisture, momentum essential. • Dropsondes & Doppler Radar play significant role in observing the depth.

  26. Future Research • More measurements • Dissipative heating • Sea spray • Ocean-atmosphere coupling

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