Scott A. Braun & Wei-Kuo Tao

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