Sensitivity of High-Resolution Simulations of Hurricane Bob

(1991) to Planetary Boundary Layer Parametrizations

Scott A. Braun & Wei-Kuo Tao

Chris BirchfieldAtmospheric Sciences, Spanish minor

What is a Hurricane?

Center of low pressure

Warm Core No fronts

attached

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

Anatomy of a Hurricane

Saffir-Simpson Hurricane Scale

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)

Hurricane Bob (1991)

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

Map of domains

Domain A – 36-km grid

Domain B – 12-km grid

C1 & C2 – moving 4-km grids

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

Min Pressure & Max Winds

Surface Wind Analysis

Shows max winds in excess of 50 m/s E

Shaded region = winds of 35< m/s

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

NEXRAD radar (Cape Hatteras, NC)

Indicates partial eyewall

Radius of ~25km Northern convective

bands showed high reflectivities

Simulated Radar Reflectivity

Each case shows defined eyewall

Precipitation less defined – more scattered

Vertical Cross Sections (1)

BT

BA

BL

MRF

Vertical Velocity Tangential Velocity

Vertical Cross Sections (2)

BT

BA

BL

MRF

Radial Velocity Equivalent Potential Energy (Θe)

Vertical Cross Sections (3)

BT

BA

BL

MRF

Avg. Tangential Velocity Temp. Tendency

Vertical Cross Sections (4)

BT

BA

BL

MRF

Water Vapor Tendencies Eddy Diffusivity Coefficient

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

Surface Fluxes and Exchange coefficients

Es & Cq = Moisture Hs & C0 = Heat τ s & CD = Momentum Ck = Enthalpy

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

Min SLP & Max Wind Speed

Original Blackadar produces weakest storm (Ck/CD) is smallest

BL/BU1 neglecting zo produces the strongest storm

BL/BU2 including zo does not produce strongest

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

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.

Future Research

More measurements Dissipative heating Sea spray Ocean-atmosphere

coupling