1. Sensitivity of High-Resolution Simulations of Hurricane Bob (1991) to Planetary Boundary Layer Parameterizations SCOTT A. BRAUN AND WEI-KUO TAO PRESENTATION BY: AUSTIN NEW

2. Overview  Braun and Tao ran the fifth- generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model to simulate hurricane Bob in a high resolution (4km) representation.  They used four different representations of the PBL in the model to determine the results and accuracy for each scheme.  The four schemes are the bulk aerodynamic, Blackadar, Medium-Range Forecast (MRF) model, and Burk–Thompson boundary layer schemes.

3. Why?  There is very little know about the PBL inside hurricanes.  With little knowledge of the PBL, forecast model representations of storms can be inaccurate and vary greatly.  It is important so we can better forecast the track, intensity, vertical structure, precipitation rate, and formation of tropical cyclones.

4. Background: The bulk aerodynamic scheme  Assumes that 86 meters is the top of the boundary layer.  This scheme also uses surface fluxes of momentum, heat, and moisture and tendencies  In the scheme the MM5 also uses a dry convective adjustment to remove unstable layers.

5. Background:The Blackadar PBL scheme  Use two regimes of turbulent mixing, a stable regime and a free convection regime.  The stable boundary level is divided into thee categories determined by bulk Richardson Number.  In the free convective regime the vertical transfers of heat, moisture, and momentum are determined by thermal structure of the whole mixed layer and the surface heat flux

6. Background: The MRF PBL Scheme  It is uses bulk characteristics of the PBL.  Included in the bulk characteristics is counter gradient transports of temperature and moisture that account for the contributions from large-scale eddies

7. Background:The Burk–Thompson scheme  The MM5 does not take into effect the effects of liquid water and countergradient terms in the fluxes of heat and moisture.  The eddie diffusivity in this scheme is a result algebraic function.

8. Hurricane Bob’s Intensity  The Burk–Thompson scheme produced a steady pressure drop with the final pressure within 3 mb of the actual pressure. The winds where slightly higher than the observed winds.  The Bulk aerodynamic scheme also the same results as the Burk-Thompson scheme.

9. Hurricane Bob’s Intensity Cont.  The Blackadar and MRF PBL produced mainly higher pressures and a weaker wind speed.  Generally, the pressure was 12-14 mb greater than the observed values.

10. Hurricane Bob’s Precipitation  In the Burk–Thompson case the eye of Hurricane Bob was 80-90 km wide. This was nearly twice as large as the actual observed eye.  The precipitation shield consisted of weakly organized cells instead of strong bands. The overall precipitation shield was smaller than the observed.

11. Hurricane Bob’s Precipitation  In the bulk aerodynamic PBL case, the model only had precipitation around half to three-quarters of the eye. The representation of the eye was a very close approximation to the observed eye of the storm.  In addition, the overall model representation of the precipitation matched actual radar observations closely.

12. Bob’s Precipitation  The Blackadar PBL scheme was very similar to the Burk-Thompson case, but with a broader precipitation shield.  In the MRF PBL case, the eye had strong convection and the overall precipitation was the broadest of the models. The storm was the weakest in this scenario.

13. Vertical structure  The vertical motion in the eyewall was the strongest in the Burk- Thompson case and weaker in the bulk aerodynamic and MRF case.  The average tangential wind speed was at a much lower level in all for schemes compared to the actual hurricane.

14. Surface fluxes  Each of the four schemes have differences in the exchange coefficients, flux algorithms, and surface roughness parameters.  These variation can cause great differences in the heat, moisture, and momentum fluxes.

15. Vertical Mixing: Structure and Intensity  Four experiments were conducted to determine the effects of vertical mixing in the PBL.  The results showed that the Burk- Thompson, Burk–Thompson surface fluxes with the Blackadar vertical mixing scheme and the Burk– Thompson surface fluxes with the bulk aerodynamic showed that intensity was mainly effected by surface fluxes instead of vertical mixing.  The MRF scheme was clearly effected by vertical mixing, causing a weaker representation of the storm.

16. Vertical Mixing: Precipitation  The Burk- Thompson surface fluxes, Burk–Thompson surface fluxes with the Blackadar vertical mixing scheme and the Burk–Thompson surface fluxes with the bulk aerodynamic fluxes representations showed little effect on precipitation from vertical mixing.  The vertical mixing had a much greater impact on the precipitation in the MRF.

17. Conclusions  The high resolution model was very sensitive to the different PBL parameterizations used.  The central pressure and wind velocity representations varied by 16 mb and 15 m/s.  The vertical structure varied less in three of the PBL schemes, but was much different in the MRF scheme.

18. Conclusions  The four schemes produced fairly significant differences in temperature, moisture, and momentum.  Because of the different scheme results, the roles in vertical mixing and fluxes are hard to determine.  Precipitation varied greatly in the different schemes showing that hurricanes are just a sensitive to PBL as to microphysical parameterizations.

19. Conclusions  We need to study and determine measurement in the PBS to better forecast tropical cyclones.  The best way to study the PBL in hurricanes is by Doppler radar, mobile radar, and dropsondes.

20. Summary  The 1999 study used four PBL schemes to model hurricane Bob.  Each scheme caused great variation in the intensity, precipitation, and vertical structure of the storm.  The PBL needs to be studies and incorporated into hurricane models to better forecast tropical cyclones.

21. Questions?