Roger Smith Ludwig-Maximilians University of Munich Collaborator: Gerald Thomsen

1. Dependence of tropical cyclone intensification on the boundary-layer parameterization in a numerical model Roger Smith Ludwig-Maximilians University of Munich Collaborator: Gerald Thomsen

2. Outline • Motivation • New insights into the role of the boundary layer • Dependence of tropical-cyclone evolution and azimuthal-mean structure on the representation of the boundary layer • Conclusions

3. Motivation In the light of our recent findings that the spin-up of the hurricane inner core occurs in the boundary layer, two important questions arise: • How sensitive is tropical cyclone intensification in a model to the boundary-layer parameterization scheme used? • How large are the differences in boundary-layer structure predicted by different schemes?

4. The primary circulation Pressure gradient force LO r v sea Centrifugal force and Coriolis force

5. Frictionally-induced secondary circulation primary circulation Secondary circulation Pressure gradient force r Friction layer v v Centrifugal force and Coriolis force are reduced by friction

6. “Tea cup” Experiment

7. Hurricane intensification • Basic principle - Conservation of absolute angular momentum: M = rv + r2f/2 f = Coriolis parameter = 2Wsin(latitude) r v v = M/r - rf/2 If r decreases, v increases! Spin up requires radial convergence

8. The basic thought experiment for intensification Initial condition Mean sounding Axisymmetric vortex p(z) T(z) q(z) V(r,z) r 27oC sea Nguyen, Smith and Montgomery calculation, QJRMS, 2008: • Idealized numerical model simulations, simple physics, MM5 • 5 km (1.67 km) resolution in the finest nest, 24s-levels

9. In print In press

10. From Montgomery, Nguyen & Smith (2009): QJRMS

11. From Montgomery, Nguyen & Smith (2009): QJRMS

12. From Montgomery, Nguyen & Smith (2009): QJRMS

13. From Montgomery, Nguyen & Smith (2009): QJRMS

14. From Montgomery, Nguyen & Smith (2009): QJRMS

15. Two mechanisms for TC intensification 15 10 z km 5 M conserved 0 50 r km 100 Mreduced by friction, but strong convergence  small r From Montgomery, Nguyen & Smith (2009): QJRMS

16. Braun & Tao (2000) Monthly Weather Review

17. To be submitted

18. Schemes

19. 6 - unmod Bulk 1 - mod Bulk 2 - Blackadar 3 – Burk Thompson 5 – Gayno-Seaman 4 - MRF 7 – unmod Gayno-Seaman

20. unmodified Bulk modified Bulk Radial and tangential wind components

21. Radial and tangential wind components

22. unmodified Bulk modified Bulk Total wind

23. Total wind

24. unmodified Bulk modified Bulk Vertical velocity

25. Vertical velocity

26. unmodified Bulk modified Bulk Net radial force = v2/r + fv (m s-1/hour)

27. Net radial force = v2/r + fv (m s-1/hour)

28. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk Comparison with Franklin’s data

29. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk Surface wind reduction factor

30. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk Surface inflow angle

31. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk Surface wind

32. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk 0 km 1.5 km Pseudo-equivalent potential temperature

33. Turbulent kinetic energy

34. Summary, conclusions and open questions • Extended the work of Braun and Tao (2000). • Examined new aspects of the boundary layer structure. • Tropical-cyclone intensification is sensitive to the scheme used including onset time of rapid intensification and the final intensity. • The boundary-layer structure varies considerably between schemes. • Which is the most realistic scheme? • Is boundary-layer theory valid in the inner core region? Answer: see Smith and Montgomery (2009)

35. Thank you for your attention

36. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk Surface latent heat flux

37. 1 – mod Bulk 2 – Blackadar 3 – Burk-Thompson 4 – MRF 5 – Gayno-Seaman 6 – unmod Bulk Surface sensible heat flux