Impact of Stable Boundary Layer on Tropical Cyclone Structure in Coupled WRF Model

1. Impact of Stable Boundary Layer on Tropical Cyclone Structure in Coupled WRF Model Chiaying Lee and Shuyi S. Chen RSMAS/Univ. of Miami

2. Outline • Introduction and motivation • Scientific questions • Results from ITOP observations • Results from coupled and uncoupled model simulations • Conclusions

3. Previous studies on TC-induced SST cooling • Observations of hurricane-induced cold wake (Lepord 1967, Shay and Elsberry 1987) • Idealized modeling on ocean response to the hurricane forcing and characteristic of cold wake (Price 1981, 1994) • Idealized axisymmetric coupled modeling of air-sea interaction in hurricanes (Chang and Anthes 1979, Schade and Emanuel 1999) • Idealized axisymmetric modeling of air-sea interaction on TC intensity (Emanuel 1986, Rotunno and Emanuel 1987) • Three dimension coupled model forecast of TCs (Bender and Ginis 2000) SST cooling

4. Lee and Chen et al. (2011) quadrant – averaged HBL Azimuthally – averaged HBL Uncoupled Coupled Inflow layer Ekman BL ML Cold wake How does the TC-induced cold wake and associated BL asymmetry affect storm structure and intensity?

5. How does Air-Sea Coupling Affect TC Structure? Coupled t = 0 Iso-surface of Tracer from cold wake region t = 120 min. t = 60 min. t = 20 min. ? Dynamic forcing? Thermodynamic forcing? Uncoupled t = 0 t = 120 min. t = 60 min. t = 20 min. 301 302 303

6. Dynamic forcing – SST gradient • Idealized dry model with axisymmetric TC flow • homogeneous SST • imposed square SST front Homogeneous SST U V Imposed SST front U V • Air parcels passing over the colder water: decelerate and turn toward lower pressure center. SST = SSTenv – 4oC Chen et al. (2009)

7. Thermodynamic forcing – Stable boundary Layer Stull (1988): the surface is cooler than the air. Pre-storm SST Staticstability z > 0 - stable = 0 – neutral < 0 - unstable Stable Unstable Neutral qv Stable Boundary Layer in Hurricanes? Post-storm SST

8. Specific questions: • Does a stable boundary layer exist in hurricanes? • Observations from the Impact of Typhoon on the Ocean in Pacific (ITOP, 2010) • How does the TC-induced cold wake and associated BL asymmetry affect storm structure and intensity? • Coupled WRF (CWRF) and uncoupled WRF simulation of Typhoon Choiwan • Distinct BL structure in CWRF • Combined tracer and trajectory analysis in CWRF and WRF

9. C130 dropped AXBTs and Dropsondes at the same location – Simultaneous profile of atmospheric and oceanic BL thermal structure

10. Fanapi B WARM COLD WARM A Air Temperature Cold air flows over warm ocean, SH goes upward. Energy transfer is reversed Air-sea interface Warm air flows over cold ocean, SH goes downward. Ocean Temperature A B

12. Coupled WRF (CWRF) • WRF V3.1.1 • Nested model grids with 12-, 4-, 1.3-km resolutions • 36 vertical levels (with 9 within the lowest 1 km) • WSM5 microphysics • YSU PBL+Donelan’s (2004) wind-dependent surface roughness • Garratt (1992) thermal exchange coefficient over water • GFS initial/lateral B.C. 12 km 4 km 1.3km

13. 3DPrice–Weller–Pinkel (3DPWP) ocean model • Fully physics • No bathymetry • same grid as WRF • moving nest • Highest resolution: 4-km • PWPin package: • Satellite SST • Operational Global HYCOM • Laplacian smooth method 12 km Linear interpolation 5 m 4 km Call 3DPWP 1.3km 10 m HYCOM Call 3DPWP 30 layers 20m Z= 390 m

14. Satellite SST HYCOM profile R L ITOP cases

15. Two ways to track the property of fluid: • Trajectory: an undiluted air parcel advected by wind • Tracer: air mass that is subjected to advection, turbulent mixing and molecular diffusion Sink term Molecular diffusion Turbulent mixing

16. Implementation of Trajectory in WRF: • Forward Trajectory Subroutine in WRF Dynamic Codes • Low frequent temporal wrfout • Three 1D array variables • Longitude, Latitude, Height • Works for moving nest • Initialized in wrfinput or wrfrst • Drawback: No backward trajectory 12 km d01 – call traj d02 – call traj 4 km 1.3km d03 – call traj Call trajectory

17. Typhoon Choiwan (2009)

18. dBZ at 80 m • Resemblance in WRF and CWRF • Similar core structure and primary rainband location • Difference in WRF and CWRF • Lesser radial extent of convections in CWRF • Persistently, less convection downwind of cold wake in CWRF WRF CWRF 29.5 29.5 Cold wake

19. W at 80 m WRF CWRF

20. Is there a reduction of convection downwind of the cold wake in reality?? WRF CWRF Yes, there is !!

21. @ 3km ~ 700mb The lack of convection in the downwind side of cold wake in CWRF could be the consequence of the cold wake and its non-local effect!

22. CWRF • Further inward turning WRF Cooling ~ 0oC • Not as drastic as Chen et al. (2009). • ~1.5 oC cooling is enough to induce the dynamic forcing Dynamic Processes

23. CWRF WRF CWRF-WRF More inward turning Stronger inflow with larger angle over cold wake Stronger inflow

24. Thermodynamic Processes @ 0600 UTC Cold wake @ 1200 UTC Cold wake: CWRF CWRF WRF WRF Environment Environment

25. CWRF WRF • Surface stability: L : Monin-Obukhovlength Z : well mixed layer • BL stability:

26. Tracer and trajectory initialization aownwind annular ring cold wake upwind • CWRF • cold wake regions, upwind, downwind • Annular rind afrea • r=150 ~ 350 km • WRF – same storm-relative locations • ~100 trajectories released in cold wake region • Tracer and trajectories released @ 80 m • Both are tracked for 6 hours.

27. How does Air-Sea Coupling Affect Hurricane Intensity? Coupled t = 0 Iso-surface of Tracer from cold wake region t = 120 min. t = 60 min. t = 20 min. Uncoupled t = 0 t = 120 min. t = 60 min. t = 20 min. 301 302 303

28. Tracer in Cold Wake CWRF WRF t = 0 Total tracer concentration t = 3 hr t = 3 hr Initial location • In WRF: tracers tends to vertically transported into the outer rainband • In CWRF: tracers tends to transported into the eyewall. t = 5 hr t = 4 hr t = 4 hr t = 5 hr t = 6 hr t = 6 hr

29. qe along the trajectories from the cold wake region CWRF WRF eye eye Colder initial value Warmer initial value

30. qealong the trajectories WRF CWRF eyewall eyewall rainband

31. Energy / tracer fluxes Outer region eyewall ~ 500 m BL Storm center RMW+20 km CWRF BL Outer region Eyewall WRF

32. Downwind Annular ring Cold wake Upwind CWRF WRF

33. Dynamic forcing – SST gradient Storm center (low pressure) Z Internal BL X Warm SST Cold SST Storm center Outer region (high pressure) Cold wake Outer region

34. Thermodynamic forcing – Stable boundary Layer WRF CWRF

35. Conclusions • ITOP observations confirm stable boundary layer in TCs and provide coherent air-sea data for coupled model verification. • Cold wake/stable BL suppress deep convection locally and downwind (in outer rainband region). • Stable BL over the cold wake keeps air in BL longer and enhances sfc flux/increases qeof the inflow into the eyewall. • SST gradient at the edge of cold wake results in larger inflow angle and stronger inflow. • Both thermodynamic and dynamic processes associated with the stable BL/cold wake affect TC structure by enhanced inflow into the inner core. • Fully coupled model is needed to accurately describe air-sea interaction and its impact on TC structure and intensity.

36. Acknowlegement: • ITOP team • Drs. R. Foster and P. Sullivan on stable boundary layer • NCAR scientists for their guidance on coupling CWRF and developing tracer/trajectory analysis in WRF: Dr. JimyDudhia, Dr. Wei Wang Dave Gill, and John Michalakes This work is supported by the ONR ITOP grant N000140810576