Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005)

1. Convective-scale Downdrafts in the Principal Rainband of Hurricane Katrina (2005) Anthony C. Didlake, Jr. COGS Seminar UW, Dept. Atmos Sci., Seattle, November 6, 2008

2. Idealized structure of a tropical cyclone downwind • Inner and Outer eyewalls • Stationary Band Complex (SBC) • principal band • secondary bands upwind Willoughby 1988

3. Overview • Background and Motivation • Description of RAINEX and dataset • Methodology: Convective separation and cross sections • Characteristics of downdrafts within principal rainband • Forcing mechanisms and immediate effects of downdrafts • Possible impacts on larger tropical cyclone • Summary and conclusions

4. Background and Motivation • Dynamic role of principal rainband in the larger storm remains uncertain • Several modeling studies suggest the principal rainband impacts the storm intensity • PV generation and inward advection • Inhibiting inflow of warm, moist air • Formation of secondary eyewall via vortex-Rossby wave dynamics • Important to understand structure and dynamics of principal rainband, so that we may better address the difficulties in forecasting tropical cyclone intensity

5. Houze et al. 2006, 2007

6. Model of Principal Rainband • Convective cells embedded in stratiform rain • Overturning updraft, two downdrafts Hence and Houze 2008

7. Downdrafts in the Principal Rainband IED • Low-level downdraft (LLD) • Inner-edge downdraft (IED) • Forcing mechanisms, immediate effects, possible impacts on larger storm? LLD

8. Downdrafts in ordinary convection Zipser 1977 • Convective-scale saturated downdraft forced by precipitation drag • Mesoscale downdraft due to evaporative cooling Palmén and Newton 1969, Biggerstaff and Houze 1993, Yuter and Houze 1995 • Convective-scale downdraft forced by buoyancy pressure gradient force (BPGF) field

9. Hurricane Katrina (2005)

10. ELDORA data Reflectivity at 2 km

11. Convective/stratiform separation • Based on local gradients in reflectivity • Similar to Steiner et al. 1995, TRMM satellite data classification Convective Stratiform Weak echoNo echo

12. Convective pixels Stratiform pixels 2D frequency distributions Reflectivity data in % of height total

13. Convective pixels Stratiform pixels 2D frequency distributions Vertical velocity data in % of height total

14. Rainband cross sections • Radial cross sections at regular angular intervals • 0.375°  109 cross sections • Cross section coordinates based on classification

15. Updrafts (m s-1) Downdrafts (m s-1) Average vertical velocity Reflectivity (dBZ) as black contours LLD IED

16. Vertical velocity (m s-1) at 42.2° LLD analysis • Located in lower levels • Embedded in heavy precipitation Reflectivity (dBZ) as black contours

17. Average downdrafts (m s-1) LLD forcing mechanism • Located in lower levels • Embedded in heavy precipitation • Zipser’s “Convective-scale saturated downdraft” • Forced down by precipitation drag • Attains negative buoyancy from continuous evaporative cooling Reflectivity (dBZ) as black contours

18. Average downdrafts (m s-1) IED analysis • IED investigation area: 8.5 km  12.5 km

19. IED analysis: Downward vertical mass flux 2D distribution in kg s-1

20. IED analysis: Conditional probability distribution of IED speeds at 1 km • Condition: 4.5 km-IED ≤ 3 m s-1 or > 3 m s-1 • Weak mid-level IED comes with weaker low-level IED, while strong mid-level IED comes with stronger low-level IED

21. IED analysis: Vertical velocity at 4 km • Intermittent pattern of convective-scale updraft and downdraft cores

22. IED analysis: Autocorrelation of vertical velocity, Lag = 4 (≈ 4.5 km) • Physical relationship between IEDs and updrafts

23. Reflectivity (dBZ) at 35.8° IED forcing mechanism • Originates above the melting level, outside of heavy precipitation • Occurs on the convective scale, rather than mesoscale Overlaid by in-plane wind vectors

24. Vertical velocity (m s-1) at 35.8° IED forcing mechanism • Originates above the melting level, outside of heavy precipitation • Occurs on the convective scale, rather than mesoscale • Initially forced by the BPGF created by the updraft Reflectivity (dBZ) as black contours

25. Reflectivity (dBZ) at 35.8° IED forcing mechanisms • Initially forced by the BPGF created by the updraft 2-step process!

26. Reflectivity (dBZ) at 35.8° IED forcing mechanisms • Initially forced by the BPGF created by the updraft • Attains negative buoyancy by evaporating heavy precipitation of rainband 2-step process!

27. IED effects: Sharp inner-edge reflectivity gradient

28. Reflectivity (dBZ) at 35.8° IED effects: Sharp inner-edge reflectivity gradient

29. Tangential wind speed (m s-1) at 35.8° IED effects: Low-level wind maximum (LLWM)

30. Tangential wind speed Vertical velocity Vertical vorticity Divergence

31. Composite tangential wind speed from Hurricane Floyd (1981) Tangential wind speed (m s-1) at 35.8° Possible impacts: Increased inward flux of angular momentum • LLWM lies in radial inflow • Increased angular momentum results in stronger vortex Barnes et al. 1983

32. Conceptual model of rainband cross section

33. Commonly observed features of principal rainband • Upwind end consists of newer, robust convective cells • Downwind end consists of older cells collapsing into stratiform precipitation • Principal rainband is often stationary relative to the storm center Hence and Houze 2008 Barnes et al. 1983

34. Vertical velocity at 42.2° Possible impacts: Growth and sustenance of principal rainband • Area of divergence near surface under LLD • Preferred region of convergence on upwind side of LLD core • Growth of updraft on upwind end of rainband Divergence Convergence LLD Background flow Plan view

35. Possible impacts: Growth and sustenance of principal rainband • Tropical storm Ophelia (2005) • Operational radar from Melbourne, FL • Discrete propagation of vertical velocity cores, rainband cells • Stationary rainband relative to storm center Radar loop

36. Conceptual model of rainband at 2 km

37. Conceptual model of rainband at 2 km

38. Conceptual model of rainband at 2 km

39. Conclusions • Principal rainband contains two repeatable convective-scale downdrafts • Low-level downdraft is forced by precipitation drag beneath heavy precipitation • Inner-edge downdraft is initially forced by pressure perturbations created by nearby buoyant updrafts, then evaporative cooling • Vorticity dynamics of updraft and IED create a low-level wind maximum that leads to increased angular momentum of storm • Interaction between updraft and two downdrafts leads to growing and sustaining convection of principal rainband • Convective-scale features allow principal rainband to continue its impact on the overall storm

40. Future Work • Analyze convective-scale structures in high-resolution model output from RAINEX • Investigate outer rainbands and compare to inner core of storm

41. Acknowledgments • Bob Houze • Deanna Hence, Stacy Brodzik • Brad Smull, Tomislav Maric, Jian Yuan, Mesoscale Group • Michael Bell, Sandra Yuter • Beth Tully • Atmos Grad 2006

42. Extra Slides

43. Convective/stratiform classification • Technique used in Steiner et al. 1995, Yuter and Houze 1997, Yuter et al. 2005 • Algorithm separates convective regions from stratiform regions by comparing local reflectivity to background reflectivity • Tuning of algorithm required to recognize convective regions; the rest is designated as stratiform

44. Convective/stratiform classification • Convective center if: • Z  Zti • Z-Zbg Zcc(Zbg) • Classified convective within R(Zbg) from convective center, remaining is classified stratiform (unless Z < Zwe) Zti= 45 dbZ; Zwe= 20 dbZ; R = 0.5+.23(Zbg-20); Rbg= 11 km; a=9; b=45

45. 4 km reflectivity

46. 2 km reflectivity

47. 2 km vertical velocity

48. 6 km vertical velocity

49. Average downdrafts for upwind half

50. Statistical significance testing • Two-sided Student’s t statistic • Significance level of 95% • Null hypothesis that true autocorrelation is zero • Number of independent samples determined by formula of Bretherton et al. (1999)