1. Comparison of secondary eyewall and principal rainband in Hurricane Rita (2005)
Not a modeling study
Several theories out there for secondary eyewall formation
Observations suggest that outer rainbands coalesce and axisymmetrize to form secondary eyewall.
Does this happen, and if so, how?
Willoughby 1988
Anthony C. Didlake, Jr. and Robert A. Houze, Jr.
29th Conference on Hurricanes and Tropical Meteorology
May 14, 2010

2. Hurricane Rita (2005)
Observed during RAINEX project using NCAR ELDORA radar
Captured both principal rainband and secondary eyewall in exceptional detail
Unique opportunity to compare the internal dynamics of the two on the convective scale
Better understanding of the structural changes that must occur during the process of secondary eyewall formation
Hurricane Rainband and Intensity Change Experiment
ELDORA radar provided data with unprecedented resolution in hurricane environment
Both rainband and secondary eyewall have direct and indirect implications on hurricane intensity

3. Principal rainband w Downdrafts
Intermittent convective cells on inner boundary of stratiform rain
Outwardly leaning reflectivity towers
Two preferred locations for tangential wind maxima
Hence and Houze (2008), Didlake and Houze (2009)

4. Along-band average of vertical velocity
Repeatable and distinct up- and downdrafts
Two separate downdraft regions, IED and LLD
Didlake and Houze (2009)
Reflectivity (dBZ) as black contours 4

5. Reflectivity at 4 km dBZ Courtesy of Michael Bell
Dataset courtesy of Michael Bell
Secondary eyewall, moat, inner eyewall
Inner edge of secondary eyewall sharp at time, lax at others
Convective cells embedded in stratiform precipitation ring, but not aligned along the inner edge or at a certain radius.
Courtesy of Michael Bell

6. Vertical velocity at 4 km
w (m/s)
Secondary eyewall contains pockets of updraft and downdraft cores
Updrafts seem more prevalent
Don’t forget to use toggling!

7. Azimuthal average of vertical velocity
Secondary eyewall
Updrafts in secondary eyewall are stronger than downdrafts
Remember the rainband draft plots?
No clear sign of the inner-edge downdraft
No distinct separation in locations of updraft and downdraft within secondary eyewall
Reflectivity (dBZ) as black contours

8. Azimuthal average of vertical velocity
Principal rainband
Updrafts in secondary eyewall are stronger than downdrafts
Remember the rainband draft plots?
No clear sign of the inner-edge downdraft
No distinct separation in locations of updraft and downdraft within secondary eyewall
Reflectivity (dBZ) as black contours

9. Azimuthal average of vertical velocity
Secondary eyewall
Updrafts in secondary eyewall are stronger than downdrafts
No clear sign of the inner-edge downdraft
No distinct separation in locations of updraft and downdraft within secondary eyewall
Reflectivity (dBZ) as black contours 9

10. Horizontal wind 10

11. Tangential velocity at 4 km
Vt (m/s)
Secondary eyewall contains tangential wind maximum
Wavenumber-1 structure
Smaller convective-scale disturbances within the secondary eyewall

12. Azimuthal average of tangential velocity (m/s)
Tangential maximum embedded in reflectivity tower just like in primary eyewall
Wind max is at around 2 km, much lower than the main jet of the PR
Although similar enhancement of tangential winds is seen in the mid levels on outer side of the reflectivity tower

13. Vertical vorticity at 4 km (wavenumbers 0 and 1 removed)
ζ’ (10-3 m/s)
Many positive and negative vorticity perturbations along the secondary eyewall in a complicated structure
Couplets can occur without any apparent organization
Couplets can be organized into a wavelike pattern, where narrow bands spiral toward the center.
When we look at this cross section….

14. Tangential velocity perturbations
Vt’ (m/s)
w’ (m/s)
downwind
Vorticity perturbations
ζ’ (10-3 m/s)
Banded features in the tangential velocity and vorticity perturbations tilt upwind with altitude.
In perturbations, updrafts tend to overlap positive vorticity and negative tangential wind.
Downdrafts tend to overlap negative vorticity and positive tangential wind.
This cross section is representative of a pattern seen commonly around the secondary eyewall.
w’ (m/s)
Vertical velocity perturbations in red/blue contours

15. Tendency equations of mean tangential momentum and mean vorticity
Cylindrical coordinates
Averaged over total storm
Pressure tendency terms are ignored.
Dominant terms are the w terms.
Cylindrical coordinates
Averaged over total storm 15

16. Vertical profiles of mean terms and perturbation terms in secondary eyewall
Perturbation and mean terms are roughly same order of magnitude for both momentum and vorticity equations.
Terms weighted by density and radius.
Mean terms are consistent with a strengthening storm.
Perturbation velocities are increasing momentum below ~4 km and decreasing vorticity below ~5 km. Mostly due to vertical velocity terms in both equations. Target altitude…transition to next slide.

17. Radial profiles of mean and perturbation terms below 4
Radial profiles of mean and perturbation terms below 4.8 km in secondary eyewall
vorticity
Mean
Terms
momentum
momentum
For mean terms, vorticity max increase is radially inward of momentum max increase
For perturbation terms, vorticity max decrease is radially outward of momentum max incresae
Perturbation
Terms
vorticity

18. Conclusions
Secondary eyewall convection differs from principal rainband.
No preferred radius for convective drafts in secondary eyewall.
Below ~5 km,
vortex-scale motions increase vorticity in inner portion of the developing eyewall,
convective-scale motions decrease vorticity in outer portion.
Mean and perturbation motions act together to build the Vt max in the new eyewall.
Rita’s secondary eyewall adopted a different set of convective-scale dynamics than a principal rainband as part of its transition into a mature eyewall.
The secondary eyewall consisted of convective cells embedded in stratiform precipitation like the principal rainband, but the reflectivity towers of these cells had no preferred shape, orientation, or radial location. Updraft and downdraft cores did not occur at distinctly different altitudes or locations as in the principal rainband; but rather, they both occurred wherever the reflectivity tower appeared and they both peaked in the mid-levels (4-6 km). There was no inner-edge downdraft to form a sharp inner boundary of the secondary eyewall.
Convective-scale elements are building the low-level tangential wind maximum embedded within the secondary eyewall’s reflectivity cells, such that the secondary eyewall can evolve into a more mature eyewall by building a connected low-level wind maximum around the storm, unlike a principal rainband
The radial location of decreasing vorticity suggests that perturbation velocities are also building the low-level wind maximum via vorticity dynamics.

19. Personal Acknowledgments
Michael Bell
Stacy Brodzik
Brad Smull

20. Current dataset ELDORA radar on board NRL aircraft 1800-1820 UTC
Wind field retrieved using global minimization technique (Gamache 1997, Reasor et al. 2009)
2-step Leise filter, ~5 km minimum resolvable wavelength
Additional fields removing wavenumber-0 and wavenumber-1
Grid size of 0.6 km in horizontal, 0.4 in vertical
Courtesy of Michael Bell?

21. This research was supported by NSF Grant ATM and NASA Grants NNX07AD59G and NNX10AH70G and the NDSEG program