1. Dynamics of Tropical Cyclone Intensification: Deep Convective Cyclonic “Left Movers”
Wallace A. Hogsett and Stacy R. Stewart
Journal of Atmospheric Sciences
Volume 71
January 2014
Connor Nelson
March 2015
AATM 741: Problems in Tropical Cyclone Research
Journal Discussion

2. General Thoughts?

3. General Thoughts
Theoretical application of dynamics rather than modeling or comparisons of observations
Unique mid-latitude mesoscale dynamics approach for examining RI
Variety of “real-world” examples (?)
Lightning data
Reflectivity
Would have like to seen more evidence in a real world or model application…

4. Introduction
Overarching Issue: Due to rapid intensification of TC’s, large forecast errors occur
Hurricane Forecast Improvement Program
TC’s intensify as their RMW’s contract (Shapiro and Willoughby, 1982)(Willoughby, 1990)
Deep convection near the RMW may play a key role
Vortical Hot Tower (VHT)  vigorous, deep, helical, updrafts are the main form of convection near TCs (Hendricks et al., 2004)
Produces a lot of vorticity via stretching
Vertical shear can impact updrafts and drive production of vorticity!

5. Problem and Solution
Problem: Little is known about convection dynamics in the inner core and how VHT’s connect to updraft scale dynamics and evolution
Goal: Develop a dynamics based theoretical model of intense updrafts in the inner core of a TC (eyewall and RMW) that would explain efficient vortex spinup and eyewall contraction using a “mid-latitude framework”
Past studies linked mid-latitude supercell thunderstorms to storms in TC periphery (e.g. Eastin and Link, 2009; McCaul and Weismann 1996)

6. Methodology
Using this “mid-latitude framework” model for intense convection near the RMW
Extend Klemp (1987) to TC inner core
Demonstrate that storm splitting will promote a “left mover” convective cell and deviant motions will occur
This left mover cell should therefore promote RI
Compare to real world scenarios
Earl (2010), Helene (2006), Bret (1999), Karl (2010), Gabrielle (2001), Erin (1995)
Compare to model output
Maria (2011), other TCs

7. Mid-Latitude Framework
Based on studies of Rotunno and Klemp (1982), Weisman and Rotunno (2000), and Bunkers et al. (2000)
In a unidirectional vertical shear environment, an initially non- rotating updraft can create vertical vorticity via tilting of vortex lines
This promotes a cyclonic and anticyclonic region on the flanks of a storm
Due to non-linear pressure perturbation effects, updrafts will form on the flanks
The new cyclonic (anticyclonic) updraft will propagate with a component to the right (left) of the shear/mean wind vector
Right mover ingests local streamwise vorticity, which promotes longevity and non-linear positive vorticity feedbacks

8. Approx. 30 Deg.
0-5 km Bulk Shear 40 m/s and Buoyant Energy of 2200 J/kg

9. What about in a TC?
In a TC, the coordinate system is also switched to cylindrical (r, θ)
Assume TC center is the origin
Shear vector is opposite the mean flow!!!
Outside of HBL, tangential winds are strongest “near surface” and decay upward
At ALL azimuths and vertical levels

10. r θ

11. Ground Relative
Updraft Relative r θ

12. r θ

13. r θ

14. r θ

15. r θ

16. r θ
Left Mover
Right Mover

17. Resulting TC Hodographs
Bunkers et al. (2000)  Strength of wind or shape of initial hodograph doesn’t matter, just the direction of the shear vector

18. Reflectivity Schematic: TC

19. Problems Addressed
Mature idealized TC vortex already exhibits cyclonic vertical vorticity at the lower troposphere where these updrafts initiate
Stretching term plays most role in vorticity production in TC low levels
Suggest tilting only aids in causing deviant motion and a positive feedback with stretching term
Rarely see anticyclonic VHT’s
Suggest decreasing of vorticity = moat
Introduce a more realistic scenario of pre-existing cyclonic rotation

20. Enhances updraft growth on radially inward side rather than split!

21. Role of the Left Mover

22. Role of the Left Mover Intensify TC  ingest streamwise vorticity
Creates low vorticity moat

23. “Real World” Examples

24. RI of Earl (2010) 1-7 km Bulk Shear
HS Fig. 7
Anticyclonic shear (~15 m/s) throughout RI of Earl! So shear opposes mean flow.

25. Radar Reflectivity Examples
HS Fig. 8
All in areas of local shear ~15 m/s
Crosses indicate likely left movers. These areas fit conceptual radar reflectivity gradient.

26. Lightning Data Example: Gabrielle (2010)
HS Fig. 9
Gabrielle was heavily sheared (westerly), which doesn’t fit conceptual model. But here lightning shows convection moving radially inward.

27. Model vs. Reality: Chanchu (2006) and Erin (1995)
HS Fig. 10
Hogsett and Zhang (2010)

28. Discussion/Conclusions
Ignoring the TC secondary circulation, support for left movers and intensifying TCs and explains dynamics of strong updrafts inside core
VHT’s near the RMW can evolve into left movers given the right vertical shear profile
It is the deviant motion from the SHEAR vector that is important, not the TC motion itself…
Lifetimes are longer than VHT’s but shorter than supercells
Allows transport of air out of eye
Problems:
The local shear vector is most likely pointed inward within the HBL and not outward
Need to study the HBL
TC updraft ≠ supercell

29. Questions
How can a real left mover ingest low level streamwise vorticity on the flank nearest the eye when radial inflow occurs from other side?
Proposed solution by Braun (2002) and Houze (2010)
Is this description of the “moat” correct?
Based on this study and given a different shear profile, is it possible to have a cyclonic right-mover in the TC?
What effects does the actual “environment” have on LMs?
Would dropsonde derived hodographs give more insight to local shear vector?
What heights are important? (Bunkers et al., 2000)