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

General Thoughts?

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…

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!

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)

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

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

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

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

r θ

Ground Relative Updraft Relative r θ

r θ

r θ

r θ

r θ

r θ Left Mover Right Mover

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

Reflectivity Schematic: TC

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

Enhances updraft growth on radially inward side rather than split!

Role of the Left Mover

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

“Real World” Examples

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.

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.

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.

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

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

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)