The structure and evolution of vortex lines in supercell thunderstorms

1. The structure and evolution of vortex lines in supercell thunderstorms Paul Markowski & Yvette Richardson Pennsylvania State University Acknowledgments: D. Dowell, R. Davies-Jones, H. Murphey, H. Cai, E. Rasmussen, J. Straka, J. Trapp, R. Wakimoto

2. Why vortex lines? • Although the vertical component of vorticity tends to be emphasized in supercell thunderstorm and tornado studies, there is some merit in systematically inspecting the distribution and orientation of three-dimensional vortex lines in observed and simulated storms. • The three-dimensional perspective provided by vortex lines can expose dynamics that may not be as apparent in inspections of only one vorticity component at a time.

3. Why vortex lines? • The presence of horizontal buoyancy gradients can complicate vortex line analyses in phenomena like thunderstorms due to the virtually unavoidable baroclinic generation of vorticity by the horizontal buoyancy gradients that accompany the precipitation regions and vertical drafts of thunderstorms. • In the presence of significant baroclinic vorticity generation, vortex lines may not even closely approximate material lines. • Nonetheless, vortex line analyses still can be enlightening in that they can suggestplausible methods of vorticity generation and reorientation (e.g., observations of vortex rings might lead one to surmise that a local buoyancy extremum is present and responsible for the generation of the rings).

4. t = 20 min Evolution of vortex lines in a simulated supercell t = 40 min t = 80 min arches

5. What do the vortex line arches tell us? Markowski et al. (2008) Straka et al. (2007)

6. 3D wind syntheses obtained via Gamache (1997) technique using ELDORA pseudo-dual-Doppler observations Markowski, P. M., J. M. Straka, E. N. Rasmussen, R. P. Davies-Jones, Y. Richardson, and J. Trapp, 2008: Vortex lines within low-level mesocyclones obtained from pseudo-dual-Doppler radar observations. Mon. Wea. Rev., 136, 3513-3535.

7. Markowski, P. M., J. M. Straka, E. N. Rasmussen, R. P. Davies-Jones, Y. Richardson, and J. Trapp, 2008: Vortex lines within low-level mesocyclones obtained from pseudo-dual-Doppler radar observations. Mon. Wea. Rev., 136, 3513-3535.

8. Rotunno and Klemp (1985)

9. What do the vortex line arches tell us? baroclinic mechanism (Straka et al. 2007; Markowski et al. 2008) barotropic mechanism (Davies-Jones 2000, 2007; Markowski et al. 2003, 2008) negative buoyancy • vortex lines are frozen in the fluid and move as material lines (Helmholtz’ Theorem) • counter-rotating vortices straddle downdraft • leads to arching vortex lines and counter-rotating vortices • in actual storms, both environmental and baroclinic vorticity are likely important

10. max downdraft Markowski, P. M., J. M. Straka, E. N. Rasmussen, R. P. Davies-Jones, Y. Richardson, and J. Trapp, 2008: Vortex lines within low-level mesocyclones obtained from pseudo-dual-Doppler radar observations. Mon. Wea. Rev., 136, 3513-3535.

11. photo by Jim Marquis

12. Is it possible that the same fundamental dynamical process (baroclinic vortex lines generated in a cool downdraft and subsequently lifted by an updraft) can produce vortices that range in size and intensity from bookend vortices to near-ground mesocyclones to tornadoes? Weisman and Davis (1998) Fujita (1979)

13. Why do tornadic supercells lack strong cold pools? • Mobile mesonet observations from Markowski et al. (2002), Shabbott and Markowski (2006), Grzych et al. (2007), Hirth et al. (2008) • Climatological studies show that tornadic supercells are favored when boundary layer relative humidity is large. tornadic nontornadic Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells. Mon. Wea. Rev., 130, 1692-1721.

14. Why do tornadic supercells lack strong cold pools? • Perhaps supercell baroclinity is another “Goldilocks” problem whereby at least some baroclinity is crucial (all thunderstorms have at least some baroclinity), but too much, especially near the ground, is detrimental in that large near-ground baroclinity would imply very cold air near the ground and thus rapid gust front motion relative to the main updraft, which might undercut it (Brooks et al. 2003) or inhibit the vorticity stretching required by tornadogenesis (Leslie and Smith 1978; Markowski et al. 2003). • If the downdraft air containing the vortex rings is too negatively buoyant, then perhaps the end result is something resembling Fujita's microburst model rather than significant lifting of the leading edge of the vortex rings to produce vertical vorticity.

16. dual-Doppler analysis of a nontornadic supercell on 12 June 2004 near Beatrice, NE view from southwest 3 km 3 km Majcen, M., P. Markowski, Y. Richardson, and J. Wurman, 2006: A dual-Doppler analysis of a nontornadic supercell observed on 12 June 2004 using ground-based mobile radars. Preprints, 23rd Conf. on Severe Local Storms, St. Louis, MO, Amer. Meteor. Soc.

17. strong shear Tornadic storms likely weak shear low RH high RH Tornadic storms unlikely courtesy of Harold Brooks

18. Why is the combination of large low-level shear and high boundary layer RH so favorable for tornadoes? • Strong low-level shear promotes stronger low-level dynamic lifting of baroclinic vortex lines? • Low LCLs promote weaker cold pools?

19. Summary • Vortex line arches seem to be a robust trait of supercell low-level mesocyclone regions • The arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches, compared to the orientation of the ambient (barotropic) vorticity, are strongly suggestive of (i) baroclinic vorticity generation within the hook echo and associated rear-flank downdraft region of the supercells; (ii) subsequent lifting of the baroclinically altered vortex lines by an updraft, rather than ambient vortex lines alone being tilted by either an updraft or downdraft to produce a low-level vertical vorticity maximum • Not necessarily new dynamics being proposed, but rather a new way of looking at things that (i) suggests dynamical similarity to larger-scale convective systems and (ii) provides possible insight into why low LCLs and strong low-level shear is a favorable combination for tornadic supercells