1. Supercell tornado environments
Supercell (mesocyclone) tornadoes:
© Craig Setzer and Al Pietrycha
Developed by
Jon Davies – Private Meteorologist – Wichita, Kansas

2. Basic environment ingredients for supercell tornadoes
(from accepted research over the last 15 years)
Basic environment ingredients for supercell tornadoes
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH) (0-1 km)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)

3. Basic environment ingredients for supercell tornadoes:
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH) (0-1 km)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)
Sizable 0-3 km CAPE (relatively low LFC heights)?
(Davies 2003)

4. Basic environment ingredients for supercell tornadoes:
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH) (0-1 km)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)

5. Need CAPE to generate an updraft for vertical stretching

6. Need low-level shear to generate horizontal vorticity (“spin”)
ground
3 km
1 km

7. Tilting and stretching of horizontal vorticity:
Low-level mesocyclones, possible tornadoes?
Combinations of
CAPE and
low-level shear

8. Energy-Helicity Index
from Johns, Davies, & Leftwich 1993
EHI = CAPE x SRH
160000
EHI = 2.0
F2+ tornadic
storms
Energy-Helicity Index

9. Energy-Helicity Index
from Johns, Davies, & Leftwich 1993
Problems
with EHI
in this area
of chart
when SRH is
large and
CAPE is small
EHI = CAPE x SRH
160000
EHI = 2.0
F2+ tornadic
storms
Energy-Helicity Index

10. Rasmussen (2003)
0-1 km EHI

11. CAPE-SRH combinations often don’t work well in these situations:
Small CAPE – large SRH (many cool season cases)
Cold core tornado situations (500 mb closed lows)
Nonsupercell/nonmesocyclone tornadoes

12. Basic environment ingredients for supercell tornadoes:
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)

13. Deep shear helps organize storms and strengthen updrafts
This is important for
most supercell tornadoes

14. from Davies and Johns 1993
30 kts kts kts
F2+ tornadic
storms

15. Basic environment ingredients for supercell tornadoes:
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)

16. Low cloud bases (low LCL heights):
clear slot/downdraft
( R F D )
updraft
inflow
low cloud bases & large humidity reduce cold pooling?
downdraft not cold - contains buoyancy
(Markowski et al. 2002)

17. From Craven and Brooks 2005
1500

18. Other environment characteristics
that may have some relevance to tornadoes:
Relatively low LFC heights?
Sizable CAPE in low-levels
(below 3 km)?
(less work for low-level parcels of air
to move upward and “stretch” in updrafts?)

20. (from 518 supercell cases using RUC profiles)
F1 - F4 tornadoes
by MLLFC range
(from 518 supercell cases using RUC profiles)
(Davies 2003)

21. Contrasting
environments
large CIN
high LFC
no CAPE0-3
low LCL
This setting would
probably be more
favorable for
tornadoes:
small CIN
low LFC
large CAPE0-3
low LCL

22. Basic environment ingredients for supercell tornadoes:
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)
Sizable 0-3 km CAPE (relatively low LFC heights)?

23. Basic environment ingredients for supercell tornadoes:
Instability (CAPE)
Enhanced horizontal vorticity near ground (SRH)
Deep-layer shear (0-6 km shear)
Relatively low cloud bases? (low LCL heights)
Sizable 0-3 km CAPE (relatively low LFC heights)?
EHI S T P

24. Environment parameter values suggesting notable support for supercell tornadoes?
0-1 km MLEHI or more? MLCAPE J kg-1 or more?*
0-1 km SRH m2s-2 or more?*
0-6 km shear kts or more?*
MLLCL heights below m?*
0-3 km MLCAPE J kg-1 or more, MLLFC less than m?
Be careful
using in
small CAPE -
large SRH
settings!
*in SPC’s STP parameter

25. Significant Tornado Parameter (updated - Thompson 2005):
STP = MLCAPE/1500
x SRH0-1/150
x shear0-6/40
x (2000-LCL)/1500
x (200+CIN)/150
set to 1.5 if shear0-6 > 60 kts
set to 0 if shear0-6 < 25 kts
set to 1 if LCL < 1000 m
set to 1 if CIN < -50 J/kg
MLCAPE in J/kg; SRH in m2/s2; shear in kts; LCL in m;
use lowest 100 mb mixed-layer lifted parcels

26. Environment parameters suggesting support for supercell tornadoes?
Be careful
using in
small CAPE -
large SRH
settings!
Use with caution… the atmosphere
doesn’t recognize thresholds!!!
0-6 km shear < 30 kts kts kts kts
0-3 km CAPE < 20 J/kg J/kg J/kg J/kg
(Surface boundaries can enhance and focus these parameters)

27. When using forecast fields
of these ingredients:
Look for areas of focus and convergence where storm development might be expected, and then assess how the parameter fields may affect that area.
Fit the parameter fields with the surface pattern !
Don’t treat them as “magic numbers” or “bulls eyes” !

28. 9 June 2005 Graham County KS tornado (w/Tim Samaras)
9 June 2005 – northwest Kansas:
Strong supercell tornadoes
(probe deployment attempt w/Tim Samaras)
8 May 2005 – central Kansas: nonsupercell/nonmesocyclone tornado

29. June 9, 2005

33. Storm relative helicity
(SRH)

34. Energy-helicity index
(EHI)

35. 0-6 km shear

36. LCL height

37. LFC height

38. (low-level instability)
0-3 km CAPE
(low-level instability)

39. MLCAPE J/kg
0-1 km SRH m2/s2
0-1 km EHI
0-6 km shear 41 kts
MLLCL m
MLLFC m
CAPE 0-3 km J/kg
STP

40. Significant Tornado Parameter
(STP)
All the basic
environment
ingredients that
suggest support
for supercell
tornadoes came
together in this
area.

41. 1st tornadic storm
develops

42. Tornado southwest of Hill City, KS ~ 4:25 pm CDT