A Study on the Environments
Associated with Significant
Tornadoes Occurring Within the
Warm Sector versus Those Occurring
Along Boundaries
Jonathan Garner
Storm Prediction Center
Norman, OK
Objectives
• Examine similarities and differences between
environments supporting significant
tornadoes (F2+/EF2+) in the warm sector
versus along boundaries during the period
1999-2010
– Basic climatological aspects associated with each
category
– Convective/tornado parameter space
Tornadoes along Boundaries
Maddox et al. (1980)
Markowski et al. (1998)
Warm Sector Significant Tornadoes
Newton 1967
Fujita
Methodology
• Storm Data, Severe Plot, and 2010 NWSFO
storm survey information
• Subjective surface analysis and visible satellite
imagery
– Classification of events (warm sector or boundary)
• Archived regional reflectivity
– Storm longevity
• Appearance of 35 dBZ echo in regional reflectivity
• Dissipation of cell or morphology into another storm type
• Level II data used to evaluate rotational characteristics
– Convective mode
Methodology
• Rapid Update Cycle (RUC) hourly analysis
proximity soundings
– 46 soundings from Thompson et al. (2003)
– 39 soundings from Thompson et al. (2007)
– 6 soundings collected during 2010
• Modified for surface temperature/dewpoint
and wind representative of the inflow sector
of each tornadic storm
– 35 Significantly Tornadic Warm Sector Events
– 56 Significantly Tornadic Boundary Events
Climatalogical Aspects
•
•
•
•
•
Tornado Path Length
Supercell Longevity
Convective Mode
Boundary Interaction
Warm Sector Initiating Boundary
Tornado Path Length
Warm Sector Sig Mean – 25 mi
Boundary Sig Mean – 10 mi
(99% Confidence Level)
Supercell Longevity
Warm Sector Mean – 4.7 hrs
Boundary Mean – 3.8 hrs
(99% Confidence Level)
Convective Mode
Discrete – well spaced cells with reflectivity
values < 25 dBZ between each cell (Dial et al.
2010)
Convective Mode
Linear – solid continuous line of 35 dBZ or
greater reflectivity values with a length to width
ratio of 5 to 1 (Dial et al. 2010)
Convective Mode
Mixed – discrete cells occurring adjacent
(usually downstream) to a linear storm mode
Convective Mode
18
51%
16
14
12
35
55%
30
34%
25
DISCRETE
10
20
MIXED
8
LINEAR
6
DISCRETE
29%
MIXED
15
LINEAR
10
4
2
3%
0
5
4%
0
Warm Sector
Boundary
Boundary Interaction
35
54%
30
25
INVERTED TROUGH
OUTFLOW BOUNDARY
20
STATIONARY FRONT
15
TRIPLE POINT
WARM FRONT
16%
12% 14%
10
5
4%
0
Boundary Events
Warm Sector Initiating Boundary
18
46%
16
14
12
37%
COLD FRONT
10
DRYLINE
PREFRONTAL TROUGH
8
TRIPLE POINT
6
4
2
9%
6%
0
Warm Sector Events
RUC Thermodynamic Parameters
Mean Values
**Difference in means is statistically insignificant for all thermodynamic
parameters except for the MLLCL height.
MLLCL Height
Warm Sector Sig Mean – 955 m
Boundary Sig Mean – 1061 m
(95% Confidence Level)
MLLCL Height
• More humid boundary layer air mass would promote
longer-lived supercells and significant
tornadogenesis
• Warm sectors which are hot, well-mixed, and less
humid would make significant tornadoes less
probable
• However, observational uncertainty suggests that the
difference in warm sector and boundary MLLCL
heights cannot operationally distinguish between the
two environments
RUC Wind Parameters
Mean Values
850 mb Wind Speed
Warm Sector Sig Mean – 41 kt
Boundary Sig Mean – 27 kt
(99% Confidence Level)
0-1 km Bulk Shear
Warm Sector Sig Mean – 29 kt
Boundary Sig Mean – 22 kt
(99% Confidence Level)
500 mb Wind Speed
Warm Sector Sig Mean – 58 kt
Boundary Sig Mean – 43 kt
(99% Confidence Level)
0-6 km Bulk Shear
Warm Sector Sig Mean – 56 kt
Boundary Sig Mean – 47 kt
(99% Confidence Level)
Bunkers (ID Method) Storm Speed
Bunkers et al. (2000)
Warm Sector Sig Mean – 39 kt
Boundary Sig Mean – 25 kt
(100% Confidence Level)
Summary
• Warm sector versus boundary events
– Stronger ambient wind fields and vertical wind
shear parameters for warm sector environments
• Weaker ambient shear would confine significant tornadoes to
boundaries…where shear and instability would be augmented
– Thermodynamic parameters were similar for both
significant tornado categories
– Mean and median MLLCL heights were lower for
warm sector significant tornado events
• Observational uncertainty suggests this result cannot
operationally distinguish between the two environments
Summary
• ID method for predicting supercell speed
– Much stronger for warm sector significantly tornadic storms
– This likely reflects upon the stronger deep-layer wind fields over the warm
sector
• Significant tornadoes observed along low-level boundaries are most
likely to occur within 10 km on the warm side to 30 km on the cold side
of a boundary (Markowski et al. 1998)
– A storm which has a slower storm motion would allow it to interact with a
boundary for a greater amount of time, increasing the potential for a
significant tornado, compared to a storm which quickly passes across the
boundary
– On the other hand, warm sector tornadoes have fast storm motions, thus
the width of the unstable air mass must be great enough to allow time for
significant tornadogenesis to occur before the parent storm moves into a
more hostile downstream environment
Future Work
• Examine in greater detail boundaries which do
and do not support tornadic storms
– What is the pattern of destabilization along preexisting boundaries which support tornadic
thunderstorms?
– How is moisture, instability and shear distributed
along boundaries?
– What is the most favored mode of storm-boundary
interaction?
Acknowledgments
Steve Weiss and Rich Thompson