A Study on the Environments Associated with Significant
Tornadoes Occurring Within the Warm Sector versus Those
Occurring Along Boundaries
Jonathan GarnerStorm 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
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
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
RUC Thermodynamic ParametersMean Values
**Difference in means is statistically insignificant for all thermodynamic parameters except for the MLLCL height.
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
Bunkers (ID Method) Storm SpeedBunkers et al. (2000)
Warm Sector Sig Mean – 39 ktBoundary 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 pre-
existing boundaries which support tornadic thunderstorms?
– How is moisture, instability and shear distributed along boundaries?
– What is the most favored mode of storm-boundary interaction?