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

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 miBoundary Sig Mean – 10 mi

(99% Confidence Level)

Supercell Longevity

Warm Sector Mean – 4.7 hrsBoundary 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

Warm Sector Boundary

34%

51%

3%

29%

55%

4%

Boundary Interaction

Boundary Events

4%

12% 14%16%

54%

Warm Sector Initiating Boundary

Warm Sector Events

37%

46%

9%6%

RUC Thermodynamic ParametersMean Values

**Difference in means is statistically insignificant for all thermodynamic parameters except for the MLLCL height.

MLLCL Height

Warm Sector Sig Mean – 955 mBoundary 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 ParametersMean Values

850 mb Wind Speed

Warm Sector Sig Mean – 41 ktBoundary Sig Mean – 27 kt

(99% Confidence Level)

0-1 km Bulk Shear

Warm Sector Sig Mean – 29 ktBoundary Sig Mean – 22 kt

(99% Confidence Level)

500 mb Wind Speed

Warm Sector Sig Mean – 58 ktBoundary Sig Mean – 43 kt

(99% Confidence Level)

0-6 km Bulk Shear

Warm Sector Sig Mean – 56 ktBoundary Sig Mean – 47 kt

(99% Confidence Level)

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?

Acknowledgments

Steve Weiss and Rich Thompson