Radar Climatology of Tornadoes in High Shear, Low CAPE Environments in the Mid-Atlantic

and Southeast

Jason Davis Matthew Parker

North Carolina State UniversityNC State-NWS CSTAR Workshop

November 16, 2012

Acknowledgements: NOAA CSTAR grant (NA10NWS4680007), AMS/NASA Earth Science Graduate Fellowship ProgramNWS collaborators, especially Justin Lane, Patrick Moore, Jonathan Blaes, and Hunter Coleman

Andy Dean from SPC

Motivation

• Issuing accurate tornado warnings in high shear, low CAPE (HSLC) environments is a difficult challenge for forecasters.

Motivation

• Smaller and shallower storms compared to higher CAPE environments.

• Storm appearance on radar can vary markedly from the typical structures/signatures.

• Quick spin-ups.• Previous radar studies of HSLC tornadoes have

primarily been limited to case studies. – i.e. McAvoy et al. 2000, Lane and Moore 2006

Methods

• For this study, a high shear, low CAPE (HSLC) environment is defined as– SBCAPE < 500 J/kg– 0-6 km bulk shear > 35 kts

• Tornado reports taken from the SPC’s severe weather database (Smith et al. 2012).

• Shear and CAPE values determined from nearest grid point in hourly SPC mesoanalysis data (Thompson et al. 2012).

• 100 HSLC severe weather events were identified by area WFOs between January 2006 and April 2011.

Outline

I. Regional climatology of HSLC tornadoesII. Climatology of convective modes for HSLC

tornadoes.III. Climatology of mesocyclones/mesovortices

for HSLC tornadoes.IV. Conclusions

Regional Climatology of HSLC Tornadoes

Convective Modes Climatology Results

Based on data from Smith et al. (2012) convective mode database.

Convective Modes Climatology Results

On days with HSLC tornadoes occurring in part of the domain, the high shear, high CAPE tornadoes in other areas of the domain have a greater percentage of supercells.

Regional climatology of convective modes

Regional climatology of convective modes

Climatology of HSLC Vortices-Methods

• Azimuthal shear used to measure the strength of the radar-observed rotation in mesocyclones/ mesovortices associated with tornadic and non-tornadic HSLC storms.– Gradient in radial velocity in the azimuthal direction.

Climatology of HSLC Vortices-Methods

• Warning Decision Support System-Integrated Information (WDSS-II) application (Lakshmanan et al. 2007) used to generate azimuthal shear using the linear least squares derivative method (Smith and Elmore 2004).

• A tracking algorithm was developed to track azimuthal shear maxima over time (see preprint).

• Only used cases when WSR-88D “super res” data was available (after summer 2008).

• Non-tornadic vortices found by using false alarm tornado warnings.

Tracking algorithm example

• 83 tornadic vortices (yellow) and 84 non-tornadic vortices (red) tracked.

• 83 tornadic vortices (yellow) and 84 non-tornadic vortices (red) tracked.

Vortices binned by range from radar

Preliminary Results

t -25min t +25min

Comparing tornadic and non-tornadic vortices

Preliminary Results

Comparing tornadic and non-tornadic vortices

Preliminary Results

Comparing tornadic and non-tornadic vortices

Vortex Vertical Structure

Vortex Vertical Structure

Vortex Vertical Structure

Vortex Vertical Structure

Conclusions

• Greater relative frequency of QLCS HSLC tornadoes and lower relative frequency of discrete supercell HSLC tornadoes.

• Azimuthal shear discriminates well at the base scan between tornadic and non-tornadic vortices close to the radar, but not very well farther from the radar.

• This tendency is also apparent at higher tilts as well.

Future Work

• Further quantitative analyses of results, including – vortex lifetime and depth– differences between supercells and QLCSs.

• Find ways to apply these results to operations.• Addition of more cases.• Compare results to higher CAPE

environments.• Study of radar reflectivity signatures.

Conclusions

• Greater relative frequency of QLCS HSLC tornadoes and lower relative frequency of discrete supercell HSLC tornadoes.

• Azimuthal shear discriminates well at the base scan between tornadic and non-tornadic vortices close to the radar, but not very well farther from the radar.

• This tendency is also apparent at higher tilts as well.

References• Lakshmanan, V., T. Smith, G. Stumpf, and K. Hondl, 2007: The Warning Decision Support

System–Integrated Information. Wea. Forecasting, 22, 596–612.• Lane J.D., and P.D. Moore, 2006: Observations of a non-supercell tornadic thunderstorm

from terminal Doppler weather radar. Preprints, 23rd Conf. Severe Local Storms, St. Louis, MO., P4.5.

• McAvoy, B. P., W. A. Jones, and P. D. Moore, 2000: Investigation of an unusual storm structure associated with weak to occasionally strong tornadoes over the eastern United States. Preprints, 20th Conf. on Severe Local Storms, Orlando, FL, 182-185.

• Smith, B.T., R.L. Thompson, J.S. Grams, and C. Broyles, 2012: Convective Modes for Significant Severe Thunderstorms in the Contiguous United States. Part I: Storm Classification and Climatology. Wea. Forecasting, 27, 1114-1135.

• Smith, T.M. and K. L. Elmore, 2004: The use of radial velocity derivatives to diagnose rotation and divergence. Preprints, 11th Conf. on Aviation, Range, and Aerospace, Hyannis, MA, Amer. Meteor. Soc., P5.6.

• Thompson, R.L., B.T. Smith, J.S. Grams, A.R. Dean, and C. Broyles: 2012 Convective Modes for Significant Severe Thunderstorms in the Contiguous United States. Part II: Supercell and QLCS Tornado Environments. Wea. Forecasting, 27, 1136-1154.

Extra Slides

Tracking algorithm

Predicted position of vortex at previous time t = t0 – Δt based on estimated motion vector

Position of vortex at initial time t = t0

Vortex motion vector uncertainty

Possible actual vortex position

Tornado touchdown point, or center of false alarm warning.

Preliminary Results

Preliminary Results

Preliminary Results