Indices of Violent Tornado Environments Ariel Cohen

April 22, 2010 *Needed to issue a concise suite of products that represented the gravity of the extreme-impact event during a spring weekend. *Phrases and terminology need to bring out the essence of a potentially dangerous situation to keep customers at a high state of readiness. *Significant tornadoes were likely. *What about specific extremes…like violent tornadoes? *Could talk about significant tornadoes, but what about “violent tornadoes,” in particular? April 24, 2010

About Violent Tornadoes (EF4 and EF5) *Using SeverePlot program for official reports (Hart and Janish 2006)…  Violent tornadoes are RARE  5.2% of all significant tornadoes (EF2+) and 1.1% of all tornadoes between 1 January 1950 and 31 December 2009  Violent tornadoes are HIGH IMPACT  3,296 deaths; 43,057 injuries; and $3.96 billion dollars of damage. *Our current understanding of high-end tornadoes is focused on significant tornadoes (EF2+). *However…understanding the near-storm environments associated with violent tornadoes (EF4+) is critical for situational awareness in case the rare circumstance arises…given their high impact.

Thompson et al. (2003, 2004) Updated STP and SCP Formulations: STP = (mixed-layer CAPE / 1500 J kg-1) * (effective bulk wind difference/ 20 m s-1) * (effective SRH / 150 m2 s-2) * ((2000 – mixed-layer LCL) / 1500 m) * ((250 + mixed-layer CIN) / 200 J kg-1)   SCP = (most-unstable CAPE / 1000 J kg-1) * (effective bulk wind difference / 20 m s-1) * (effective SRH / 50 m2 s-2)

Thompson et al. (2003) RUC-2 Proximity Sounding Results (413 cases) Sig Tor Weak Tor No Tor Marginal Non-supercell Sig Tor Weak Tor No Tor Marginal Non-supercell Sig Tor Weak Tor No Tor Sig Tor Weak Tor No Tor (0-1 km) (0-1 km) (0-1 km) (0-3 km) (0-3 km) (0-3 km) Sig Tor Weak Tor No Tor Marginal Non-supercell

Effective Storm-Relative Helicity Thompson et al. (2007) Effective Storm-Relative Helicity Definition: *Start by lifting the surface parcel, and then continue by lifting a parcel at each successive level with increasing height. *The level at which the lifted parcel generates at least 100 J kg-1 of CAPE AND CINH less negative than -250 J kg-1 is the effective surface. *The depth over which the aforementioned constraint is met for lifting parcels from succeeding vertical levels is computed. * The helicity calculated from this depth is the effective helicity, and is known for best distinguishing between significantly tornadic and nontornadic supercells. Effective- layer Fixed- layer

Significant Tornado Work and Motivation *Provides analysis on significant tornado NSEs, but does not highlight the NSEs specifically associated with the smaller subset of violent tornadoes. *Violent tornadoes only comprise 5.2% of significant tornadoes, so their weight in the overall significant tornado database is not substantial. *So we will focus on the violent tornado subset, and Thompson’s work provides an excellent starting point with regard to the parameters consider. *Purpose of the present work  To derive guidance for recognizing an environment supportive of yielding violent tornadoes in a nowcast sense. When NWP model consensus converges on the resulting values, EXTREME IMPACT wording could be mentioned in products based on the results here.

Now Focus on Violent Tornadoes Since 2003 *Forty-six violent tornado cases studied. *Used RUC-2 Mesoscale Analysis from SPC to assess the near-storm environment within 1 hour of each of the tornadoes.  Mostly automated documentation, except involved manual classifications for the 2010 data, which introduces some error but not more than that from analysis in a nowcasting sense. *Considered many of the variables studied by Thompson. *Spatial distributions of STP were investigated for 20 selected violent and 20 selected strong tornado cases.

Twenty Selected Violent Tornadoes NW MAX E W *Commonalities: many are not in the sig tor maximum, generally in the northern semicircle gradient with CINH less negative than -25 J kg-1.

Twenty Selected Strong Tornadoes MAX NW W NE SW N *Similar theme as with the violent tornadoes, except “the bulls’ eye” value is not as large.

Rough Spatial Analysis and “The Bulls’ Eye” *Upon stratification of tornado locations based on their occurrence on the STP gradient, most occurred on the northern gradient. *This likely reflects interaction of low-level boundaries (near gradients) and violent tornadogenesis. *Bulls’ eye STP values are around 2 units higher for violent tornado NSEs than strong tornado NSEs. Parameter Values 75th Percentile Mean 50th Percentile 25th Percentile

The NSE Results – STP and SCP (all 46 violent tornado cases) *Interquartile range for NSE STP (not necessarily areal max) is larger for violent tornadoes and is at the upper end of the distribution of significant tornadoes. *Though, many events featured lower values  we don’t necessarily need much higher STP values for violent tornadoes... reflection of the gradient.

CAPE *Similar to the values identified in Thompson’s work. *Extremely high values of CAPE are not necessary… overall limiting threshold of 750 J kg-1. *Absence of substantially higher values of CAPE likely needs some compensating effect for violent tornadoes.

Low-Level SRH *Consistently at the upper end of the distributions identified in Thompson’s work.

SRH Ratios *The majority of 0-3 km shear is contained in the 0-1 km layer where strong directional shear is likely present (consistently 75-90%... reflects the large streamwise vorticity in the lowest part of the hodograph). *This confirms some of the work by Estherheld and Guiliano (2008) who identified the importance of the strong low-level shear characterized by a nearly 90-degree angle between the storm-relative inflow vector and a long straight-line hodograph in the surface-to-500-m layer.

Other Variables *Long-track tornadoes have been found to occur in environments with very strong 0-8 km bulk shear (fast-moving storms) and with substantial low-level moisture (thus small near-surface dewpoint depressions and low MLLCL heights) – Garner (2007). *Similar interquartile range for 0-8 km bulk-shear between violent tornadoes and long-track tornadoes… increasing potential for violent tornadoes is associated with an increasing potential for long-track tornadoes. *MLLCL heights are at the lower (more moist) end of the broader distribution amongst sig tors.

Conclusions on Violent Tornadoes NSEs Look for northern gradient in STP…values at least 3. Don’t necessarily look for extreme values of STP…probably won’t be in the maximum. Bulls’ eye values for violent tornadoes are larger than for strong tornadoes. Look for SBCAPE/MLCAPE values of at least 750 J kg-1. Look for very high values of low-level helicity to compensate… especially 0-1 km SRH and ESRH (at least 300 m2 s-2) and 0-1 km shear (at least 15 m s-1)…and high ratios of 0-1 km SRH to 0-3 km SRH (at least 75%). Look for 0-8 km bulk wind shear of at least 35 m s-1 (long tornado path lengths) and MLLCL heights below 950 m.

Applications of Violent Tornado Indices at WFO Jackson, MS Given a parameter value output from model forecasts…wanted to provide an easy way for JAN forecasters to quickly reference the corresponding percentile value for violent tornado environments. Goal – Provide spatial representations of predicted percentiles given model output. Determined the line of best fit for percentile versus parameter values using the previous distributions. Example: MLCAPE Percentile = slope * (MLCAPE value) + constant  Areas of MLCAPE ~ 2000 J kg-1 are painted ~ 50 percentile, areas of MLCAPE ~ 1200 J kg-1 are painted ~ 25 percentile, etc. Plots, displayable in AWIPS, use a color scale that covers the 25th-75th percentile of each parameter.  Represents the broad middle part of the distributions while acknowledging that outlier values may not necessarily ideal for characterizing previous violent tornado environments.  Spatial overlap of this range from multiple parameters could be a clue that violent tornado mention may be warranted in products  example: 27 April 2011 NWS Jackson, MS

00h Initialization of the 18Z GFS 27 April 2011 MLCAPE SBCAPE 0-3-km SRH 0-1-km SRH

00h Initialization of the 18Z GFS 27 April 2011 STP value STP percentile SCP

References Bunkers, M. J., B. A. Klimowski, J. W. Zeitler, R. L. Thompson, and M. L. Weisman, 2000: Predicting supercell motion using a new hodograph technique. Wea. Forecasting, 15, 61–79.   Esterheld, J. M. and D. J. Guiliano, 2008: Discriminating between tornadic and non-tornadic supercells: A new hodograph technique. Electronic Journal of Severe Storms Meteorology, 3 (2), 1–50. Hart, J. A., and P. R. Janish, cited 2006: SeverePlot: Historical severe weather report database. Version 2.0. Storm PredictionCenter, Norman, OK. [Available online at http://www.spc.noaa.gov/software/svrplot2/.] Garner, J., 2007: A preliminary study on environmental parameters related to tornado path length. National Weather Association Electronic Journal of Operational Meteorology, 2007-EJ5. (http://www.nwas.org/ej/2007-EJ5/) Markowski, P. M., C. Hannon, J. Frame, E. Lancaster, A. Pietrycha, R. Edwards and R.L. Thompson, 2003a: Characteristics of vertical wind profiles near supercells obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1262-1272. Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. M. Markowski, 2003: Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243–1261. _____, _____, and C. Mead, 2004: An Update to the Supercell Composite and Significant Tornado Parameters, Preprints, 22nd Conf. of Severe Local Storms, Hyannis, MA. (CD-ROM). _____, _____, _____, 2007: Effective Storm-Relative Helicity and Bulk Shear in Supercell Thunderstorm Environments. Wea. Forecasting, 22, 102-115. Storm Prediction Center cited 2010: SPC Hourly Mesoscale Analysis. [Available online at http://www.spc.noaa.gov/exper/ma_archive/.].