Indices of Violent Tornado

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Indices of Violent Tornado Environments Ariel
Cohen
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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
extremeslike violent tornadoes? Could talk
about significant tornadoes, but what about
violent tornadoes, in particular?
April 24, 2010
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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).
• Howeverunderstanding the near-storm
  environments associated with violent tornadoes
  (EF4) is critical for situational awareness in
  case the rare circumstance arisesgiven their
  high impact.

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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)
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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
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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
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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 Thompsons 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.
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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.
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Twenty Selected Violent Tornadoes
Commonalities many are not in the sig tor
maximum, generally in the northern semicircle
gradient with CINH less negative than -25 J kg-1.
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Twenty Selected Strong Tornadoes
N
Similar theme as with the violent tornadoes,
except the bulls eye value is not as large.
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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
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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 dont
necessarily need much higher STP values for
violent tornadoes... reflection of the gradient.
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CAPE
Similar to the values identified in Thompsons
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.
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Low-Level SRH
Consistently at the upper end of the
distributions identified in Thompsons work.
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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.
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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.
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Conclusions on Violent Tornadoes NSEs

• Look for northern gradient in STPvalues at least
  3.
• ?Dont necessarily look for extreme values of
  STPprobably wont 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.

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Applications of Violent Tornado Indices at WFO
Jackson, MS

• Given a parameter value output from model
  forecastswanted 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

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00h Initialization of the 18Z GFS 27 April 2011
MLCAPE
SBCAPE
0-3-km SRH
0-1-km SRH
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00h Initialization of the 18Z GFS 27 April 2011
STP value
STP percentile
SCP
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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,
6179.   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), 150. 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/. G
arner, 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,
12431261.   _____, _____, 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/..