Transcript Document

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
Sig Tor Weak Tor No Tor
(0-1 km) (0-1 km) (0-1 km)
Marginal Non-supercell
Sig Tor
Weak Tor
No Tor
Sig Tor
Weak Tor No Tor
(0-3 km) (0-3 km) (0-3 km)
Sig Tor
Weak Tor
No Tor
Marginal
Marginal
Non-supercell
Non-supercell
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.
Effectivelayer
Fixedlayer
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
E
E
MAX
NW
N
MAX
NW
N
NW
MAX
E
MAX
MAX
N
W
N
N
NW
N
MAX
*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
W
N
NW
N
NW
N
E
N
W
MAX
NE
MAX
W
N
E
NE
NW
W
N
SW
*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.
Parameter
Values
75th Percentile
Mean
50th Percentile
25th Percentile
*Bulls’ eye STP values are
around 2 units higher for
violent tornado NSEs than
strong tornado NSEs.
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
SBCAPE
0-1-km SRH
MLCAPE
0-3-km SRH
00h Initialization of the 18Z GFS 27 April 2011
STP
percentile
SCP
STP
value
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/.].