MiniatureSupercells

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Miniature-Supercells

• 2006 NWS Southern Region SOO Conference
• 11July 2006

Jeffrey M. Medlin National Weather Servi
ce
Mobile, Alabama
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Definition

• What is a miniature supercell?
• Supercell Is synonymous with the term
  mesocyclone and represents a thunderstorm whose
  updraft rotation is
  
• Relatively deep compared to overall updraft
  depth
  
• Temporally persistent
• Possesses a minimum strength (or rate) of
  rotation
  
• In the early 1990s, an attempt was made to
  classify storms according to their internal
  dynamic organization and NOT- cellular
  composition

First, lets examine the term
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Definition

• What is a miniature supercell?
• Supercell - Two historical references
• Long before thunderstorm observations by Doppler
  radar, Brooks (1949) found that the tornado
  cyclone was often closely associated with the
  rotating updraft of a supercell thunderstorm.
• The structure was labeled a mesocyclone and was
  essentially found to be the parent circulation of
  the tornado (Fujita, 1985).

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Definition
Kansas

• Nowwhat makes a mesocyclone miniature?
• Is it strictly visual?

Photo courtesy Jonathan Davies
http//members.cox.net/jondavies1/tornado_fcsting/
tor_minisprcll.jpg
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Definition
Coastal Alabama

• versus formation in Coastal Alabama!

13 Oct 2001- Mesocyclone prior to producing
F3 intensity tornado near Foley, AL
Photo courtesy NWS Mobile, AL (SKYWARN Spotter
name unknown)

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Definition
This image compares the KMOB 0.5 deg storm
relative velocity image(SRM) valid 1819 UTC, with
a spotter photograph of the F3 tornado taken
around the same time.
Photo courtesy NWS Mobile, AL (SKYWARN Spotter
name unknown)

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Definition

• What makes a mesocyclone miniature?
• oris it defined by certain radar attributes
  like diameter of rotation?
  

Photo courtesy NWS Mobile, AL
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Definition

• What makes a mesocyclone miniature?

Storm Top
and storm top?
Reduced CAPE lower EL
Photo courtesy NSSL SWAT Homepage
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Definition

• What makes a mesocyclone miniature?
• Is it the depth of rotation?

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Definition

• What makes a mesocyclone miniature?
• They even seem
• to form within a
• variety of
• Synoptic settings

Nebraska
500 mb Cold-Core mini-supercell over Nebraska
18 April 2003
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Definition
Hurricane Frances - Sep 2004 -

• What makes a mesocyclone miniature?
• within different geographic locales and at
  different times of the year!
  

Photo courtesy NWS Raleigh, NC
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Definition
and even occur throughout the world!...
Adapted from Suzuki et al., 2000 (MWR, Vol. 128,
No. 6, pp.1868-1882)

Typhoon 9019 and mini-supercells on the Kanto
Plain, Japan
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Definition
Formal literature searches yield very little
Your search criteria Journal(s) All Date Rang
e All Words from Title (ALL) mini supercells
Sort by Relevance Search Results Articles Fo
und 1
Your search criteria Journal(s) All Date Rang
e All Words from Title (ALL) miniature superc
ells Sort by Relevance Search Results Articl
es Found 0
Your search criteria Journal(s) All Date Rang
e All Words from Title (ALL) shallow mesocy
clones Sort by Relevance Search Results Arti
cles Found 0
Your search criteria Journal(s) All Date Rang
e All Words from Title (ALL) shallow superc
ell Sort by Relevance Search Results Article
s Found 1
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Then, I ask, What is ultimately important?

• After numerous examples of mini-supercells, and
  at the expense of not further compartmentalizing
  storms based on their sampled appearance, we
  need to ask ourselves, What is ultimately most
  important to those tasked with radar warning
  responsibility?
• Lets more closely examine
• Associated pre-storm environmental Conditions
• Associated radar reflectivity and velocity
  characteristics
  
• Radar Sampling Limitations

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Pre-storm Environmental Conditions1

• Modest Thermodynamic Instability resulting in
• CAPE range - 300-1500 J kg-1
• CAPE average - 600-1000 J kg-1
• Foster et. al., (1995) and Wicker and Cantrell,
  (1996)
  
• Moderate to strong vertical wind shear resulting
  in
  
• Sr Helicity range - 200-700 m2 s-2
• Sr Helicity average - 200-400 m2 s-2
• Foster et al., (1995), Guerrero and Read (1993)
  and Korotky (1988)
  
• Thus, forecasters have the ability to anticipate
  days when mini-supercells may occur (i.e.,
  modest CAPE and moderate-high Sr Helicity values).

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Pre-storm Environmental Conditions2

• It has been shown, however, that one cannot
  assess the total value of CAPE alone, but rather,
  what is considered most important is the vertical
  distribution of thermodynamic instability with
  height
• Eugene W. McCaul Jr. and Morris L. Weisman. 2001
  The Sensitivity of Simulated Supercell Structure
  and Intensity to Variations in the Shapes of
  Environmental Buoyancy and Shear Profiles.
  Monthly Weather Review Vol. 129, No. 4, pp.
  664687.
• David O. Blanchard. 1998 Assessing the Vertical
  Distribution of Convective Available Potential
  Energy. Weather and Forecasting Vol. 13, No. 3,
  pp. 870877.
• Eugene W. McCaul Jr. and Morris L. Weisman. 1996
  Simulations of Shallow Supercell Storms in
  Landfalling Hurricane Environments. Monthly
  Weather Review Vol. 124, No. 3, pp. 408429.
• Eugene W. McCaul Jr.. 1991 Buoyancy and Shear
  Characteristics of Hurricane-Tornado
  Environments. Monthly Weather Review Vol. 119,
  No. 8, pp. 19541978.Blanchard (1998), WAF Note

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Pre-storm Environmental Conditions3

• Normalizing the CAPE by varying integration
  depths allows for a more direct assessment of
  updraft vertical acceleration as related to
  updraft stretching of horizontal vorticity after
  having been tilted into the vertical (and the
  eventual development of dynamic pressure
  perturbations which promotes storm longevity).
• McCaul and Weisman (1996) found in their zero
  wind simulations using vertical profiles with the
  same CAPE (800 J kg-1), that by varying the
  height at which the maximum buoyancy occurs,
  updraft magnitude nearly doubled (35 m/s, at
  2.75 km versus 19 m/s at 5.82 km).
• Wicker and Cantrell (1996) also showed that the
  coupling of low-level vertical shear and
  low-level CAPE (i.e, lowest 1-2 km) appeared to
  be more important to the development of updraft
  rotation than did large values of CAPE throughout
  a deeper convective layer (sfc-LFC).

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Radar Reflectivity and Velocity Characteristics

• Radar observation has shown mini-supercells are
  low-topped
  
• (6-8 km) convective storms with small
  rotational diameters

2.7 kft
3.7 kft
1.2 kft
5.5 kft
9.6 kft
7 kft
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Radar Reflectivity and Velocity Characteristics

• and tend to possess lower maximum radar
  reflectivity with both reduced horizontal and
  vertical extent
  

2.7 kft
3.7 kft
1.2 kft
5.5 kft
9.6 kft
7 kft
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Radar Reflectivity and Velocity Characteristics
2.7 kft
4.1 kft
1.2 kft
Divergent component
5.5 kft
9.6 kft
7 kft
Photos courtesy NSSL SWAT Homepage
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Radar Reflectivity and Velocity Characteristics
and we have already seen that WERs, BWERs and
Hooks can occur, (although reduced in horizontal
and vertical extent)
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Radar Reflectivity and Velocity Characteristics

• Compared to traditional supercells, Burgess1 et
  al. (1995) found that mini-supercells possessed
  weaker (44 vs. 28 kt) rotational velocities
  throughout their mature stages, but both had the
  same relative increase in rotation rate between
  their organizing and mature stages.
• Wicker and Cantrell (1996), Grant and Prentice
  (1996) and Darbe and Medlin (2005) more closely
  examined horizontal shear (to include observed
  smaller diameters). When smaller diameters were
  considered, less of a difference existed between
  mini-supercells rotation rates and their classic
  counterparts (practical experience).

1- Burgess et al. (1995) studied these within 55
n mi of a single-Doppler radar. He examined
the organizing, mature and dissipating stages for
each group.
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Radar Reflectivity and Velocity Characteristics
Note the near sudden doubling in Max Vr Shear
from T0-2 to T0
Grant and Prentice (1996) 16 tornado-producing

mesocyclones assoc/w mini-supercells
Darbe and Medlin (2005) six tornado-
producing mesocyclones assoc/w
mini-supercells 13 Oct 2001
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Radar Sampling Limitations
New VCPs do not entirely solve the problem!
Radar Horizon
VCP-12
Aspect Ratio
Un-balanced Mesocyclone Signatures
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Concluding Remarks

• Provided a brief documentary on mini-supercells
  
  
• (as much as 15 minutes would allow) to
  address
  
• Naming convention - Avoid further
  compartmentalization
  
• How did we arrive to where we are now?
• Citing of significant research and formal
  references
  
• Very briefly discussed most significant
  operational impacts
  
• Definition of the mesocyclone.
• Pre-storm environmental characteristics.
• Reduced vertical and horizontal dimensions and
  impact of radar sampling.
  
• Questions and/or Comments?