Miniature-Supercells 2006 NWS Southern Region SOO Conference 11July 2006 Jeffrey M. Medlin Jeffrey M. Medlin National Weather Service National Weather

Miniature-SupercellsMiniature-Supercells

2006 NWS Southern Region SOO Conference2006 NWS Southern Region SOO Conference

11July 2006 11July 2006

Jeffrey M. MedlinJeffrey M. Medlin National Weather ServiceNational Weather Service Mobile, AlabamaMobile, Alabama

DefinitionDefinition

What is a ‘What is a ‘miniature supercellminiature supercell?’?’

‘‘Supercell’Supercell’ – Is synonymous with the term ‘ – Is synonymous with the term ‘mesocyclonemesocyclone’ and ’ and represents a thunderstorm whose updraft rotation is:represents a thunderstorm whose updraft rotation is:

Relatively deep compared to overall updraft depthRelatively deep compared to overall updraft depth Temporally persistentTemporally persistent Possesses a minimum strength (or rate) of rotationPossesses a minimum strength (or rate) of rotation

In the early 1990s, an attempt was made to classify storms In the early 1990s, an attempt was made to classify storms according to their internal dynamic organization and according to their internal dynamic organization and –NOT-–NOT- cellular compositioncellular composition

First, let’s examine the term…First, let’s examine the term…


What is a ‘What is a ‘miniature supercellminiature supercell?’?’

‘‘Supercell’Supercell’ - Two historical references - Two historical references::

Long before thunderstorm observations by Doppler Long before thunderstorm observations by Doppler radar, Brooks (1949) found that the tornado cyclone radar, Brooks (1949) found that the tornado cyclone was often closely associated with the rotating updraft was often closely associated with the rotating updraft of a supercell thunderstorm.of a supercell thunderstorm.

The structure was labeled a ‘The structure was labeled a ‘mesocyclone’ mesocyclone’ and was and was essentially found to be the parent circulation of the essentially found to be the parent circulation of the tornado (Fujita, 1985).tornado (Fujita, 1985).


Now…what makes a Now…what makes a mesocyclone ‘mesocyclone ‘miniatureminiature?’?’

Is it Is it strictly visual?strictly visual?

Photo courtesy:Photo courtesy: Jonathan Davies Jonathan Davieshttp://members.cox.net/jondavies1/tornado_fcsting/tor_minisprcll.jpg

KansasKansas

DefinitionDefinition… … versus versus formation in formation in Coastal Coastal Alabama!Alabama!

Photo courtesy:Photo courtesy: NWS Mobile, AL (SKYWARN Spotter name unknown) NWS Mobile, AL (SKYWARN Spotter name unknown)

13 Oct 2001- Mesocyclone prior to producing Mesocyclone prior to producing F3 intensity tornado near Foley, ALF3 intensity tornado near Foley, AL

Coastal AlabamaCoastal Alabama


Photo courtesy:Photo courtesy: NWS Mobile, AL (SKYWARN Spotter name unknown)

This image compares the KMOB 0.5 This image compares the KMOB 0.5 deg storm relative velocity image(SRM) deg storm relative velocity image(SRM) valid 1819 UTC, with a spotter valid 1819 UTC, with a spotter photograph of the F3 tornado taken photograph of the F3 tornado taken around the same time.around the same time.


What makes a What makes a mesocyclone mesocyclone ‘‘miniatureminiature?’?’

……or…is it or…is it defined by defined by certain radar certain radar attributes like attributes like diameter of diameter of rotationrotation??

Photo courtesy:Photo courtesy: NWS Mobile, AL NWS Mobile, AL

DefinitionDefinitionWhat makes a What makes a mesocyclone mesocyclone ‘‘miniatureminiature?’?’

Photo courtesy:Photo courtesy: NSSL SWAT Homepage NSSL SWAT Homepage

and and storm topstorm top??

Storm Top

**Reduced CAPE = lower ELReduced CAPE = lower EL


Is it the Is it the depth depth of rotation?of rotation?


They even seemThey even seem

to form within a to form within a

variety of variety of

Synoptic settings…Synoptic settings…

‘‘500 mb Cold-Core’ mini-supercell over Nebraska – 18 April 2003500 mb Cold-Core’ mini-supercell over Nebraska – 18 April 2003

NebraskaNebraska


What makes a What makes a mesocyclone mesocyclone ‘‘miniatureminiature?’?’

……within different within different geographic geographic locales and at locales and at different times different times of the year!of the year!

Hurricane Frances - Sep 2004 -Hurricane Frances - Sep 2004 -

Photo courtesy:Photo courtesy: NWS Raleigh, NC NWS Raleigh, NC


Adapted from Suzuki et al., 2000 (MWR, Vol. 128, No. 6, pp.1868-1882)

……and even occur throughout the world!...and even occur throughout the world!...

Typhoon 9019 and mini-supercells on the Kanto Plain, JapanTyphoon 9019 and mini-supercells on the Kanto Plain, Japan

DefinitionDefinition … … Formal literature searches yield very little…Formal literature searches yield very little…

Your search criteria:Journal(s): All Date Range: All Words from Title (ALL) : mini supercells Sort by: Relevance Search Results: Articles Found: 1

Your search criteria:Journal(s): All Date Range: All Words from Title (ALL) : miniature supercells Sort by: Relevance Search Results: Articles Found: 0

Your search criteria:Journal(s): All Date Range: All Words from Title (ALL) : shallow mesocyclones Sort by: Relevance Search Results: Articles Found: 0

Your search criteria:Journal(s): All Date Range: All Words from Title (ALL) : shallow supercell Sort by: Relevance Search Results: Articles Found: 1

Then, I ask, “What is ultimately important?”Then, I ask, “What is ultimately important?”

After numerous examples of mini-supercells, and at the After numerous examples of mini-supercells, and at the expense of not further expense of not further ‘compartmentalizing’‘compartmentalizing’ storms based on storms based on their sampled appearance, we need to ask ourselves, their sampled appearance, we need to ask ourselves, “What is “What is ultimately most important to those tasked with radar warning ultimately most important to those tasked with radar warning responsibilityresponsibility?”?”

Let’s more closely examine:Let’s more closely examine:

Associated pre-storm environmental ConditionsAssociated pre-storm environmental Conditions Associated radar reflectivity and velocity characteristicsAssociated radar reflectivity and velocity characteristics Radar Sampling LimitationsRadar Sampling Limitations

Pre-storm Environmental ConditionsPre-storm Environmental Conditions11

Modest Thermodynamic Instability resulting in:Modest Thermodynamic Instability resulting in:CAPE range -> 300-1500 J kgCAPE range -> 300-1500 J kg-1-1

CAPE average -> 600-1000 J kgCAPE average -> 600-1000 J kg-1-1

Foster et. al., (1995) and Wicker andFoster et. al., (1995) and Wicker and Cantrell, (1996)Cantrell, (1996)

Moderate to strong vertical wind shear resulting in: Moderate to strong vertical wind shear resulting in: SSrr Helicity range -> 200-700 m Helicity range -> 200-700 m22 s s-2-2

SSrr Helicity average -> 200-400 m Helicity average -> 200-400 m22 s s-2-2

Foster et al., (1995), Guerrero and Read (1993) and Korotky (1988)Foster et al., (1995), Guerrero and Read (1993) and Korotky (1988)

Thus, Thus, forecasters have the abilityforecasters have the ability to anticipate days to anticipate days when ‘mini-supercells’ may occur (i.e., modest CAPE when ‘mini-supercells’ may occur (i.e., modest CAPE and moderate-high Sand moderate-high Sr r Helicity values).Helicity values).


It has been shown, however, that one cannot assess the It has been shown, however, that one cannot assess the total value of CAPE alone, but rather, total value of CAPE alone, but rather, what is considered what is considered most important is the vertical distribution of thermodynamic most important is the vertical distribution of thermodynamic instability with heightinstability with height::

Eugene W. McCaul Jr. and Morris L. Weisman. 2001Eugene W. McCaul Jr. and Morris L. Weisman. 2001: : The Sensitivity of Simulated The Sensitivity of Simulated Supercell Structure and Intensity to Variations in the Shapes of Environmental Supercell Structure and Intensity to Variations in the Shapes of Environmental Buoyancy and Shear Profiles.Buoyancy and Shear Profiles. Monthly Weather ReviewMonthly Weather Review: Vol. 129, No. 4, pp. 664–: Vol. 129, No. 4, pp. 664–687.687.

David O. Blanchard. 1998:David O. Blanchard. 1998: Assessing the Vertical Distribution of Convective Assessing the Vertical Distribution of Convective Available Potential Energy.Available Potential Energy. Weather and ForecastingWeather and Forecasting: Vol. 13, No. 3, pp. 870–877.: Vol. 13, No. 3, pp. 870–877.

Eugene W. McCaul Jr. and Morris L. Weisman. 1996:Eugene W. McCaul Jr. and Morris L. Weisman. 1996: Simulations of Shallow Simulations of Shallow Supercell Storms in Landfalling Hurricane Environments.Supercell Storms in Landfalling Hurricane Environments. Monthly Weather Monthly Weather ReviewReview: Vol. 124, No. 3, pp. 408–429.: Vol. 124, No. 3, pp. 408–429.

Eugene W. McCaul Jr.. 1991Eugene W. McCaul Jr.. 1991: : Buoyancy and Shear Characteristics of Hurricane-Buoyancy and Shear Characteristics of Hurricane-Tornado Environments.Tornado Environments. Monthly Weather ReviewMonthly Weather Review: Vol. 119, No. 8, pp. 1954–1978.: Vol. 119, No. 8, pp. 1954–1978.Blanchard (1998), WAF NoteBlanchard (1998), WAF Note


Normalizing the CAPE by varying integration depths Normalizing the CAPE by varying integration depths allows for a more direct assessment of updraft vertical allows for a more direct assessment of updraft vertical acceleration as related to updraft stretching of horizontal acceleration as related to updraft stretching of horizontal vorticity after having been tilted into the vertical (and the vorticity after having been tilted into the vertical (and the eventual development of dynamic pressure eventual development of dynamic pressure perturbations which promotes storm longevity).perturbations which promotes storm longevity).

McCaul and Weisman (1996)McCaul and Weisman (1996) found in their zero wind found in their zero wind simulations using vertical profiles with the same CAPE simulations using vertical profiles with the same CAPE (800 J kg(800 J kg-1-1), that by varying the height at which the ), that by varying the height at which the maximum buoyancy occurs, updraft magnitude nearly maximum buoyancy occurs, updraft magnitude nearly doubleddoubled ( (~35 m/s, at 2.75 km~35 m/s, at 2.75 km versus versus ~19 m/s at 5.82 km~19 m/s at 5.82 km).).

Wicker and Cantrell (1996)Wicker and Cantrell (1996) also showed that the coupling also showed that the coupling of low-level vertical shear and low-level CAPE (i.e, of low-level vertical shear and low-level CAPE (i.e, lowest 1-2 km) appeared to be more important to the lowest 1-2 km) appeared to be more important to the development of updraft rotation than did large values of development of updraft rotation than did large values of CAPE throughout a deeper convective layer (sfc->LFC).CAPE throughout a deeper convective layer (sfc->LFC).

Radar Reflectivity and Velocity CharacteristicsRadar Reflectivity and Velocity Characteristics Radar observation has shown mini-supercells are low-topped Radar observation has shown mini-supercells are low-topped

(6-8 km) convective storms with small rotational diameters(6-8 km) convective storms with small rotational diameters

1.2 kft 2.7 kft 3.7 kft

5.5 kft 7 kft 9.6 kft

Radar Reflectivity and Velocity CharacteristicsRadar Reflectivity and Velocity Characteristics … …and tend to possess lower maximum radar reflectivity with both reduced and tend to possess lower maximum radar reflectivity with both reduced

horizontal and vertical extenthorizontal and vertical extent

1.2 kft 2.7 kft 3.7 kft

5.5 kft 7 kft 9.6 kft

Radar Reflectivity and Velocity CharacteristicsRadar Reflectivity and Velocity Characteristics

1.2 kft2.7 kft 4.1 kft

5.5 kft7 kft

9.6 kft

Photos courtesy:Photos courtesy: NSSL SWAT Homepage NSSL SWAT Homepage

Divergent component

Radar Reflectivity and Velocity CharacteristicsRadar Reflectivity and Velocity Characteristics……and we have already seen that WERs, BWERs and Hooks can occur, and we have already seen that WERs, BWERs and Hooks can occur, (although reduced in horizontal and vertical extent)(although reduced in horizontal and vertical extent)


Compared to traditional supercells, Compared to traditional supercells, BurgessBurgess11 et al. (1995) et al. (1995) found found that ‘mini-supercells’ possessed weaker (44 vs. 28 kt) rotational that ‘mini-supercells’ possessed weaker (44 vs. 28 kt) rotational velocities throughout their mature stages, but both had the velocities throughout their mature stages, but both had the same relative increase in rotation rate between their organizing same relative increase in rotation rate between their organizing and mature stages. and mature stages.

Wicker and Cantrell (1996),Wicker and Cantrell (1996), Grant and Prentice (1996)Grant and Prentice (1996) and and Darbe and Medlin (2005)Darbe and Medlin (2005) more closely examined horizontal more closely examined horizontal shear (to include observed smaller diameters). When smaller shear (to include observed smaller diameters). When smaller diameters were considered, less of a difference existed diameters were considered, less of a difference existed between mini-supercells rotation rates and their classic between mini-supercells rotation rates and their classic counterparts (practical experience).counterparts (practical experience).

1- Burgess et al. (1995) 1- Burgess et al. (1995) studied these within 55 n mi of a single-Dopplerstudied these within 55 n mi of a single-Doppler radar. He examined the organizing, mature and dissipating stages for eachradar. He examined the organizing, mature and dissipating stages for each group.group.


Note the near sudden doubling in Max Vr Shear from TNote the near sudden doubling in Max Vr Shear from T00-2 to T-2 to T00

Grant and Prentice (1996) – Grant and Prentice (1996) – 16 tornado-producing 16 tornado-producing mesocyclones assoc/w mesocyclones assoc/w mini-supercellsmini-supercells

Darbe and Medlin (2005) – Darbe and Medlin (2005) – six tornado-six tornado-producing mesocyclones assoc/w producing mesocyclones assoc/w mini-supercells – 13 Oct 2001mini-supercells – 13 Oct 2001

Radar Sampling LimitationsRadar Sampling Limitations

Radar HorizonRadar Horizon

Un-balanced Mesocyclone SignaturesUn-balanced Mesocyclone SignaturesAspect RatioAspect Ratio

New VCPs do not entirely solve New VCPs do not entirely solve the problemthe problem!

VCP-12VCP-12

Concluding RemarksConcluding RemarksProvided a brief ‘documentary’ on mini-supercells Provided a brief ‘documentary’ on mini-supercells

(as much as 15 minutes would allow) to address:(as much as 15 minutes would allow) to address:

Naming convention - Avoid ‘further compartmentalization’Naming convention - Avoid ‘further compartmentalization’

How did we arrive to where we are now?How did we arrive to where we are now?

Citing of significant research and formal referencesCiting of significant research and formal references

Very briefly discussed most significant operational impacts:Very briefly discussed most significant operational impacts:

Definition of the mesocyclone.Definition of the mesocyclone.

Pre-storm environmental characteristics.Pre-storm environmental characteristics.

Reduced vertical and horizontal dimensions and impact of radar sampling.Reduced vertical and horizontal dimensions and impact of radar sampling.

Questions and/or Comments?Questions and/or Comments?

Thank You!Thank You!

Documents

Miniature-Supercells 2006 NWS Southern Region SOO Conference 11July 2006 Jeffrey M. Medlin Jeffrey M. Medlin National Weather Service National Weather