MiniatureSupercells 2006 NWS Southern Region SOO Conference 11

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

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

Definition What is a ‘miniature supercell? ’ n ‘Supercell’ - Two historical references: Long before thunderstorm observations by Doppler radar, Brooks (1949) found that the tornado cyclone was often closely associated with the rotating updraft of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).

Definition Now…what makes a mesocyclone ‘miniature? ’ n Kansas Is it strictly visual? Photo courtesy: Jonathan Davies http: //members. cox. net/jondavies 1/tornado_fcsting/tor_minisprcll. jpg

Definition … versus formation in Coastal Alabama! Coastal Alabama 13 Oct 2001 - Mesocyclone prior to producing F 3 intensity tornado near Foley, AL Photo courtesy: NWS Mobile, AL (SKYWARN Spotter name unknown)

Definition This image compares the KMOB 0. 5 deg storm relative velocity image(SRM) valid 1819 UTC, with a spotter photograph of the F 3 tornado taken around the same time. Photo courtesy: NWS Mobile, AL (SKYWARN Spotter name unknown)

Definition What makes a mesocyclone ‘miniature? ’ …or…is it defined by certain radar attributes like diameter of rotation? Photo courtesy: NWS Mobile, AL

Definition What makes a mesocyclone ‘miniature? ’ and storm top? *Reduced CAPE = lower EL Photo courtesy: NSSL SWAT Homepage Storm Top

Definition What makes a mesocyclone ‘miniature? ’ Is it the depth of rotation?

Definition What makes a mesocyclone ‘miniature? ’ They even seem to form within a variety of Synoptic settings… ‘ 500 mb Cold-Core’ mini-supercell over Nebraska – 18 April 2003 Nebraska

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

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

Definition … 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) : shallow mesocyclones Sort by: Relevance Search Results: Articles Found: 0 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 supercell Sort by: Relevance Search Results: Articles Found: 1

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? ” Let’s more closely examine: n n n Associated pre-storm environmental Conditions Associated radar reflectivity and velocity characteristics Radar Sampling Limitations

Pre-storm Environmental Conditions 1 n 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) n Moderate to strong vertical wind shear resulting in: Sr Helicity range -> 200 -700 m 2 s-2 Sr Helicity average -> 200 -400 m 2 s-2 Foster et al. , (1995), Guerrero and Read (1993) and Korotky (1988) n Thus, forecasters have the ability to anticipate days when ‘mini-supercells’ may occur (i. e. , modest CAPE and moderate-high Sr Helicity values).

Pre-storm Environmental Conditions 2 n 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. Mc. Caul 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. 664– 687. David O. Blanchard. 1998: Assessing the Vertical Distribution of Convective Available Potential Energy. Weather and Forecasting: Vol. 13, No. 3, pp. 870– 877. Eugene W. Mc. Caul Jr. and Morris L. Weisman. 1996: Simulations of Shallow Supercell Storms in Landfalling Hurricane Environments. Monthly Weather Review: Vol. 124, No. 3, pp. 408– 429. Eugene W. Mc. Caul Jr. . 1991: Buoyancy and Shear Characteristics of Hurricane. Tornado Environments. Monthly Weather Review: Vol. 119, No. 8, pp. 1954– 1978. Blanchard (1998), WAF Note

Pre-storm Environmental Conditions 3 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). Mc. Caul 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).

Radar Reflectivity and Velocity Characteristics Radar observation has shown mini-supercells are low-topped (6 -8 km) convective storms with small rotational diameters 1. 2 kft 5. 5 kft 2. 7 kft 3. 7 kft 9. 6 kft

Radar Reflectivity and Velocity Characteristics …and tend to possess lower maximum radar reflectivity with both reduced horizontal and vertical extent 1. 2 kft 5. 5 kft 2. 7 kft 3. 7 kft 9. 6 kft

Radar Reflectivity and Velocity Characteristics 1. 2 kft 2. 7 kft 4. 1 kft Divergent component 5. 5 kft Photos courtesy: NSSL SWAT Homepage 7 kft 9. 6 kft

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

Radar Reflectivity and Velocity Characteristics Compared to traditional supercells, Burgess 1 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 minisupercells 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.

Radar Reflectivity and Velocity Characteristics Note the near sudden doubling in Max Vr Shear from T 0 -2 to T 0 Grant and Prentice (1996) – 16 tornado-producing mesocyclones assoc/w mini-supercells Darbe and Medlin (2005) – six tornadoproducing mesocyclones assoc/w mini-supercells – 13 Oct 2001

Radar Sampling Limitations Radar Horizon New VCPs do not entirely solve the problem! problem VCP-12 Aspect Ratio Un-balanced Mesocyclone Signatures

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