Supercell tornado environments

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Presentation transcript:

Supercell tornado environments Supercell (mesocyclone) tornadoes: © Craig Setzer and Al Pietrycha Developed by Jon Davies – Private Meteorologist – Wichita, Kansas

Basic environment ingredients for supercell tornadoes (from accepted research over the last 15 years) Basic environment ingredients for supercell tornadoes Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) (0-1 km) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights)

Basic environment ingredients for supercell tornadoes: Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) (0-1 km) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights) Sizable 0-3 km CAPE (relatively low LFC heights)? (Davies 2003)

Basic environment ingredients for supercell tornadoes: Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) (0-1 km) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights)

Need CAPE to generate an updraft for vertical stretching

Need low-level shear to generate horizontal vorticity (“spin”) ground 3 km 1 km

Tilting and stretching of horizontal vorticity: Low-level mesocyclones, possible tornadoes? Combinations of CAPE and low-level shear

Energy-Helicity Index from Johns, Davies, & Leftwich 1993 EHI = CAPE x SRH 160000 EHI = 2.0 F2+ tornadic storms Energy-Helicity Index

Energy-Helicity Index from Johns, Davies, & Leftwich 1993 Problems with EHI in this area of chart when SRH is large and CAPE is small EHI = CAPE x SRH 160000 EHI = 2.0 F2+ tornadic storms Energy-Helicity Index

Rasmussen (2003) 0-1 km EHI

CAPE-SRH combinations often don’t work well in these situations: Small CAPE – large SRH (many cool season cases) Cold core tornado situations (500 mb closed lows) Nonsupercell/nonmesocyclone tornadoes

Basic environment ingredients for supercell tornadoes: Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights)

Deep shear helps organize storms and strengthen updrafts This is important for most supercell tornadoes

from Davies and Johns 1993 30 kts 40 kts 50 kts F2+ tornadic storms

Basic environment ingredients for supercell tornadoes: Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights)

Low cloud bases (low LCL heights): clear slot/downdraft ( R F D ) updraft inflow low cloud bases & large humidity reduce cold pooling? downdraft not cold - contains buoyancy (Markowski et al. 2002)

From Craven and Brooks 2005 1500

Other environment characteristics that may have some relevance to tornadoes: Relatively low LFC heights? Sizable CAPE in low-levels (below 3 km)? (less work for low-level parcels of air to move upward and “stretch” in updrafts?)

(from 518 supercell cases using RUC profiles) F1 - F4 tornadoes by MLLFC range (from 518 supercell cases using RUC profiles) (Davies 2003)

Contrasting environments large CIN high LFC no CAPE0-3 low LCL This setting would probably be more favorable for tornadoes: small CIN low LFC large CAPE0-3 low LCL

Basic environment ingredients for supercell tornadoes: Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights) Sizable 0-3 km CAPE (relatively low LFC heights)?

Basic environment ingredients for supercell tornadoes: Instability (CAPE) Enhanced horizontal vorticity near ground (SRH) Deep-layer shear (0-6 km shear) Relatively low cloud bases? (low LCL heights) Sizable 0-3 km CAPE (relatively low LFC heights)? EHI S T P

Environment parameter values suggesting notable support for supercell tornadoes? 0-1 km MLEHI 2.0-3.0 or more? MLCAPE 500-1000 J kg-1 or more?* 0-1 km SRH 150-200 m2s-2 or more?* 0-6 km shear 30-35 kts or more?* MLLCL heights below 1200-1500 m?* 0-3 km MLCAPE 40-60 J kg-1 or more, MLLFC less than 2000-2500 m? Be careful using in small CAPE - large SRH settings! *in SPC’s STP parameter

Significant Tornado Parameter (updated - Thompson 2005): STP = MLCAPE/1500 x SRH0-1/150 x shear0-6/40 x (2000-LCL)/1500 x (200+CIN)/150 set to 1.5 if shear0-6 > 60 kts set to 0 if shear0-6 < 25 kts set to 1 if LCL < 1000 m set to 1 if CIN < -50 J/kg MLCAPE in J/kg; SRH in m2/s2; shear in kts; LCL in m; use lowest 100 mb mixed-layer lifted parcels

Environment parameters suggesting support for supercell tornadoes? Be careful using in small CAPE - large SRH settings! Use with caution… the atmosphere doesn’t recognize thresholds!!! 0-6 km shear < 30 kts 30 - 35 kts 36 - 44 kts 45+ kts 0-3 km CAPE < 20 J/kg 20-39 J/kg 40-59 J/kg 60+ J/kg (Surface boundaries can enhance and focus these parameters)

When using forecast fields of these ingredients: Look for areas of focus and convergence where storm development might be expected, and then assess how the parameter fields may affect that area. Fit the parameter fields with the surface pattern ! Don’t treat them as “magic numbers” or “bulls eyes” !

9 June 2005 Graham County KS tornado (w/Tim Samaras) 9 June 2005 – northwest Kansas: Strong supercell tornadoes (probe deployment attempt w/Tim Samaras) 8 May 2005 – central Kansas: nonsupercell/nonmesocyclone tornado

June 9, 2005

http://www.spc.noaa.gov

Storm relative helicity (SRH) http://www.spc.noaa.gov

Energy-helicity index (EHI) http://www.spc.noaa.gov

0-6 km shear http://www.spc.noaa.gov

LCL height http://www.spc.noaa.gov

LFC height http://www.spc.noaa.gov

(low-level instability) 0-3 km CAPE (low-level instability) http://www.spc.noaa.gov

MLCAPE 3780 J/kg 0-1 km SRH 140 m2/s2 0-1 km EHI 3.2 0-6 km shear 41 kts MLLCL 1290 m MLLFC 1610 m CAPE 0-3 km 90 J/kg STP 2.4

Significant Tornado Parameter (STP) All the basic environment ingredients that suggest support for supercell tornadoes came together in this area. http://www.spc.noaa.gov

1st tornadic storm develops

Tornado southwest of Hill City, KS ~ 4:25 pm CDT