Radar – Supercell Convection Conceptual Models

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Analysis & Diagnosis 1Radar Palette Home Supercell Convection

Supercells

Radar Palette Home Supercell Convection

Well developed Supercell with Rear Anvil

MAMMATUS CLOUDS (in the overhanging anvil)

RAINRAIN

HEAVYHEAVY RAINRAIN

HAILHAIL

RAIN FREE BASERAIN FREE BASE

SHORT FLANKING TOWERS (no rain)

OVERSHOOTING OVERSHOOTING TOPSTOPS

MAMMATUSMAMMATUS

Banding

Wall Cloud

Tail Cloud

Supercell Identification

• Cloud Features

LP Supercell

• Less common supercell

HP Supercell

• More common supercell

Supercell Modeling - Satellite Features - Low Level Flow Boundaries• Outflow boundary interaction...

Supercell Modeling - Satellite FeaturesMid Level Flows relating to Low Level Flows• V- notch and other flows...

Storm Propagation

• Regeneration• Propagation• Train-echo systems

Supercell Splitting - One

• Storm splitting in straight line shear

Supercell Splitting - Two

• Low level baroclinicity increases mid level meso

Supercell Splitting - Three

• Veering hodo favours the right mover...

Supercell Splitting - Four

• straight line shear• cyclonic shear with height

Supercell Structure

• Potential satellite and radar clues to a supercell

Supercell Prediction

Instability

• Thermodynamic parameters• The most important include: • CAPE• LI• Cap• Dewpoint depression 700 through 500 mb

Moisture - Dewpoints

• Greater than 24C (75F) Incredibly juicy• 18-23C (65-74F) Juicy• 12-17C (55-64F) Semi-juicy• Less than 11C (55F) Low moisture content

Conceptual Model for Supercell Tornadogenesis

• Positive shear in the 0 to 3km above ground level. Units are in time to the negative 1.

• 0 to 3 weak • 4 to 5 moderate • 6 to 8 large • 9+ very large

Speed Shear

• Causes updrafts to tilt in the vertical thus leading to supercell storms.

• Speed shear also causes tubes of horizontal vorticity, which can be ingested into thunderstorms.

Cell Splitting

0-3km VWS

• Directional Shear• Cause horizontal vorticity • Also produces differential advection• Best case… SE at sfc… SW at 700 mb

Right Propagating Supercells

Tornadogenesis and the RFD

Storm-Relative 500 mb Winds

• 500 mb level) storm-relative (S-R) winds useful to help differentiate between tornadic and non-tornadic supercells within the overall environment

• Balance between Low-Level Inflow and• Low-level Rear Flank Downdraft

Storm-Relative 500 mb Winds

• 500 mb S-R winds = 16 kts (8 m/s) Lower limit for tornadic supercells.

• 500 mb S-R winds = 40 kts (20 m/s)

Aprx upper limit for tornadic supercells.

Vorticity Generation

• Advection + Tilting + Stretching• Stretching term is the ONLY term capable of

amplifying vorticity to tornadic magnitudes

500 millibar vorticity

• Vorticity is a function of curvature, earth vorticity, and speed gradients.

• If the values of vorticity are being rapidly advected, divergence will "in the real world" be much more than if the winds through the vorticity maximum are stationary or moving slowly.

Low Level Jet - LLJ

• Strong low level winds will quickly advect warm and moist air into a region if it is associated with the low level jet

• Low level convergence along LLJ

Upper level Jet Stream

• Greater 200 knots Incredible divergence• 150 to 200 knots Large divergence• 100 to 149 knots Good divergence• 70 to 99 knots Marginal divergence• Less 70 knots Small divergence

Lake Breeze Boundaries – Guelph Tornado

Maximum Updraft Speed

• W-max = square root of [2(CAPE)]• CAPE of 1500-2500 J/kg gives a w-max range of

about 50-70 m/s (100-140 kts).• due to water loading, mixing, entrainment, and

evaporative cooling, the actual w-max is approximately one-half that calculated

CAPE Distribution

• A longer, narrower profile represents the potential for a slower updraft acceleration but taller thunderstorms which is best for high precipitation efficiency

• A shorter, fatter profile would lead to a more rapid vertical acceleration which would be important for potential development of updraft rotation within the storm.

Convective Inhibition - CINH

• negative area on a sounding. A large cap or a dry planetary boundary layer will lead to high values of CINH and stability

• Cap strength in degrees Celsius• Cap needs to be less than 2 in general before it

can be broken

Mesocyclone and the Updraft

• If RFD is too cold and strong then the updraft may be undercut before tornadogenesis can begin

• If the RFD is relatively warm, the tornadoes can be long lived and violent.

Precipitation Drag RFD’s

• In moist thermodynamic profiles, evaporative cooling potential minimal even if heavy PCPN is close to the updraft… precipitation drag may drive the RFD.

Cyclic Mesocyclones

Tornado Events

• Likely isolated supercells but can develop within line segments

• High Ambient SRH (0-2km>200 m2s2) except when high CAPE and deviant storm motion locally creates helicity

• Mid-upper level winds (4-6km >15kts) aid tornado development and longevity

• CAPE/CAPE Distribution/LFC/LCL and Evaporative cooling (RFD)

• Boundaries – local helicity and possibly lower LCL

Discrete Supercells

• Convergence more localized than linear• If CIN/CAP weak, convergence trigger can be very

subtle• Strong CIN/CAP (50 J/kg) under strong convergence• Shear is through a deep layer (0-6km)• Mean Shear Vector oriented at a relatively large angle to

the initiating boundary• Discrete Supercells can evolve into lines but rarely from

lines to discrete

LFC/LCL Heights

• For greater tornado threat, relatively low LCL heights (<6000 ft)

• High LCL heights associated with dry boundary layers promote– Convective downbursts– Outflow dominated convection

Horizontal Convective Rolls

• Align with the mean wind• Forced by surface heating• Identified by cloud streets• Contribute to convection initiation

Horizontal Roll Conceptual Model

Upper Patterns

• SWLY upper flow ahead of sharp upper trof• WSWLY upper flow associated with low

amplitude S/W trofs embedded with a progressive WLY flow pattern

Torndogenesis Failure

• LFC too high• PBL too dry• Storms evolve into lines too quickly• Too little CAPE above the LFC• Too little helicity in the absence of boundaries• Deep Layer shear too strong for the CAPE• Mid Level storm relative shear too weak

Nowcasting Tornado Potential

• Monitor vertical shear (VWP data)• Jet Streaks• Surface Pressure Tendencies• Surface Analysis Boundaries• Satellite – Breaks in clouds along line• Deviant Storm Motion – Right Movers• Rapid Destabilization/ Pressure Falls

Improving Warnings for Areas with No Radar

• Weather Watcher Coverage• Anticipation based on Diagnosis• Upstream Warnings/Prior Events• Satellite/Surface Observations

– Enhanced-V notch– Boundaries– Destabilization

Radar – Supercell Convection Conceptual Models

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