Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued

Shear vs. CAPE

• Need a balance between Shear and CAPE for supercell development • Without shear: single, ordinary, airmass thunderstorm which lasts 20 minutes • If shear is too strong (gust front moves too fast) : multicellular t-storms or low topped severe thunderstorms

CAPE and Shear

Bulk Richardson Number

BRN = CAPE ½ (U z 2 ) Where U z = the vertical wind shear (averaged over 3-6km layer) • In general: 15-40 favors supercell development >40 favors multicellular type storms • Explains the balance between wind shear and convective energy

Bulk Richardson Number (BRN)

BRN= CAPE 1 / 2 U z 2 (where U z is a measure of the vertical wind shear)

Shear Classification:

• Two Main types: Speed and Directional • Bulk Shear: The Boundary Layer through 6 km (or higher) above ground level shear vector denotes the change in wind throughout this height.

• Usually given in units of knots • Thunderstorms tend to become more organized and persistent as vertical shear increases. Supercells are commonly associated with vertical shear values of 35-40 knots and greater through this depth • Doesn’t take into account elevated parcels: • Effective Shear (kts) • Bulk Richardson Number shear (m^2/s^2)

DIRECTIONAL SHEAR

• Veering vs. Backing winds with height • Clockwise vs. Counterclockwise with height • Warm Air advection vs. Cold Air Advection • Northern vs. Southern Hemisphere

Shear Just Right

• 2-D equilibrium: squall line develops A B A B • 3-D equilibrium: right moving and left moving supercells Left Mover Right Mover

V

Hodographs

U •Draw wind vectors in direction they are going •This is opposite of how the wind barbs are drawn Wind speed

How a Hodograph is Created!

• Lets open our Sounding COMET Module…

Example

Straight Line Shear

700 • Storm Splitting: – R and L storm cells move with mean wind but drift outward 900 850 1000 500

Curved Hodograph

• Emphasizes one of the supercells – Veering (clockwise curve): • right moving supercells • warm air advection in northern hemisphere – Backing (counter clockwise curve): • left moving supercells • warm air advection in southern hemisphere 700 500 300 850 900 1000

Straight Line Hodograph Clockwise Curved hodograph

Helicity

• Can be thought of as a measure of the “corkscrew” nature of the winds.

H = velocity dotted with vorticity = V • ζ = u (d y w - d z v) - v (d x w - d z u) + w (d x v - d y u) • Higher helicity values relate to a curved hodograph.

– large positive values--> emphasize right cell – large negative values--> emphasize left cells • Values near zero relate to a straight line hodograph.

Lets Review Main Points

• Shear in two parts: Direction and Speed • Hodographs: characterize shear and help us to visualize the vertical shear profile • Helicity = V • ζ • Positive values favor right cells • Negative values favor left cells • 2-d equilibrium calls for developing squall line • 3-d equilibrium calls for storm splitting to occur.

• Now lets take a look at some radar loops to see these processes in action!!!!

The “Granddaddy” of them all!!!

Not the Rose Bowl • Super Tornado Outbreak of March 11 th - 12 th , 2006 • Early season event, Strongly dynamically driven • 105 confirmed tornadoes, 13 fatalities • Longest lived singular supercell in recorded weather history!

• Supercell tracked 800 miles across Oklahoma, Kansas, Missouri, Illinois, Indiana, and Michigan.

• Supercell lasted 17.5 hours • Accounted for nearly a 1/3 of the tornadoes on March 12 th !

• In a few weeks we will cover the physical and dynamical processes involved in creating and maintaining a supercell through its complete life-cycle.

CAPE and Helicity •Plainfield, IL tornado: CAPE=7000 Helicity=165 Energy Helicity: (

EHI

= 160,000 )

In Class Case Study Tornado Outbreak

Missouri, Illinois, Indiana 3/08/09

Synoptic Setup:

300mb

500mb

700mb

850mb

925mb

Surface

Springfield, Missouri

Lincoln, Illinois

SPC really jumps on the bandwagon…Yee Haw!!!

SPC really jumps on the bandwagon…Yee Haw!!!

1600z SPC Mesocale Discussion

Radar Loops

http://vortex.plymouth.edu/nids.html

Radar Loops http://vortex.plymouth.edu/nids.html

What to take away from this event…?

Dynamics/Shear dominate heat energy!

• Strong mid/upper level shortwave trough • Strong dynamical forcing/frontal forcing • Strengthening surface low • Cold temperatures: <70F • Very small CAPE values: <1000J/kg • Bulk Shear: 80+ kts !!!

• Helicity: 500+ m^2/s^2 !!!

• Clockwise Hodographs favoring right moving cells • Perfect situation for low topped discrete supercells capable of producing tornadoes near triple point.

Stability Indices

K Index

• This index uses the values for temperature (t) and dew point temperature (t d ), both in o C at several standard levels.

K = t 850 - t 500 + t d850 - t 700 + t d700 K value <15 15-20 21-25 26-30 31-35 36-40 >40 T-Storm Probability 0% <20% 20-40% 40-60% 60-80% 80-90% >90%

Vertical Totals

VT = T 850 - T 500 • A value of 26 or greater is usually indicative of thunderstorm potential.

CT

18-19 20-21 22-23 24-25 26-29 >29

Cross Totals

CT =T d850 - T 500

T-Storm Potential

Isolated to few moderate scattered moderate, a few heavy scattered moderate, a few heavy and isolated severe scattered heavy, a few severe; isolated tornados scattered to numerous heavy, few to scattered severe, a few tornados numerous heavy, scattered showers, scattered tornadoes

TT

44-45 46-47 48-49 50-51 52-55 >55

Total Totals (TT)

TT = VT + CT =T 850 + T d850 - 2 T 500

T-Storm Potential

Isolated to few moderate scattered moderate, a few heavy scattered moderate, a few heavy and isolated severe scattered heavy, a few severe; isolated tornados scattered to numerous heavy, few to scattered severe, a few tornados numerous heavy, scattered showers, scattered tornadoes

SWEAT (severe weather threat) Index

SWI = 12D + 20(T - 49) + 2f 8 + f 5 + 125(S + 0.2) where: D=850mb dew point temperature ( o C) (if D<0 then set D = 0) T = total totals (if T < 49 then set entire term = 0) f 8 =speed of 850mb winds (knots) f 5 = speed of 500mb winds (knots) S = sin (500mb-850mb wind direction) And set the term 125(S+0.2) = 0 when any of the following are

not

true 1.

850mb wind direction is between 130-250 2.

3.

4.

500mb wind direction is between 210-310 500mb wind direction minus 850mb wind direction is positive 850mb and 500mb wind speeds > 15knots

SWEAT (severe weather threat) Index

SWI = 12D + 20(T - 49) + 2f 8 + f 5 + 125(S + 0.2) <300 300-400 >400 Non-severe thunderstorms Severe thunderstorms possible Severe thunderstorms, including possible tornados

Lifted Index (LI)

• Compares the parcel with the environment at 500mb.

LI = (T env -T parcel ) 500 Lifted Index >+2 0 to +2 -2 to 0 -4 to –2 < -4 Thunderstorm Potential No convective activity Showers probable, isolated thunderstorms possible Thunderstorms probable Severe thunderstorms possible Severe thunderstorms probable, tornados possible

• Best Lifted Index – Uses the highest value of q e troposphere.

or q w in the lower – Use the highest mixing ratio value in combination with the warmest temperature.

• SELS Lifted Index – Use the mean mixing ratio and mean q of the lowest 100mb – If using a 12z sounding add 2 o – Start parcel at 50mb above the surface

Showalter Index (SI)

• Compares a parcel starting at 850mb with the environment at 500mb.

SI = (T env -T parcel ) 500 SI Thunderstorm Possibility > +3 1 to 3 -2 to 1 -6 to –2 < -6 No convective activity Showers probable, isolated thunderstorms possible Thunderstorms probable Severe thunderstorms possible Severe thunderstorms probable, tornados possible

Supercell Index

• Weights various parameters which are indicative of possible supercell development

Important Points to Remember

• Severe weather is more dependent on dynamical forcing than instability!

• No one parameter tells the full tale!

• 12z soundings usually predict afternoon convection better than 00z soundings predict evening convection.

Links

• • • • • • http://www.geocities.com/weatherguyry/swx2.html

http://avc.comm.nsdlib.org/cgi-bin/wiki.pl?Severe_Weather_Indices

http://www.theweatherprediction.com/severe/indices/ http://www.theweatherprediction.com/habyhints/315/ http://www.spc.noaa.gov/exper/mesoanalysis/ http://mocha.meteor.wisc.edu/table.12z.html