Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued

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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

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### 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
Bulk Richardson Number

BRN = CAPE

½ (Uz2)

Where Uz = 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

1/2Uz2

(where Uz 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
• Northern vs. Southern Hemisphere
Shear Just Right
• 2-D equilibrium: squall line develops
• 3-D equilibrium: right moving and left moving supercells

A

B

A

B

L

Left Mover

L

Right Mover

V

Hodographs

North

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

U

West

East

Wind speed

South

How a Hodograph is Created!
• Lets open our Sounding COMET Module…
Storm Splitting:

R and L storm cells move with mean wind but drift outward

Straight Line Shear

500

700

850

900

1000

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

300

500

850

900

1000

Straight Line Hodograph

Clockwise Curved hodograph

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.

Helicity

H = velocity dotted with vorticity

= V • ζ

= u (dyw - dzv) - v (dxw - dzu) + w (dxv - dyu)

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 11th- 12th , 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 12th!
• 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
• CAPE=7000
• Helicity=165
• Energy Helicity:

### In Class Case StudyTornado OutbreakMissouri, Illinois, Indiana

3/08/09

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 (td), both in oC at several standard levels.

K = t850 - t 500 + td850 - t700 + td700

Vertical Totals

VT = T850 - T500

• A value of 26 or greater is usually indicative of thunderstorm potential.
Cross Totals

CT =T d850 - T500

Total Totals (TT)

TT = VT + CT

=T850 + T d850 - 2 T500

SWEAT (severe weather threat) Index

SWI = 12D + 20(T - 49) + 2f8 + f5 + 125(S + 0.2)

where: D=850mb dew point temperature (oC)

(if D<0 then set D = 0)

T = total totals (if T < 49 then set entire term = 0)

f8=speed of 850mb winds (knots)

f5= 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

• 850mb wind direction is between 130-250
• 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) + 2f8 + f5 + 125(S + 0.2)

Lifted Index (LI)
• Compares the parcel with the environment at 500mb.

LI = (Tenv-Tparcel)500

Best Lifted Index
• Uses the highest value of qe or qwin the lower troposphere.
• 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 2o
• Start parcel at 50mb above the surface
Showalter Index (SI)
• Compares a parcel starting at 850mb with the environment at 500mb.

SI = (Tenv-Tparcel)500

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.