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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

Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued

shear vs cape
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
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
Bulk Richardson Number (BRN)

BRN= CAPE

1/2Uz2

(where Uz is a measure of the vertical

wind shear)

shear classification
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
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
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

hodographs

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

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

Straight Line Shear

500

700

850

900

1000

curved hodograph
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

slide14

Straight Line Hodograph

Clockwise Curved hodograph

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

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
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
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 and Helicity
  • Plainfield, IL tornado:
  • CAPE=7000
  • Helicity=165
  • Energy Helicity:
what to take away from this event
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.
k index
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
Vertical Totals

VT = T850 - T500

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

CT =T d850 - T500

total totals tt
Total Totals (TT)

TT = VT + CT

=T850 + T d850 - 2 T500

sweat severe weather threat index
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 index1
SWEAT (severe weather threat) Index

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

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

LI = (Tenv-Tparcel)500

slide45
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
Showalter Index (SI)
  • Compares a parcel starting at 850mb with the environment at 500mb.

SI = (Tenv-Tparcel)500

supercell index
Supercell Index
  • Weights various parameters which are indicative of possible supercell development
important points to remember
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
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
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