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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. 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

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  1. Soundings and Adiabatic Diagrams for Severe Weather Prediction and Analysis Continued

  2. 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

  3. CAPE and Shear

  4. 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

  5. Bulk Richardson Number (BRN) BRN= CAPE 1/2Uz2 (where Uz is a measure of the vertical wind shear)

  6. 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)

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

  8. 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

  9. 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

  10. How a Hodograph is Created! • Lets open our Sounding COMET Module…

  11. Example

  12. Storm Splitting: R and L storm cells move with mean wind but drift outward Straight Line Shear 500 700 850 900 1000

  13. 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

  14. Straight Line Hodograph Clockwise Curved hodograph

  15. 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)

  16. 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!!!!

  17. 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.

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

  19. In Class Case StudyTornado OutbreakMissouri, Illinois, Indiana 3/08/09

  20. Synoptic Setup:300mb

  21. 500mb

  22. 700mb

  23. 850mb

  24. 925mb

  25. Surface

  26. Springfield, Missouri

  27. Lincoln, Illinois

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

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

  30. 1600z SPC Mesocale Discussion

  31. Radar Loopshttp://vortex.plymouth.edu/nids.html

  32. Radar Loopshttp://vortex.plymouth.edu/nids.html

  33. 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.

  34. Stability Indices

  35. 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

  36. Vertical Totals VT = T850 - T500 • A value of 26 or greater is usually indicative of thunderstorm potential.

  37. Cross Totals CT =T d850 - T500

  38. Total Totals (TT) TT = VT + CT =T850 + T d850 - 2 T500

  39. 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

  40. SWEAT (severe weather threat) Index SWI = 12D + 20(T - 49) + 2f8 + f5 + 125(S + 0.2)

  41. Lifted Index (LI) • Compares the parcel with the environment at 500mb. LI = (Tenv-Tparcel)500

  42. 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

  43. Showalter Index (SI) • Compares a parcel starting at 850mb with the environment at 500mb. SI = (Tenv-Tparcel)500

  44. Supercell Index • Weights various parameters which are indicative of possible supercell development

  45. 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.

  46. 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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