1. Convective Storm types James LaDue FMI Severe Storms Workshop June 2005 James LaDue FMI Severe Storms Workshop June 2005

2. OutlineOutline Single cell convectionSingle cell convection –Ordinary cell convection –Sheared cell convection Multicell convectionMulticell convection

3. Fundamental Concepts of Convection

4. Ordinary cell convection Dominate when the vertical shear is smallDominate when the vertical shear is small Dominated by buoyancy processesDominated by buoyancy processes Mogollon Rim, AZ 1999 James LaDue

5. Ordinary cell evolution TCU + 7 min -10° C

6. Ordinary cell evolution TCU + 14 min -10° C

7. Ordinary cell evolution TCU + 21 min -10° C

8. Ordinary cell evolution TCU + 28 min -10° C

9. Pulse storm downbursts TCU + 35 min -10° C

10. Radar and visual view of an ordinary cell thunderstorm Look for onset of elevated reflectivity core as the updraft reaches the freezing levelLook for onset of elevated reflectivity core as the updraft reaches the freezing level Note the time when the intense reflectivity reaches groundNote the time when the intense reflectivity reaches ground Note the time of dissipationNote the time of dissipation Link to loop

11. What CAPE is the storm realizing? CAPE = 1490 J/kg Wmax = (2CAPE) 1/2 = 54 m/s Assume 50% or 27 m/s EL temp = -60  C But does this storm appear to have a 27 m/s updraft and an EL = -60  C?this storm

12. What CAPE is the storm realizing? A more realistic parcel path is more like the new curve Causes? Dry air entrainment Lower initial parcel  e

13. Influence of CAPE profiles Which sounding is most likely to produce a stronger updraft?Which sounding is most likely to produce a stronger updraft? Sounding ASounding A –Stronger initial acceleration –Less precipitation drag CAPE (A) = CAPE (B)

14. Influence of CAPE profiles CAPE density = CAPE/depthCAPE density = CAPE/depth –When high, expect rapid upward parcel acceleration –Occurs with steep lapse rates above and below the LFC

15. Influence of CAPE profiles Two temperature profiles with the same moisture.Two temperature profiles with the same moisture. Both yield 800 j/kg of CAPEBoth yield 800 j/kg of CAPE Lowering the maximum buoyancy level increases updraft strength at low levels.Lowering the maximum buoyancy level increases updraft strength at low levels. Z b = 5.5 km Z b = 2.5 km After McCaul and Weisman 2000 - MWR

16. DowndraftsDowndrafts Commonly initiate in the 3 – 5 km AGL layerCommonly initiate in the 3 – 5 km AGL layer Initiated by precipitation loadingInitiated by precipitation loading Evaporational cooling adds significant contributionEvaporational cooling adds significant contribution Precipitation loading becomes strong with reflectivity > 55 dBZ

17. Downdraft buoyancy From evaporational coolingFrom evaporational cooling Measured by Downdraft CAPE (DCAPE)Measured by Downdraft CAPE (DCAPE) Similar to CAPE but in reverseSimilar to CAPE but in reverse Average  w of the downdraft Average  w of the 700-500 mb layer  w of the updraft Downdraft initiation level DCAPE

18. Downdraft buoyancy Larger DCAPE mostly means stronger downdraftsLarger DCAPE mostly means stronger downdrafts However, stronger CAPE can result in stronger precipitation loadingHowever, stronger CAPE can result in stronger precipitation loading Average  w of the downdraft Average  w of the 700-500 mb layer  w of the updraft Downdraft initiation level DCAPE

19. Downdraft buoyancy DCAPE is never fully utilized by the downdraftDCAPE is never fully utilized by the downdraft Downdrafts are not saturated and do not follow the to the surfaceDowndrafts are not saturated and do not follow the  w to the surface Average  w of the downdraft Average  w of the 700-500 mb layer  w of the updraft Downdraft initiation level DCAPE DCAPE is still a good starting place to estimate downdraft strength

20. Downdrafts in single cells Downdraft strengthDowndraft strength –amount of DCAPE –precipitation loading –Nonhydrostatic vertical pressure profiles

21. Updraft/shear interactions

22. Shear interactions with updrafts Updraft tiltUpdraft tilt Causes separation of precipitation and updraftCauses separation of precipitation and updraft Precipitation loading a lesser threat to integrity of the updraftPrecipitation loading a lesser threat to integrity of the updraft

23. Shear interactions with updrafts Updraft tilt is a function of its strengthUpdraft tilt is a function of its strength –Given same shear, a weaker updraft tilts more Updraft tilt also a function of shear strengthUpdraft tilt also a function of shear strength

24. Origins of updraft rotation from straight shear Incipient updraft tilts horizontal vorticityIncipient updraft tilts horizontal vorticity Result is a counterrotating twin vortex on either side of an updraftResult is a counterrotating twin vortex on either side of an updraft

25. Origins of updraft rotation from straight shear Vortices generate ‘dynamic’ lows at the points of maximum rotation (usually in midlevels)Vortices generate ‘dynamic’ lows at the points of maximum rotation (usually in midlevels)

26. Origins of updraft rotation from straight shear Dynamic midlevel lows encourage new updraft growth within the rotation axis.Dynamic midlevel lows encourage new updraft growth within the rotation axis. Updraft appears to move right and left of the shear vector.Updraft appears to move right and left of the shear vector. Core initiates a downdraft in the middle.Core initiates a downdraft in the middle. Result is a rotating updraft.Result is a rotating updraft.

27. Origins of updraft rotation from straight shear The result is a splitting stormThe result is a splitting storm The left and right moving members rotating in opposite directionsThe left and right moving members rotating in opposite directions

28. Directional shear L H L H Low High

29. Directional shear From COMET (1996)

30. Behavior of rotating storms from curved shear Clockwise turning shear with height favors the cyclonically rotating supercellClockwise turning shear with height favors the cyclonically rotating supercell Counter clockwise turning shear with height favors the anticyclonically rotating supercellCounter clockwise turning shear with height favors the anticyclonically rotating supercell

31. Estimating supercell motion The Internal Dynamics (ID) methodThe Internal Dynamics (ID) method –Plot the 0-6 km mean wind –Draw the 0-6 km shear vector –Draw a line orthogonal to the shear vector through the mean wind –Plot the left (right) moving storm 7.5 m/s to the left (right) of the mean wind along the orthogonal line.

32. Estimating supercell motion The Internal Dynamics (ID) methodThe Internal Dynamics (ID) method –Plot the 0-6 km mean wind –Draw the 0-6 km shear vector –Draw a line orthogonal to the shear vector through the mean wind –Plot the left (right) moving storm 7.5 m/s to the left (right) of the mean wind along the orthogonal line.

33. Estimating supercell motion The Internal Dynamics (ID) methodThe Internal Dynamics (ID) method –Plot the 0-6 km mean wind –Draw the 0-6 km shear vector –Draw a line orthogonal to the shear vector through the mean wind –Plot the left (right) moving storm 7.5 m/s to the left (right) of the mean wind along the orthogonal line.

34. Estimating supercell motion The Internal Dynamics (ID) methodThe Internal Dynamics (ID) method –Plot the 0-6 km mean wind –Draw the 0-6 km shear vector –Draw a line orthogonal to the shear vector through the mean wind –Plot the left (right) moving storm 7.5 m/s to the left (right) of the mean wind along the orthogonal line. Bunkers et al. (2000)

35. Other types of supercells High precipitationHigh precipitation ClassicClassic Low PrecipitationLow Precipitation

36. LP supercells No official definitionNo official definition Poorly efficient precipitation producersPoorly efficient precipitation producers Generate outflows too weak to generate strong low level mesocyclonesGenerate outflows too weak to generate strong low level mesocyclones

37. LP supercells Tornado/wind threat is smallTornado/wind threat is small Large hail threat is largeLarge hail threat is large Near zero fl flood threatNear zero fl flood threat

38. Classic supercells No official definitionNo official definition More efficient precipitation producersMore efficient precipitation producers Generate sufficient outflows to generate strong low level mesocyclonesGenerate sufficient outflows to generate strong low level mesocyclones

39. Classic supercells Tornado/wind threat is largeTornado/wind threat is large Large hail threat is largeLarge hail threat is large Increasing fl flood threat for slow moving cellsIncreasing fl flood threat for slow moving cells

40. High Precipitation supercells No official definitionNo official definition Most commonMost common Moderate efficient precipitation producersModerate efficient precipitation producers Strong outflows generate strong low-level mesos but mostly short-livedStrong outflows generate strong low-level mesos but mostly short-lived

41. HP supercells Tornado threat is largeTornado threat is large Damaging wind threat is largerDamaging wind threat is larger Large hail threat is largeLarge hail threat is large Most likely responsible for flash floodsMost likely responsible for flash floods 30 Apr 2000 – Olney, TX - J. LaDue

42. HP Supercells Adapted from Moller et al., 1990

43. Cold pool/shear interactions This is most significant when considering multicell behaviorThis is most significant when considering multicell behavior –Motion, longevity, severity

44. Cold pool/shear interactions This side is where environmental and cold pool vorticity inhibit deep lifting. This side is where environmental and cold pool vorticity enhance deep lifting. Based on theory by Rotunno, Klemp and Weisman, 1988 (RKW)

45. Cold pool shear interactions RKW theory shows how the shear/cold pool interactions affect the depth of liftingRKW theory shows how the shear/cold pool interactions affect the depth of lifting Strength of surface convergence does not indicate depth of liftingStrength of surface convergence does not indicate depth of lifting

46. Cold pool shear interactions According to RKW theory, the shear component perpendicular to the orientation of the line helps determine line longevityAccording to RKW theory, the shear component perpendicular to the orientation of the line helps determine line longevity Either of the top two examples have good component of shearEither of the top two examples have good component of shear

47. Other cold pool/shear considerations RKW theory tested with idealized model multicell initiationRKW theory tested with idealized model multicell initiation Other studies such as Coniglio and Stensrud suggest shear layer deeper than RKW is better for anticipating long-lived multicell eventsOther studies such as Coniglio and Stensrud suggest shear layer deeper than RKW is better for anticipating long-lived multicell events Cold pool shear interactions only one factor in determining convective initiation potential in multicellsCold pool shear interactions only one factor in determining convective initiation potential in multicells

48. Multicell Motion Determined by which side of the cold pool initiates the most convectionDetermined by which side of the cold pool initiates the most convection Affected byAffected by 1) Shear-cold pool interactions 2) Instability gradients 3) Low-level convergence (SR sense) 4) 3-D boundary interactions

49. Multicell Motion 2) Instability effects –Can modulate propagation of multicells toward areas of higher instability From Richardson (1999)

50. Multicell Motion 3)Low-level convergence effects Use original MBE Vector (“Corfidi”) Technique After Corfidi et al. (1996) To see where low-level convergence is located, and help predict system motion

51. Multicell Motion 4)Boundary interactions Modulates/enhances development of new convectionModulates/enhances development of new convection Blue = steering layer flow Green=triple pt motion Red = multicell motion (Weaver, 1979)

52. The Rear Inflow Jet Squall lines w/ nondescending Rear Inflow Jets (RIJs) live longer, but also consider deep-layered shear. Note the vorticity induced by the nondescending RIJ can counteract that of the cold pool

53. Dynamics of a RIJ Strength of RIJ depends on CAPE (incr. temp excess) and Shear (erect updraft with more direct heat into anvil)Strength of RIJ depends on CAPE (incr. temp excess) and Shear (erect updraft with more direct heat into anvil)

54. Three classes of multicells SR line-parallel wind comp. SR line-perpendicular comp. Speed After Parker and Johnson (2000)

55. Bow echoes Bowing structure accompanying severe wind eventsBowing structure accompanying severe wind events Often has similar hodographs to that of supercellsOften has similar hodographs to that of supercells

56. SRH vs shear as a supercell forecasting tool Shear can be used without knowing storm motionShear can be used without knowing storm motion Once storm motion is known, use SRH to estimate supercell strengthOnce storm motion is known, use SRH to estimate supercell strength 60 km VrVr C

57. SummarySummary Buoyancy considerationsBuoyancy considerations –Convective Available Potential Energy (CAPE) –Low vs. high CAPE density can alter updraft speed by changing precipitation loading –Different buoyancy profiles can alter strength of low-level updraft even though CAPE is the same –Downdraft CAPE, or DCAPE is a measure of downdraft strength potential but does not include precipitation loading

58. SummarySummary Updraft/Shear considerationsUpdraft/Shear considerations –Causes updrafts to tilt lessoning precipitation loading –Results in updraft rotation –Straight shear results in counterrotating supercells –Clockwise (counterclockwise) curved shear enhances the cyclonically (anticyclonically) rotating supercell

59. SummarySummary Cold pool/Shear considerationsCold pool/Shear considerations –Results in updraft rotation –Straight shear results in counterrotating supercells –Clockwise (counterclockwise) curved shear enhances the cyclonically (anticyclonically) rotating supercell