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Convective Storm types

Convective Storm types . James LaDue FMI Severe Storms Workshop June 2005. Outline. Single cell convection Ordinary cell convection Sheared cell convection Multicell convection. Fundamental Concepts of Convection. Ordinary cell convection. Dominate when the vertical shear is small

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Convective Storm types

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

  2. Outline • Single cell convection • Ordinary cell convection • Sheared cell convection • Multicell convection

  3. Fundamental Concepts of Convection

  4. Ordinary cell convection • Dominate when the vertical shear is small • Dominated by buoyancy processes Mogollon Rim, AZ1999 James LaDue

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

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

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

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

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

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

  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?

  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? • Sounding A • Stronger initial acceleration • Less precipitation drag CAPE (A) = CAPE (B)

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

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

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

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

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

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

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

  21. Updraft/shear interactions

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

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

  24. Origins of updraft rotation from straight shear • Incipient updraft tilts horizontal vorticity • Result 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)

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

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

  28. Directional shear H High L L Low H

  29. Directional shear From COMET (1996)

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

  31. Estimating supercell motion • The 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) 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) 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) 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 precipitation • Classic • Low Precipitation

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

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

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

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

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

  41. HP supercells • Tornado threat is large • Damaging wind threat is larger • Large hail threat is large • Most 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 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 lifting • Strength 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 longevity • Either of the top two examples have good component of shear

  47. Other cold pool/shear considerations • RKW 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 events • Cold 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 convection • Affected by • Shear-cold pool interactions • Instability gradients • Low-level convergence (SR sense) • 3-D boundary interactions

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

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

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