EAS 486 Lecture Content for Days 13, 14: Applications of Weisman and Klemp Isolated Storm Models

EAS 486 Lecture Notes for Day 14: Applications of Weisman and Klemp Isolated Storm Models

Funk et al (1999) WAF - Evolution of a squall line
1. Radar signatures
  1. Line Echo Wave Pattern (LEWP - Nolen 1959)
  2. Bow Echo (Fujita 1978)
  3. Derecho (Johns and Hirt 1987) - serial occurrence of wind damage events, involving bow echoes and squall lines
  4. Most likely occurrence of wind damage - location of echo bulging
    1. See eroding of back edge of precipitation as mid-level dry air evaporates the falling rain in the cloud
  5. Case studies - Can have shear vortices or cyclonic circulations along or near bow apex producing tornadoes
  6. Rear inflow jet - crucial role when intersecting the bowing echo
    1. Weisman (1992)
      1. Jet stays elevated until it hits updraft-downdraft couplet
        
        Requires CAPE (> 2000 J kg-1) and ambient low-level wind shear (>=20 m s-1)
        
        Long lived bowing system that may contain rotational vortices and tornadoes
        
        Example: Monticello-Big Lake wind damage in late 1990's
      2. Jet descends immediately
        
        Weakly sheared environments
        
        Less organized, shorter-lived system
2. Case study: 14-15 April 1994
  1. Long-lived convective system that produced a derecho with many bowing segments, wind damage, and several tornadoes.
  2. Set-up: "dynamic pattern:" strong short-wave trough at 500 mb with induced low-level jet in warm sector. Crossing upper-level jets over moisture stream produces squall line
  3. Sounding shows huge CAPE
    1. 2400 J/kg at Paducah
  4. Hodograph shows huge clockwise turning hodograph to 1.3 km, then slow clockwise change to 3.3 km.
    1. 0-2 km wind shear 25 m s-1
    2. 0-2 km Storm-relative helicity of 300 m2/s2
  5. Serial type derecho outbreak (see notes on derechos later)
3. Radar echo time series show 7 circulation centers
  1. Weak echo region seen developing as one cell becomes supercellular, then weakens
  2. Vortices with full rotation seen at times (clockwise at north end)
  3. Some segments showed bookend rotation centers
McCaul and Weisman (2001) MWR
1. Follow-up on Storm Behavior with changes in CAPE
2. Key parameters varied:
  1. E-values: Small CAPE (800) and large CAPE (2000 J/kg)
  2. V: Magnitude of average lower troposphere wind and shear
    1. Varied slightly to keep Ri<45 (in supercell range)
    2. 12 m/s for low CAPE
    3. 14 m/s for high CAPE
  3. Curved vs Straight hodograph
  4. Lapse rate and shear distribution specific to amount of CAPE
3. Small CAPE cases
  1. Zb - lapse rate parameter
    1. 2.5 km for large lapse rate
    2. 5.5 km for small environmental lapse rate
  2. Zv - shear parameter
    1. 2.5 km - enhanced low-level shear
    2. 5.5 km - weak low-level shear
  3. All seem to be CAPE starved
  4. Curved hodograph
    1. Only large lapse rate, strong shear case becomes a supercell
    2. Other cases - weaker supercells
      1. Low-level shear too strong relative to low-level buoyancy to sustain supercell
    3. Large lapse rate cases
      1. Updrafts stronger, larger, more numerous
      2. Surface precipitation shaft larger
      3. Surface outflow more fully developed
      4. Enhanced shear cases/large lapse rate cases show more downwind displacement of precipitation from updraft
  5. Straight hodograph
    1. Only large low-level lapse rate, strong low-level shear case becomes a supercell
      1. Thought to resemble conditions in landfalling hurricanes, shallow supercell cases (like the November case in the Carolinas)
    2. Large lapse rate, weak shear
      1. Updraft seems to tilt backwards into outflow air
      2. Develops a high-precipitation supercell structure
      3. Outflow crucial to regulating updraft development
    3. Small lapse rate, weak shear
      1. small conventional supercell
    4. Small lapse rate, strong shear slightly stronger than in curved hodograph case, but short of supercell limits.
  6. Overall
    1. larger lapse rates a bigger factor in increasing peak updraft strength than stronger shear
    2. Near surface peak vorticity (VSFC) tends to strength as the cold pool gets colder relative to the mean temperature (-TMIN)
      1. Note: ambient cold pool temperature can be estimated by comparing the minimum theta-E at midlevels with those at cloud base
4. Large CAPE simulations
  1. Have to increase V to 14 to maintain supercell development
  2. Simulations done for V=12 produced multicell storms
  3. Zb - lapse rate parameter
    1. 4.1 km for large lapse rate
    2. 7.1 km for small environmental lapse rate
  4. Zv - shear parameter
    1. 4.1 km - enhanced low-level shear
    2. 7.1 km - weak low-level shear
  5. Overall - all cases more intense than for small CAPE
  6. All seem to be a bit shear-starved
  7. Less variation due to stratification of CAPE than for small CAPE simulations
  8. Curved hodograph
    1. Strong shear
      1. Well developed supercells
      2. Cyclonically curved updraft-downdraft couplets
        
        large low-level buoyancy has vertical vorticity> 0.04 s-1
  9. Straight hodograph
    1. Strong shear
      1. large low-level buoyancy has multicell with principal cell an HP supercell with vorticity approaching 0.04 s-1
      2. Convection organized by strong surface outflow boundary
    2. Most cases allow storm cells to propagate at speeds that keep up with the outflow boundary
    3. Weak shear cases can't keep up
  10. Peak updrafts stronger in all cases than for small CAPE
  11. Large low-level lapse rate cases tend to reach an early maximum. then redevelop more episodically (transition to multicell regime)
    1. Weak low-level shear enhances this trend
5. Precipitation tends to intrude into undisturbed inflow when low-level shear is strong
6. Precipitation pushed backwards into outflow when low-level shear is weak
7. Role of low-level CAPE, low-level shear
  1. Stratification of CAPE huge factor for small CAPE, becomes less important in larger CAPE cases
  2. If CAPE held constant, increasing low-level lapse rate acts like increasing the total CAPE
  3. Justification for examining 0-1, 0-2 km profiles more intently
8. For a given hodograph, increasing low-level shear acts like increasing the magnitude of all shear
Bunkers et al (2000) WAF
1. Development of the "Bunkers" vector to predict movement of supercells
  1. Previous work
    1. Maddox (1976): 30R75 rule
    2. Davies and Johns (1993): 20R85 rule
2. Galilean invariant
  1. Depends on hodograph shape, not actual wind vectors themselves
  2. Improvement on parameter like storm-relative helicity
  3. Works for left-moving supercells
    1. Rare in US, can be favored in Australia
3. Used hodographs from 138 right-moving supercells noted in Thompson (1998)
4. Theory
  1. Both an advective and propagation component (see eqns. 1,2)
  2. Method for each sounding shown in Figure 2
  3. Mean absolute error determined the velocity of each deviant movement, the mean shear and the D factor (deviation of mean cell movement from mean wind perpendicular to the mean 0-6 km wind shear)
5. Net result
  1. Plot 0-6 km mean wind
  2. Draw wind shear vector between 0-0.5 km wind and 5.5-6.0 km wind
  3. Draw a line both perpendicular to shear and passing through the mean wind
  4. Right-mover: 7.5 m s-1 from mean wind along perpendicular line to right of vertical wind shear
  5. Left-mover: 7.5 m s-1 from mean wind to left of vertical wind shear
  6. See format in SPC soundings
6. Exceptions
  1. Supercell intersects a boundary
  2. Dynamics at boundary
    1. Vertical wind shear altered
    2. Convergence and buoyancy enhanced
  3. Net effect: supercell may propagate towards or move along boundary
  4. Orographic effects
    1. Empirical trends
      1. Enhanced convergence on windward or leeward side of mtns.
      2. Can lead to "orographically anchored storms"
      3. Can also have consistent movement relative to supercells

Last updated: March 26, 2009 12:11 PM

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