EAS 486 Lecture Content for Day 11: Klemp and Weisman Isolated Storm Model

EAS 486 Lecture Content for Day 11: Klemp and Weisman Isolated Storm Model

The lecture content included:

Behavior of Isolated Storms cont.

See Ray (1991), Mesoscale Meteorology and Forecasting, Chapter 15 for figures.

Klemp and Weisman (1983) model results
- Initial updraft initiated in model (cause unknown)
- Goal: To watch changes in storm evolution due to variations in vertical wind shear and CAPE
- Bulk Richardson Number used (R)
  - Ratio of buoyant instability to vertical wind shear
    - CAPE used as buoyant instability parameter
      - CAPE constant = 2200 J/kg throughout domain unless noted
      - Variation of R due to changing wind shear unless otherwise noted.

Wind shear = 0.5 U**2
- U defined as the difference between:
  - the density-weighted average wind in the lowest 6 km
  - a representative surface wind, defined as the mean wind in the lowest 500 mm
Bulk Richardson Number since parameters allowed to stand for differentials
- Full Richardson Number calculation would evaluate each layer of sounding differently

Diagrams show result of initial updraft at 40, 80, and 120 minutes into the simulation
Reminder of hodograph appearance
- Hodograph connects tips of wind vectors
- Both clockwise turning and straight hodograph imply veering (warm advection), although not valid in this simulation since surface conditions are the same everywhere
- For zero wind shear, hodograph is a point (all wind vectors on top of one another)

Short-lived Multicell Storm (R=89)
1. Clockwise hodograph through 5 km, but shear magnitude small (maximum wind 12 m/s)
2. Initial updraft produces fairly intense storm, but outflow boundary begins to outrun area of new cell development
3. By 120 minutes, left with weaker new cells trying to form entirely within downdraft air (not as favorable as original air that produced the original thunderstorm)
Supercell on south end of multicellular line (R=22)
1. Same shaped hodograph as in Case A, but with double the magnitude of the shear
2. Initial and redeveloping storms now can keep up with outflow boundary, ensuring continuous access to warm, humid air
3. Shear now sufficient to create vertical pressure deficit in updraft on right side of original storm
4. Initial cell now evolves into quasi-steady supercell that moves to the right and more slowly than the mean wind
  1. Supercell shows hook echo in rain water contours at 120 mn.
5. Left-moving cells produce continuous redevelopment along gust front
  1. Left-moving cells move to the left of the mean wind and more quickly than mean wind speed
6. Result is a short squall line with a supercell on the southern end.
Two cases with near similar shear
1. Right-flank supercell split from weaker left-flank storm (R=15)
  1. Same magnitude and depth of wind shear as in Case B, but have straight hodograph between 2.5 and 5 km
  2. Get storm fully split
    1. Right-moving cell again becomes cyclonically-rotating supercell
    2. Effect of reduced curvature: Left-moving cells weaker and become more isolated
2. Mirror-image supercells (R=12)
  1. Linear hodograph (not the same as "speed shear), rather than clockwise turning in lowest 2.5 km
  2. Left-mover becomes anticyclonically-rotating supercell, moves faster than mean wind
  3. Right-mover becomes cyclonically-rotating supercell, moves slower than mean wind
Right-flank Supercell (R=14)
1. Same shear as in Case C1, but linear shear extends to 7.5 km
  1. Note: In previous cases, no wind shear (same wind vector) from 5 km up
2. Increased wind shear pushes rain area farther downshear in right-moving supercell
3. Most distinct hook echo in supercell
4. Left-flank activity even weaker than in previous cases
Weak squall line (R=34)
1. Clockwise-turning hodograph as in previous cases, but shear stops at 2.5 km
2. Strong shear, but not enough to maintain a supercell
3. Constant redevelopment of new, unsteady updrafts moving roughly with mean wind
4. At 120 min, get a 50 km-long squall line
5. Cold outflow moves slightly faster than precipitation area, but new cells still reach moderate intensities
Squall line - Spearhead echo evolves into bow and comma echoes (R=34)
1. Same shaped hodograph as in Case E, but 50% increase in shear magnitude
2. Increase in CAPE: low-level moisture increased from 14 g/kg to 15 g/kg
3. Extreme shear magnitude sufficient to produce a quasi-steady updraft on right flank of storm
4. Supercell still weaker than case with deeper shear layer
5. Resembles Fujita's (1981) model for severe downburst-producing storm system
6. At 80 min, model looks like Fujita's (1977) "spearhead echo" configuration
7. At 120 min, get 60 km-long squall line with echo bowing out in center and a cyclonically-rotating comma head on northern flank
  1. Note: this is produced at mid-levels just by wind shear (no dry layer in model)
8. Surface winds along intense rain water gradient reach 35-40 m/s at 80-120 min into simulation
9. Opposite organization than supercell:
  1. Worst weather at front of storm
    1. Strongest updraft at front of storm (front-to-rear flow at top of storm)
    2. Quickly changes to downdraft air beneath roll or shelf cloud
      1. Possibility of a "gustnado"
    3. Damaging winds just behind gust front, followed by heaviest rain and hail
  2. "Rear-to-front" jet, bringing strong dry mid-level winds into system, fueling downdraft
  3. Tornadoes possible near comma head before strongest straight line winds organize
Points not covered in model
1. Weaker instability (lower CAPE) could result in stronger shear mixing storm out
2. Gradient of mid-level moisture could change strength of surface outflow
  1. Drier air in mid-levels fuel stronger downdraft
  2. Storm structure can be enhanced by extra convergence at outflow boundary
3. Storms just short on vertical wind shear can produce multicell storms with some supercell characteristics

Last updated: 03-Apr-2008 7:51 AM

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