1. Characteristics of Isolated Convective Storms
Austin Cross
Weisman, M.L., and J.B. Klemp, 1986: Characteristics of Isolated Convective Storms. In Mesoscale Meterorology and Forecasting, Ray, P. (ed), Amer. Meteor. Soc., Boston

2. Main Topics Models of different wind shear profiles
What’s left out of models
Determining storm type
Importance of storm motion
Local effects

3. Models of Wind Shear
Updraft redevelopment and motion depends on environmental wind shear
Dependence explains precipitation patterns
Klemp-Wilhelmson cloud model used to demonstrate for variety of hodographs
Storms initiated by isolated warm bubble in horizontally homogenous environment

4. Sounding

5. Case A: Short-Lived Multicell80
120 40
During initial half hour, warm bubble produces updraft
Downdraft develops, rain begins to reach surface, cold pool
After 40 min, updraft near heaviest rainfall
After 80 min, updraft has disappeared, new updraft from gust front convergence
After 120 min, second updraft gone. New, weaker updrafts

6. Case B: Supercell on Multicellular Line
Hodograph same shape, twice as much shear
Enough to produce strong vertical pressure gradient forces on flank of initial updraft
Induces initial cell to become supercell

7. Case B: Supercell on Multicellular Line
At 80 min, new rain centers develop on left flank and continue to redevelop
Right flank supercell stronger
Gust front never far from rain and updrafts
At 120 min, appearance similar to squall line, with supercell on southern end

8. Case C: Right Flank Supercell Split from Weaker Left-Flank Storm
Same magnitude and depth of shear as B
Hodograph now straight from 2.5 to 5km
With less curvature, left flank more isolated
If straight line (dashed), storm split is mirror image

9. Case D: Right Flank Supercell
Now straight line portion extends to 7.5 km
Right moving supercell now has rain drawn around by strong upper level flow
Left flank is even weaker than in case C

10. Case E: Weak Squall Line
Shear profile same as B, truncated at 2.5km
Shear is not deep enough to have pressure forcing for supercells
New updrafts grow on gust front
By 120 min, appearance of squall line

11. Case F: Squall Line
Shear profile same as case E, except magnitude of shear increased
Extreme magnitude of shear produces steady updraft on right flank

12. Case F: Squall Line
Supercell is weaker than that with deep layer shear
At 80 min, spearhead config
At 120 min, 60km squall line with rotating comma north flank

13. Observed Splitting Storm
Hodograph shows veering wind below 1km
Omnidirectional shear above 1km
Storm splitting expected (like cases C and D) and curvature favors right flank

14. Thermodynamic Stratification
Thermodynamic stratification was kept same in models
Instability or mid level dryness can affect updraft strength
E.g. weakly unstable environment can’t support convection in strong shear cond.

15. More Thermodynamic Stratification
Mid level moisture may alter strength of outflow; that could alter speed of gust front relative to updraft
Low level structure could alter if gust front can trigger new convection: if LFC high, convergence on gust front must be over deeper layer or over longer time

16. Determining Storm Type
Identifying type of storm can help to predict effects
Type was shown to be strongly dependent on vertical wind shear, but can be modified by thermodynamic considereations
Weisman and Klemp (1982, 1984) show this can be represented by bulk Richardson number…

17. Bulk Richardson Number
B is buoyant energy (eq. 15.1)
U is vertical wind shear
Unsteady, multicells occur with R > 30
Supercells occur with 10 < R < 40
Environment could support multicells and supercells simultaneously as in cases B and C

18. Bulk Richardson Number

19. Downsides of Richardson Number
Indicates type of convection might occur but not severity
Small buoyant energy and moderate shear could have R in supercell range, but lack of buoyancy might mean no severe weather
Large buoyant energy and moderate shear could have large R, but produce tornadoes or large hail in unsteady storm

20. 12Z Sounding to Find Mesocyclone and Tornado Potential
Tornadic storms only in high buoyancy, moderate to strong shear
Mesocyclones w/o tornadoes in lower buoyancy, strong shear

21. Storm Motion
Multicell updrafts tend to move with mean wind of lowest 5-7km, with redevelopment downshear
Supercells have significant propagation perpendicular, may appear to move to the right or left
Since ground-relative motion dependent on ground-relative winds, examples can be used to portray conditions for flash flooding

22. Flash Flooding For heavy rain, systems must be steady
Either steady supercells or unsteady cells that redevelop in same area
Occurs in all examples except case A, if moving slow enough

23. Local Effects
Knowledge of local variations and terrain on storm structure and evolution limited
Most storms are triggered by such effects

24. Local Effects In low shear In strong shear
Pre-existing boundaries as effective as storm’s outflow in redevelopment
Enhanced surface moisture or instability also influence location of updraft
In strong shear
Steadiness and propagation of supercell updraft depends on storm mesolow forcing air up, apart from surface outflow
Supercell propagation less affected by environment boundaries

25. The End

26. Case A

27. Case B

28. Case C

29. Case D

30. Figure 15.6