Presented by John P. Monteverdi Professor of Meteorology Department of Geosciences

2. Shear and Buoyancy Associated with 70 Tornadic and Non-TornadicThunderstorms in Northern and CentralCalifornia, 1990-1994 Presented by John P. Monteverdi Professor of Meteorology Department of Geosciences San Francisco State University Visiting Scientist Spring 2000 National Severe Storms Lab Norman, Oklahoma National Weather Service Forecast Office San Francisco Bay Area

3. Collaborators on this research Charles Doswell III National Severe Storms Laboratory Norman, Oklahoma Gary Lipari MS Thesis Candidate San Francisco State University

4. Organization of Talk • Purpose of Study • Overview of Analysis Procedures • Role of Shear in Tornadic Thunderstorms • Types of Tornadic Thunderstorms • Results of Study • Implications for Operations: Possible Thresholds

5. Purposes of Study • To determine if buoyancy played a significant role in distinguishing between tornadic and non-tornadic thunderstorms in the study period • To determine if shear, particularly in the 0-1 km and 0-2 km layers, was a distinguishing characteristic between tornadic and non-tornadic thunderstorms AND between the weaker and stronger tornadic events • To determine if the data array and the statistical analyses of the results suggested possible “threshold values” to be used operationally No! Yes! Possibly

6. Analysis Technique • Used soundings from OAK (mostly 00Z) (one VBG, one MFR), modified by surface conditions at site closest to event • Considered 3 different event types for period 1990-1994, inclusive • NULL cases … all cases in which thunder observed at SAC or FAT but no observed tornadoes in California • F0 tornado cases (suspect most non-supercells) • F1+ tornado cases (suspect many/most supercells)

7. All cases included, not just the cool season events, although most tornado events (28 of 30) were in the cool season (November-April) • Nearly half of null events (19/40) were warm season • Buoyancy calculated via “SHARP” program, updated with obs from nearest surface site • Shears calculated two ways: • Positive shear calculated by SHARP (portion of hodograph in which wind veers and speed increases with height) (Monteverdi and Lipari portion of study) • as vector differences between top and bottom of the layers (0-1, 0-2, 0-3, and 0-6 km … all AGL), updated with surface observations (Doswell and Monteverdi portion of study)

8. Current Directions of Research • Expansion of California data set in two phases: 1995-present and 1950-1989 (with C. Doswell III) • Comparison with low-buoyancy high-shear cases in Australia (with C. Doswell III and B. Hanstrum, Australian Meteorological Services)

9. Review of Vertical Shear Concepts

10. What isVertical Shear? • Is a measure of the change in wind direction and speed with height This case shows a clockwise CURVED HODOGRAPH. Shear associated with a veering wind with height is called POSITIVE SHEAR. Positive Shear values are greatest in curved hodographs (in which the wind shear vectors also veer with height). The length of the hodograph is proportional to the magnitude of the shear through the layer The dots represent the tips of the wind observations at each level. • Is estimated visually best from a hodograph Arrows joining wind observations at various levels show the shear vector in the intervening layer. In this case, the wind and the wind shear vectors are veering with height

11. Straight Hodogaph Wind Veers and Increases In Strength Through Lowest Layers There is positive shear in this straight hodograph. But note that the wind shear vector does not veer with height. That is why positive shear values tend to be less for straight hodographs. However, Wind Shear Vector Does NOT Veer To Any Great Degree

12. Importance of Shear • Removes precipitation from updraft area and shunts it down wind (updraft is not suppressed and becomes more long-lived) • Deep layer shear can create horizontal vorticity which can be tilted into the vertical by the updraft and transformed to vertical vorticity (storm scale rotation--mesocyclone) • In certain configurations of positive shear, updraft is augmented to such a degree, that the buoyancy can be magnified by a factor of two to three times • In certain configurations of positive shear, updraft strength can be augmented greatly on right flank of storm, causing the storm to “deviate” from motions of other storms (developing strong storm relative helicity and a greater tendency to become tornadic)

13. Positive Shear • Advantages • Is largest for veering wind shear vector profiles (typical shear environments for right moving supercells) • Is calculated as a matter of course by programs like SHARP (still used in many offices) • Disadvantages • Is not displayed routinely as part of AWIPS package • Is not easily calculated by “back of envelope” calculations, as bulk shear is (vector difference between wind at upper end and bottom ends of layer in question) • May distract forecaster from consideration of atypical cases (e.g., Sunnyvale May 1998 F2 anticyclonic supercellular tornado)

14. Review • In short, storms growing in an environment of “rich” positive shear have a greater likelihood of being SEVERE and in some configurations of wind shear tend to “create” their own rotation. SUPERCELLS • Storms growing in an environment without shear tend not to be severe and can only become tornadic by intercepting and ingesting pre-existing rotation. NON-SUPERCELLS • Either may be tornadic, but the strongest tornadoes and most severe weather occur in association with supercells.

15. Review of Tornadic Thunderstorm “Types”

16. Types of Tornadic Thunderstorms Observed in California Minimal Deep Layer Shear: Non-supercell Tornadic Storms (tornadic rotation associated with misocyclones) • Landspout Single Cell Storms (includes what are called “cold core” or “high-based” funnels) • Multi-cells (Hodogaphs of small length-) Moderate to Strong Deep Layer Shear With Straight Hodograph: Supercell “Line” Storms ( tornadic rotation tends to develop when storm ingests misocyclone or “shear” funnels develop at intersection of bows) • Line (Bowed Segment) Storms • Splitting isolated supercells (generally outflow dominated) Great Deep Layer Shear With Curved Hodograph: Isolated Supercell Tornadic Storms (tornadic rotation tends to be mesocyclone-induced) • Those occurring in low buoyancy environments tend to have relatively small dimensions: “low topped” or “mini-supercells”) Great Deep Layer Shear With “Flawed” Curved Hodograph: Isolated Supercell Tornadic Storms (mesocyclone/misocyclone hybrid) • Supercell intercepts pre-existing low level rotation

17. Weak Deep Layer Shear: Single Cell Non-supercell tornadic storms: Landspout Hypothesis

18. Thunderstorm does not have pre-existing rotation. Rotation exists in low level environment because of intersection of boundaries, horizontal shear along fronts or squall-lines, generation of vortices by topography. There may be a greater tendency for such low level rotation to develop and be intensified in an environment of LARGE low level (0-1 km) positive shear.

19. Small Hodograph Length

22. Moderate Deep Layer Shear: Straight Hodograph with Large Low Level Shear Large Deep Layer Shear: Straight Hodograph With Large Low Level Shear and Storm Motions Parallel To Line/Boundary Bow Echoes: “Squall Line” With Bowed Segments

23. January 9, 1995 Straight Hodograph, But Large Speed Shear

25. Dry Layer In Mid Troposphere Moist Unstable Layer Near Ground

26. Prototype Wet Microburst/Bow Echo Sounding vs Sacramento 1/9/95

27. 7PM PST January 9, 1995 Bow Segments Schematic Showing Strongest Reflectivity along Line With Bowed Segments Sites of Possible Rotation/ Tornadoes

28. Position of Subsynoptic Trough -- Storm Motion Parallel To Line Interferring Outflow Boundaries Produce Bowed Segments-- Bows Move Slightly To Right OfAnd Slower Than Mean Wind Initial Storm Motion On The Hodograph And Similar To Mean Wind KDAX Radar Reflectivity 7:00PM PST 1/9/95

30. Storm Relative Velocities 6:30 PM 1/9/95 Evidence of rotation at tip of bow

31. Large Deep Layer Shear(Curved Hodograph): Supercell Thunderstorm • A thunderstorm with a deep and persistant mesocyclone • Deep is generally taken to mean 1/4 to 1/3 depth of precipitation echo • Persistancy is generally taken to imply that the mesocyclone lasts at least 15 minutes

35. Outmoded Notions • Supercells must be large with tops >30000 • Supercells must be associated with large buoyancy • All supercells tend to be tornadic (<20% of supercells are associated with tornadoes) • Supercells are rare (if buoyancy and shear are in proper ranges, both modeling and observational studies show that supercells are the dominant mode of convection).

36. Supercell Tornadic Storms: Cascade Paradigm Vertical Shear Allows Precipitation To Be Removed From Updraft Area If low level (0-3 km) Shear Vector Veers Sufficiently (curved hodograph), Updraft And Rotation Will Be Augmented on Right Flank (with Respect to hodograph) Vertical Shear Sufficient To Generate Horizontal Rotation Which Is Tilted Into Vertical To Form Persistent Midlevel Mesocyclone

37. Supercell Tornadic Storms: Cascade Paradigm Mean Wind Hook Echo Storm Motion

38. Supercell Tornadic Storms: Cascade Paradigm • Convective updraft converts 0-6 km shear into vertical vorticity at midlevels (mesocyclone) • Persistant mesocyclone causes precipitation hook to rear flank • Rear flank downdraft (RFD) develops in association with hook • Dynamic Pipe Effect associated with descending TVS adjacent to RFD • Interaction of RFD with highly sheared inflow air (shear in 0-1 km layer) under upshear (usually northwest) side of mesocyclone associated with development of tornado rotation at surface

39. Supercell Tornadic Storms: Cascade ParadigmOutmoded Notion • Cascade Process Takes Too Long…supercell storms in California have too brief a life cycle to experience “cascade” to conventional supercell tornado Observational Studies from VORTEX 1995, 1999 show that time elapsed from mesocyclone formation to tornado is as short as ~ 15 minutes

41. November 22, 1996 Sfc subsynoptic trough Straight hodograph--moderate deep layer shear Sfc northwesterlies Sfc southeasterlies Curved hodograph--favorable deep layer shear Subsident westerlies Upper and mid-tropospheric jet Sfc leeside trough

43. Buoyancy Associated With California Thunderstorms • is typically “low” (SBCAPE ~<750 J/kg) • this relatively low (when compared to warm season Great Plains values) CAPE was and is used by many as a reason to discount tornado risk in the state • traditionally estimated poorly anyway because of propensity of some forecasters to use 500 mb Lifted Index as ONLY indicator of instability

44. What clues can be found in the research literature that might help us understand the California tornado problem? Unless California tornadoes are “different animals” than those observed elsewhere, the shear values observed with the “low buoyancy” California storms probably fit in this range.

45. “…results indicate that for moderate to high vertical shears and parcel buoyancy (limited to the layer) below 500 mb, the simulated supercells generate similar mesocyclones (compared to high buoyancy Great Plains’ cases), even though the total CAPE was a factor of 2-3 times smaller for mini-supercell cases…” Wicker and Cantrell, 1996 “…although parcel buoyancy is often small, its concentration in the strongly sheared lower troposphere promotes the development of vertical pressure forces comparable to those seen in simulated Great Plains supercells…” McCaul and Weisman, 1996

46. Analogies to California Shear/Buoyancy Combination Low Buoyancy Strong Low Level Shear Case High Buoyancy, Moderate Low Level Shear Case Davies and Johns, 1990 Both Associated With F4 Tornadoes

47. Synoptic Features for Favorable Hodographs • Strong southwesterly (to northwesterly) mid and upper tropospheric flow • Position of mid and upper tropospheric trough axis “forces” surface southeasterly flow (either directly or by means of topography) • Along coast frontal boundaries and ahead/along post-frontal trough lines

48. Schematic Synoptic Pattern Central Valley Thunderstorms

49. Central Valley: Great Plains West The “Great Plains” of California

50. Combination of surface southeasterly flow and barrier-induced low level jet can yield strongly clockwise-curved hodographs in Sacramento and San Joaquin Valleys. Topographic channeling evident in coastal valleys as well.