PowerPoint PresentationShear and Buoyancy Associated with 70 Tornadic and Non-Tornadic Thunderstorms in Northern and Central California, 1990-1994Collaborators on this researchSlide 4Purposes of StudyAnalysis TechniqueSlide 7Current Directions of ResearchReview of Vertical Shear ConceptsSlide 10Straight HodogaphImportance of ShearPositive ShearReviewReview of Tornadic Thunderstorm “Types”Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Prototype Wet Microburst/Bow Echo Sounding vs Sacramento 1/9/95Slide 27Slide 28Slide 29Slide 30Large Deep Layer Shear (Curved Hodograph): Supercell ThunderstormSlide 32Slide 33Slide 34Outmoded NotionsSlide 36Slide 37Supercell Tornadic Storms: Cascade ParadigmSupercell Tornadic Storms: Cascade Paradigm Outmoded NotionSlide 40November 22, 1996Slide 42Slide 43Slide 44Slide 45Analogies to California Shear/Buoyancy CombinationSynoptic Features for Favorable HodographsSchematic Synoptic Pattern Central Valley ThunderstormsCentral Valley: Great Plains WestSlide 50Example of Favorable Shear Profile Caused by Surface Southeasterly Flow Surmounted by Low Level JetSlide 52Topographic “Channeling” Can Contribute to Curved Hodographs Even Without Low Level JetIn Many Cases, Channeling Effects May Produce Straight Hodographs Weak-Mod 0-6 km Shear -- Lines Strong 0-6 km Shear -- Splitting SupercellsNorthern and Central California Tornadoes 1990-94Slide 56Results of StudySlide 58Bulk Shear Values Showed Similar RangesWith Much Caution Warranted Due to Small Sample Size Some Thresholds Are SuggestedSlide 61Slide 62ThresholdsSlide 64Implications for ForecastingSlide 66Conservative Operational RecommendationsSlide 6812Shear and Buoyancy Associated with 70 Tornadic and Non-TornadicThunderstorms in Northern and CentralCalifornia, 1990-1994Presented byJohn P. MonteverdiProfessor of MeteorologyDepartment of GeosciencesSan Francisco State UniversityVisiting Scientist Spring 2000 National Severe Storms LabNorman, OklahomaNational Weather Service Forecast OfficeSan Francisco Bay Area3Collaborators on this research Charles Doswell III National Severe Storms Laboratory Norman, Oklahoma Gary Lipari MS Thesis Candidate San Francisco State University4Organization of Talk• Purpose of Study• Overview of Analysis Procedures• Results of Study• Implications for Operations: Possible Thresholds• Role of Shear in Tornadic Thunderstorms• Types of Tornadic Thunderstorms5Purposes 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 operationallyNo!Yes!Possibly6Analysis 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)8Current 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)9Review of Vertical Shear Concepts10What isVertical Shear?• Is a measure of the change in wind direction and speed with height• Is estimated visually best from a hodographThe length of thehodograph is proportionalto the magnitude of the shear through the layerArrows joining windobservations at variouslevels show the shear vectorin the intervening layer.In this case, the wind and the wind shear vectors are veeringwith heightThe dots representthe tips of the windobservations at eachlevel.This case shows a clockwise CURVED HODOGRAPH. Shear associated with a veering wind with heightis called POSITIVE SHEAR. Positive Shear valuesare greatest in curved hodographs (in which thewind shear vectors also veer with height).11Straight HodogaphWind Veers and IncreasesIn Strength ThroughLowest LayersHowever, Wind Shear Vector Does NOT VeerTo Any Great Degree 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.12Importance 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)13Positive 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