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Shear and Buoyancy Parameters for California Tornadoes 1951—2011

Shear and Buoyancy Parameters for California Tornadoes 1951—2011. Chris Stumpf Masters Proposal. Thesis Committee: Dr. John Monteverdi (Advisor) Dr. Dave Dempsey Dr. Oswaldo Garcia. Outline of Proposal. Introduction Background and Previous Work Research Objectives Project Plan Timeline.

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Shear and Buoyancy Parameters for California Tornadoes 1951—2011

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  1. Shear and Buoyancy Parameters for California Tornadoes 1951—2011 Chris Stumpf Masters Proposal Thesis Committee: Dr. John Monteverdi (Advisor) Dr. Dave Dempsey Dr. Oswaldo Garcia

  2. Outline of Proposal • Introduction • Background and Previous Work • Research Objectives • Project Plan • Timeline

  3. California TornadoClimatology • Peak in March • Mainly occur during the peak heating hours Blier and Batten 1994

  4. Blier and Batten noted four areas of high tornado frequency North-Central Coast Central Valley LA Basin Desert Region

  5. The Fujita Scale • Developed in 1971 by Ted Fujita • Way to estimate wind speeds inside a tornado without direct observations • Done by investigating the damage to structures and objects in the tornado path

  6. Outline of Proposal • Introduction • Background and Previous Work • Research Objectives • Project Plan • Timeline

  7. Buoyancy • Convective Available Potential Energy (CAPE) is required for thunderstorm development • CAPE values in California are lower than those typically found in the Great Plains • Lack of high CAPE values is a result of cooler SST’s along the California coast • Resulting in low dewpoint temperatures

  8. Buoyancy • However, the magnitude does not correspond to the strength of tornadoes • Something must account for this overall lack of buoyancy Maximum, 75th, and 25th percentile and minimum values of SBCAPE observed for the null, F0, and F1/F2 bins. Monteverdi et al. 2003

  9. Wind Shear • Vertical change in wind speed and direction • Vector difference between different levels = Wind Shear

  10. Wind Shear Profile (Hodograph)

  11. Strong wind shear causes pressure perturbations and begins to strengthen the vertical pressure gradient force (PGF) • Stronger PGF can make up for a lack of buoyancy and enhance updraft speeds • Specific profiles of wind shear encourage the formation of rotating updrafts called mesocyclones

  12. Supercell Thunderstorm

  13. Previous Work • Braun and Monteverdi 1991 – Sept. 1986 tornado environment • Weak CAPE but strong wind shear values similar to those found elsewhere in the country • Blier and Batten 1994 – California tornado climatology 1950-1992 • Monteverdi and Quadros 1994 – Buoyancy and shear parameters associated with three tornado episodes in Dec. 1992 • Lipari and Monteverdi 2000 - Shear profiles of California tornadoes from 1990-1994 consistent with supercell thunderstorms found in the Great Plains • Monteverdi et al. 2003 – Strong low level shear (0-1 km) is a good indicator of tornadic potential

  14. Outline of Proposal • Introduction • Background and Previous Work • Research Objectives • Project Plan • Timeline

  15. Objectives • Build upon previous research of shear and buoyancy parameters of California tornadoes to include 1951-2011 • Perform case studies on the 1951 Sunnyvale F2 tornado and the 1983 LA Convention Center F2 tornado • Time Permitting: use the Rapid Update Cycle (RUC) gridded forecast model archives to generate atmospheric profiles near each tornado event

  16. Lipari’s Study and Proposed Study

  17. Case Studies • Before the 1980’s the F-scale rating of tornadoes was under reported • Stubborn California meteorologists • Inaccurate understanding of ingredients associated with California tornadoes • Damage is not consistent with rating given • Language used in storm survey reports shows a bias towards under reporting

  18. Sunnyvale 1951 F2 Tornado

  19. LA Convention Center F2 Tornado Taken from The Los Angeles, California Tornado of March 1, 1983

  20. Outline of Proposal • Introduction • Background and Previous Work • Research Objectives • Project Plan • Timeline

  21. Weather Balloons • Balloons or (soundings) are launched at 1200 and 0000 GMT everyday • Record vertical profile of T, Td, Pres., WS, and WD • The National Climatic Data Center maintains an archive of these launches • Use ArcGIS to link a tornado event with a particular balloon launch

  22. SurfaceObservations • Hourly surface observations of T, Td, WS, and WD • Use ArcGIS to match a tornado event to the closest surface observation station

  23. Sounding Surface Modification • To obtain buoyancy and shear parameters for all 371 cases • I will use software to modify the surface of each sounding to reflect the near storm environment • Provide a wind shear profile and buoyancy values for each event • This method makes assumptions that the environment near the storm is “homogeneous” • When convection begins the environment is far from homogenous

  24. Timeline • Early June: Pilot study of 1980’s shear and buoyancy parameters completed • Summer 2011: Sounding surface data modification for all events • Fall 2011: Analyze shear and buoyancy parameters • December 2011: First draft of thesis submitted to advisor • Spring 2012: Submit thesis to advisor and work on submitting a paper to the AMS

  25. Questions?

  26. References • Hales, J. E., Jr., 1985 Synoptic features associated with Los Angeles tornado occurrences. Bull. Amer. Meteor. Soc., 66, 657-662. • Braun, S.A. andJ.P. Monteverdi, 1991: An investigation of a mesocyclone-induced tornado occurrence in northern California. Wea. Forecasting, 6, 13-31. • Monteverdi, J. P., C. A. Doswell, and G. S. Lipari, 2003: Shear Parameter Thresholds for Forecasting Tornadic Thunderstorms in Northern and Central California. Wea. Forecasting, 18, 357-370

  27. Atmospheric Profiling • Radiosondes (weather balloons) are launched everyday at 12 and 00 UTC • Records a vertical profile of: • Temperature • Dewpoint • Wind direction and speed • Pressure

  28. What is Shear? Speed Shear Directional Shear Strong vertical shear is the combination of strong speed shear and veering directional shear

  29. California tornadoes are part of the climatological history Sacramento Tornadoes February 2005 From sacbee.com, Photo by Matthew Adams From sacbee.com, Photo by Stuart Hooper

  30. Tornado near Sacramento, CA 2005

  31. Riverside, CA May 2008 From LA Times, Photo by Michael Ritter

  32. Future Work • Use Rapid Update Cycle (RUC) forecast model sounding archives to compare soundings generated by the model to my method of sounding surface modification to check for validity and accuracy • Submit paper to Weather and Forecasting

  33. Project Plan • Retrieve radiosonde data for each case to show the environment before the tornado • Radiosonde data provided by John Monteverdi and NCDC

  34. Project Plan • Retrieve radiosonde data for each case to show the environment before the tornado • Talk about differentiating between stations and time of observations • Insert picture of tornadoes and radiosonde sites

  35. Remains of a large truck wrapped around telephone after an F5 tornado

  36. April 4th, 1974 Xenia, Ohio F5 Photo by Fred Stewart, courtesy of NOAA

  37. Say something about bigger picture? • A better understanding of the processes that create tornadoes • Will allow forecasters to make better predictions • Increase warning times • Hopefully save lives

  38. While tornadoes in California are not as frequent and as strong as they are in the Great Plains they can and do occur • The only difference is the high population density in California makes the impacts of a tornado much larger • Since 1951 371 tornadoes have occurred • On average 6 tornadoes occur every year in California • Amazingly no fatalities have been associated with these tornadoes

  39. Morning sounding before tornado event • 12Z sounding from Oakland on 12/11/83

  40. Modified 12Z Sounding using surface observations • 12Z sounding from Oakland on 12/11/83

  41. Synoptic Pattern Charateristics • Strong jet at 300 mb entering California • Upward vertical motions created by quasi-geostrophic forcing • Closed circulation centers from the surface to 500 mb and located generally offshore • Increasing wind speeds and veering of the wind with height • Majority of tornadoes occurred well behind cold front Taken from Braun and Monteverdi 1991

  42. Surface Characteristics of Central Valley Tornadoes • Quasi-stationary trough axis located on lee side of coastal ranges • Creates S to SE flow in valley • Advection of warmer temperatures into valley • Enhances low level wind shear Taken from Braun and Monteverdi 1991

  43. Surface Characteristics of LA Basin Tornadoes • SE flow ahead of surface trough axis • Due to frictional convergence • Convergence associated with local topography • Mountains surrounding the LA Basin • Islands located offshore create eddies in wind flow Surface low pressure and wind barbs Taken from Hales 1985

  44. What is Buoyancy? • Similar to a cork in water an air parcel that is buoyant will shoot to the top of the atmosphere • A measure of instability in the atmosphere known as CAPE • Air parcels in most cases must be forced upwards by a lifting mechanism to become buoyant

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