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Thunderstorms and Tornadoes Lecture 10 November 11, 2009 Review from last week At upper levels… Ridge Trough Ridge Review Vorticity < 0 Vorticity < 0 Vorticity > 0 Vorticity is simply a measure of how much the air rotates on a horizontal surface Positive vorticity is a counterclockwise (i.e. cyclonic) rotation Negative vorticity is a clockwise (i.e. anticyclonic) rotation Negative vorticity advection Positive vorticity advection Review *Positive vorticity advection (PVA) results in divergence at the level of advection… So downstream of an upper tropospheric trough, there is divergence, resulting in ascent (upward motion). * Negative vorticity advection (NVA) results in convergence at the level of advection…So downstream of an upper tropospheric ridge, there is convergence, resulting in subsidence (downward motion). Upper Tropospheric Flow and Convergence/Divergence • Downstream of an upper tropospheric ridge axis is a favored location for a surface high pressure, and of course, downstream of an upper tropospheric trough axis is a favored location for a surface low pressure center. Review How do we know if the surface cyclone will intensify or weaken? • If upper tropospheric divergence > surface convergence, the cyclone will intensify (the low pressure will become lower) • If surface convergence > upper tropospheric divergence, the cyclone will weaken, or “fill.” • Think of an intensifying cyclone as exporting mass, and a weakening cyclone as importing mass. Definition of a Severe Thunderstorm • The National Weather Service defines a severe thunderstorm as a thunderstorm that produces – Hail of 1 inch diameter (in central US) or larger And/or – Wind gusts 58 mph or great – A tornado • Note: this does not include lightning or heavy rain! When do thunderstorms occur? • During the spring and summer, certain atmospheric conditions can be set up which drive severe convective storms: • Conditionally unstable atmosphere • Moisture • Upward vertical motion (“Lifting”) • Wind shear Conditional Instability • Recall from a few weeks ago…the atmosphere is conditionally unstable if the environmental lapse rate is less than the dry adiabatic lapse rate and greater than the moist adiabatic lapse rate • A “dry” (unsaturated) air parcel will cool at the DALR when lifted. It will always be colder than its environment, and sink back to its original level – STABLE • A “moist” (saturated) air parcel will cool at the MALR when lifted. It will always be warmer than its environment, and continue to rise - UNSTABLE Moisture • Since a conditionally unstable atmosphere is only unstable with respect to an saturated air parcel, some moisture source is required to create severe weather Cyclones can advect warm, moist air from the Gulf of Mexico into the central plains Other sources of moisture include the Pacific and Atlantic oceans http://www.srh.noaa.gov/jetstream//tstorms/images/moisture.jpg Warm, moist air is being advected from the Gulf into the Plains Vertical Lifting • An unstable atmosphere will only generate severe weather when it is given a “push” • Unstable air parcels can be lifted by the following mechanisms: – Convection – Convergence – Frontal forcing – Topography Wind Shear • Wind shear is a term which describes how the speed and direction of the wind change with height • It is critically important for the formation of severe weather, because wind shear will keep upward motion and downward motion in the storm separated, allowing the storm to survive Directional Wind Shear • Wind changes direction with height • Wind is backing if the wind direction rotates counterclockwise with height • Wind is veering if the wind direction rotates clockwise with height • Generally, severe weather will only be found if the wind is veering with height Directional Shear Speed shear often causes severe weather with strong straight-line winds Life Cycle of a Thunderstorm • Building block for any thunderstorm is a thunderstorm cell • Typical thunderstorm lasts approx. 30 minutes • Three stages – Developing stage – Mature Cumulus stage – Dissipating Stage Developing Stage • Warm, humid air rises and develops an updraft • Air parcels saturate and form a towering cumulus cloud • Little or no rainfall • Lasts near 10 minutes • No severe weather yet Mature Stage • Precipitation begins to fall, creating downward motion (downdraft) • When downdraft hits the ground, it spreads out and creates a “gust front” • Storm develops overshooting (“anvil”) top • Lasts an average of 1020 minutes. • Most likely time for severe weather (large hail, gusty winds, tornadoes) • The change in the wind with height forces the downdraft to be separate from the updraft, which allows the storm to live longer and become severe Mature Thunderstorm • http://www.geography.hunter.cuny.edu/~tbw/wc.notes/10.thunderstorms.tornadoes/thunder storm.jpg Roll cloud in Dodge County -not attached to parent thunderstorm cloud -associated with thunderstorm gust front. Sign of possible microburst activity Shelf Cloud - Associated with a thunderstorm gust front - Attached to parent thunderstorm cloud - Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. Dissipating Stage • Downdraft dominates and shuts off the updraft • Gust front moves out ahead of the storm and cuts off inflow of warm, moist air • Severe weather threat diminished • Lightning still a threat • Other storms may develop along outflow boundary Types of Thunderstorms • • • • Ordinary Cell Multi-cell Cluster Multi-cell Line (aka Squall line) Supercell thunderstorm Tornadoes • A tornado is defined as “a violently rotating column of air descending from a thunderstorm and IN CONTACT with the ground.” -NWS • Can sometimes last for more than an hour and travel several miles • Most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more What causes tornadoes? • Tornadoes in the winter and early spring are often associated with strong frontal systems that form in the Central US and move east • During the spring in the Central Plains, thunderstorms develop along a dryline – separates very warm, moist area to the east from hot, dry air to the west. • Tornadoes may form as thunderstorms fire along the dryline during the afternoon hours How Do Tornadoes Form? • Before thunderstorms develop, a change in wind shear creates an invisible, horizontal spinning effect near the surface. How Do Tornadoes Form? • Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical http://www.nssl.noaa.gov/edu/safety/tornadoguide.html How Do Tornadoes Form? • An area of rotation, 2-6 miles wide, now extends through much of the storm. • Most strong and violent tornadoes form within this area of strong rotation http://www.nssl.noaa.gov/edu/safety/tornadoguide.html Tornado Facts • Some tornadoes may form during the early stages of rapidly developing thunderstorms. – Most common along range of Rocky Mountains, the Plains, and the western US • Tornadoes may appear nearly transparent until dust and debris are picked up • Two or more tornadoes may occur at the same time from the same storm. Tornado Facts • The average tornado moves SW to NE but tornadoes have been known to move in every direction. • The average forward speed of a tornado is 30 mph • Tornadoes can accompany tropical storms and hurricanes as they move over land • Tornadoes are most likely to occur between 3 pm and 9 pm Tornado on Radar Wedge Tornado http://greek.wunderground.com/data/wximagenew/m/MikeTheiss/453.jpg Rope Tornado http://www.outdoor.com/wp-content/uploads/2008/01/ropetornado.jpg Tornadoes in Wisconsin Stoughton, 8/2005 Tornado - Storm Chaser Scott A. Kampas Cool Tornado Video • http://www.weather.com/multimedia/videop layer.html?clip=14517&from=tv_program_ vortex