RotatingRotating Storms:Storms: SupercellsSupercells,, Tornadoes,Tornadoes, andand TropicalTropical StormsStorms AT 351 Lab 12 April 14, 2008 SupercellsSupercells ! Characterized by rotating updrafts (called a mesocyclone) ! Differ from multicell cluster because smaller updrafts merge into a main rotated updraft rather than developing separate and competing cells ! Can persist for 12 hours and travel hundreds of miles ! Forms in environments of strong winds aloft ! Winds veer with height from the surface ! Can be classified as either High Precipitation (HP) or Low Precipitation (LP) SupercellSupercell HPHP SupercellsSupercells •Dark rain and hail core •Prolific producers of lightning and flash floods LPLP SupercellsSupercells ! Typically associated with a dry line ! Typically small and lacking in rainfall TornadoesTornadoes ! Formation ! Life Cycle ! Definition ! Types ! Damage ! EF-scale FormationFormation ! We know relatively little about the formation of tornadoes, known as tornadogenesis ! However, we do know the basic steps along the way ! It is the details we are missing, but they are very crucial details VorticityVorticity ! Vorticity is the name we give to a natural spin in the air ! Caused mainly by wind shear ! Just like a pinwheel Non-SupercellNon-Supercell TornadoTornado FormationFormation ! Vertical wind shear crucial VorticityVorticity TiltingTilting ! After horizontal rotation is established, the storm’s updraft works to tilt it upright ! Now the storm has a vertically rotating component SuctionSuction VorticesVortices ! Many violent tornadoes contain smaller whirls that rotate inside them ! Rotate faster, and do a great deal of damage ! How these form is still not completely understood SupercellSupercell TornadoTornado FormationFormation ! Remember: supercell is a long lived storm ! Contains a rotating updraft ! Rotating updraft can contribute to the tornado, as well as vertical wind shear ! How does this rotating updraft form ! Examine the winds at different heights SupercellSupercell TornadoTornado FormationFormation ! Can also get vertical wind shear at surface, that adds or forms this rotation SupercellSupercell TornadoTornado FormationFormation SupercellSupercell TornadoTornado FormationFormation ! Why rear part of storm ! SW winds aloft usually blow precipitation NE ! Also, mesocyclone can circulate precipitation counter- clockwise StrongerStronger UpdraftUpdraft ! As the storm matures, its updraft will strengthen ! The updraft creates a low pressure center under the storm ! The new pressure center begins to draw in air from around the storm ! This inflow includes cold air from the downdraft that cannot get away WallWall CloudCloud ! Since some of the air being drawn in is colder than the rest of the storm, it will reach saturation and condense sooner on its way up ! Since it condenses lower, it makes the cloud appear to be descending ! This is called a wall cloud ! The sighting of a wall cloud will prompt a tornado warning FunnelFunnel CloudCloud ! The storm’s downdraft does two things ! Pushes the rotating column to the ground ! In stretching it, makes the rotation stronger ! As the rotating air descends, it forms the familiar funnel shape we all know and love GroundGround ContactContact ! Once the rotation reaches the ground, the downward moving air will spread out ! Some will go back toward the center of the funnel, converging and forcing it back up ! The upward motion will begin to kick up debris ! At this point, the rotation has made a complete column to the ground and is officially called a tornado DissipationDissipation ! At this point, the downdraft that brought the rotation to the ground to begin with is wrapped around the tornado ! Rear Flank Downdraft (RFD for short) ! Cuts off warm inflow, thus killing the very tornado it created ! Causes funnel to thin out into a ropelike shape Ingredients to Make a Stronger Tornado ! The same things that make thunderstorms will cause tornadoes to be stronger ! More wind shear creates longer lasting rotation ! High CAPE gives the storm plenty of energy ! Warm, moist air gives the tornado plenty of fuel to last longer and grow stronger DamageDamage ! The highest winds on Earth are found inside tornadoes ! The strongest tornado ever recorded had winds over double that of the strongest hurricane ! Damage can be beyond devastation FujitaFujita ScaleScale ! In 1973, Ted Fujita of the Univ. of Chicago devised a scale for rating the intensity of a tornado ! Subjective damage scale that classified a tornado on a scale from F0 to F5 ! F5 is the highest – don’t listen to anyone that says anything different ! Assessed by going to damage sites and using a checklist EnhancedEnhanced FujitaFujita ScaleScale ! Proposed in early 2005, adopted in 2007 ! Replaces Fujita Scale ! Uses more criteria to assess damage ! Has 28 “damage indicators” that surveyors look at OPERATIONAL EF FUJITA SCALE DERIVED EF SCALE SCALE F Fastest 1/4- 3 Second Gust EF 3 Second EF 3 Second Number mile (mph) (mph) Number Gust (mph) Number Gust (mph) 0 40-72 45-78 0 65-85 0 65-85 1 73-112 79-117 1 86-109 1 86-110 2 113-157 118-161 2 110-137 2 111-135 3 158-207 162-209 3 138-167 3 136-165 4 208-260 210-261 4 168-199 4 166-200 5 261-318 262-317 5 200-234 5 Over 200 http://www.spc.noaa.gov/efscale/ef-scale.html EF0EF0 “Light damage” ! Tree branches broken ! Chimney damage ! Sign damage EF1EF1 ! “Moderate Damage” ! Roof damage ! Windows broken ! Mobile homes damaged or overturned EF2EF2 ! “Considerable damage” ! Good houses lose roof ! Cars lifted off ground ! Mobile homes destroyed EF3EF3 ! “Severe damage” ! Houses lose entire stories ! Trains overturned ! Bark ripped from trees ! Heavy cars thrown EF4EF4 ! “Devastating damage” ! Well built houses leveled ! Cars ---> missiles EF5EF5 ! “Incredible damage” ! Only one happened this year (Greensburg, KS) ! Strong frame houses obliterated and scattered over county ! Cars thrown over 100m AA FewFew FactsFacts ! The EF scale is not a wind scale; the wind speeds are merely a benchmark ! The last F5 happened in 1999, and before that in 1998 (actually 2, eight days apart) ! There were 53 official F5’s from 1950 to 2006 ! And now one EF5 TheThe 0303 MayMay 19991999 TornadoTornado ! Hit Bridge Creek and Moore, OK in the evening (around 19:00 CDT) ! Moved through southern Oklahoma City ! Killed 36 people ! Contained strongest winds ever recorded on Earth ! 318 mph, measured by Joshua Wurman and the Doppler on Wheels DamageDamage inin MooreMoore ! Homes completely removed from foundations, leaving only a slab of concrete ! Asphalt pulled off of Interstate 35, leaving only dirt behind ! Trucks lofted high enough to be picked up on radar, then wrapped all the way around telephone poles 0404 MayMay 20072007 TornadoTornado ! Destroyed Greensburg, KS in the night ! Hit at 21:40, square on ! Demolished 95% of buildings in the town ! 10 deaths ! Entire town evacuated afterward ! Responsible for 4th “Tornado Emergency” ever declared TropicalTropical StormsStorms What is required for TS formation? ! Warm and moist air ! A really good reason why they form over tropical oceans – lots of latent heat! ! Typically tropical storms only form over areas of the ocean with a temperature higher than 26°C ! Rotation ! Comes from coriolis force ! Tropical storms form near, but not on, the equator ! Continued fuel ! A tropical storm can strengthen if it continues to move over warm water, and will weaken once it hits dry land ! NO (or very weak) vertical wind shear ! Unlike storms in the mid-latitudes which need shear to organize, tropical storms will be sheared apart and prevented from organizing StagesStages ofof developmentdevelopment ! Tropical disturbance (tropical wave) ! Formation of group of thunderstorms, typically in association with an easterly wave ! Tropical depression ! When winds exceed 20 knots ! Beginning of formation of closed low pressure center ! Appear on satellite as cluster of thunderstorms ! Will be assigned a number (e.g. “tropical depression one”) StagesStages ofof developmentdevelopment ! Tropical Storm ! Winds exceed 35 knots ! Central pressure drops ! On satellite, more organized, circular shape observed ! Storm gets a name ! Hurricane ! Sustained winds exceed 64 knots ! Surface pressure continues to drop ! On satellite imagery, well- defined cyclonic rotation around the central circulation, with observation of eye StagesStages ofof DevelopmentDevelopment DissipationDissipation ! Weaken rapidly when move over colder water ! Lose heat source ! Dissipate rapidly when move over land ! Lose energy source ! Winds decrease in strength (due to added friction) and blow more directly into center, causing central pressure to rise ! May cause flooding as continues to move over land StructureStructure ofof aa HurricaneHurricane ! Surface: ! Inflow of moist, tropical air, rises, condenses, releases latent heat ! Rising motion creates center of low pressure ! Produces a cyclonic circulation ! Aloft: ! Air diverges, producing an anticyclonic circulation ! As outflow reaches periphery of storm, begins to sink and warm, creating clear skies TheThe eyeeye ! Coriolis deflects converging wind around the center ! Vigorous convection in the eye wall warms the air due to release of latent heat ! Produces slightly higher pressures aloft, initiating downward motion within the center ! Eye characterized by warm air (due to compression), clear skies (sinking motion), and low wind speeds StormStorm SurgeSurge ! Storm surge has historically brought 90% of the death and destruction