Published on October 3, 2007
Lecture 18 : Lecture 18 Chapter 11 Thunderstorms and Tornadoes Thunder Storms: Thunder Storms Cluster of clouds producing heavy rain, lightning, thunder, hail or tornados Require large amounts of energy Moist air, strong convection Vary in length, precipitation and windiness Thunderstorm Requirements: Thunderstorm Requirements Warm moist air Lifting – mountains or frontal cyclones Thunderstorms often follow midlatitude storm tracks Satellite View: Satellite View Satellite View II: Satellite View II Growth and Development: Growth and Development Affected by Unstable atmosphere Environmental Temperature Humidity Wind speed and direction (surface to tropopause) Vertical Wind Shear – adds spin Nocturnal Jet – moisture and energy Capping inversion – the lid on a boiling pot Lifting Index: Lifting Index A measure of convective potential Compares Tparcel to Tenvironment When Tp >Te, convection is possible Te-Tp -3 to -6 marginal instability -6 to -9 moderate instability < -9 very unstable air Types of Thunderstorms: Types of Thunderstorms Composed of cells Ordinary- short lived and small Super- large, last for hours Single Cell Multi Cell Ordinary Single Cell: Ordinary Single Cell Short-lived, last for ~1 hour, localized Stages Cumulus Mature Dissapating Cumulus stage: Cumulus stage Moist surface air rises and cools at DALR until Lifting Condensation Level (LCL) is reached Entrainment from dry environmental air Evaporation of droplets, helps cool air Variability in droplet size If cloud is higher than freezing point mixed precipitation can form Mature Stage: Mature Stage Precipitation begins to fall Lightning, hail and rain maximized Updrafts strongly organized Falling precipitation occurs when air is unsaturated, promotes downdrafts of cool dense air Dissapating stage: Dissapating stage Updraft Collapses Downdraft dominates, creates drag, snuffs updraft Moisture source lost, convection slows Dry environmental air entrains Cloud dissapates Ordinary Single Cell: Ordinary Single Cell Multi Cell Systems: Multi Cell Systems Number of seperate individual cells at differing stages Last several hours 2 basic types Squall lines mesoscale Shelf cloud at gust front: Shelf cloud at gust front Squall line: Squall line Line, following frontal pattern Boundaries of unstable air 6 to 12 hours long Span several states Wind shear separates updraft, downdraft Shelf cloud Conditions for Squall line: Conditions for Squall line Divergence aloft Most low level inflow Squall lines appear ahead of cold fronts Squall Line: Squall Line Squall line: Squall line Mesoscale Convective Complex: Mesoscale Convective Complex Complex arrangement of individual storms 100 K Km2 (Iowa) High pressure in upper levels Do not require high wind shear Long lived Afternoon maturation Die in early morning (dawn) MMC requirements: MMC requirements Low level moisture source Low level jet Jet rises over downdrafts Jet weakens during sunrise, MMC breaks up Important source of water for US Great Plains Super Cell: Super Cell Rotating Single Cell system Development depends on instability and wind shear (low level southerly, upper level westerly) Updrafts and downdrafts are separate Produces dangerous weather Rain, hail, lightning, Tornadoes Super Cell Structure: Super Cell Structure Structure of Supercell: Structure of Supercell Updraft goes in at rain free base, moves ahead and downwind Anvil and overshooting tops indicate strong updrafts Upper level winds help maintain movement Downdraft in precipitation core Auntie Em, it’s a twister: Auntie Em, it’s a twister Tornadoes: Tornadoes Rapidly Rotating columns of high wind around a low beneath a thunderstorm Visible Funnel due to condensation, dust and debris in rapidly rising air Funnel cloud is not a tornado Funnel Cloud: Funnel Cloud Tornado: Tornado Just the facts: Just the facts ~1.6 km wide Short lived <30 minutes Hard to understand due to violent nature Related to slowly rotating super cell thunderstorms Movement with storm track, NE in US Rotation: Rotation Begins in interplay between updrafts and downdrafts Air spins around horizontal axis near front Meso cyclone (5 to 20km wide) Updrafts lift column and 2 columns form Vertical axis Left and Right movers Vertical stretching increases spin Spinning air lifted: Spinning air lifted Not a nice day for fishing: Not a nice day for fishing A twister is born: A twister is born Cloud under spinning updraft lowers in a rotating cloud wall Small compared to meso cyclone Funnel Cloud Water vapor makes circulation visible Touchdown - start of tornado Touchdown!! Extra point is no good!: Touchdown!! Extra point is no good! Life Cycle: Life Cycle Organizing Mature Shrinking Rope Tornado Winds: Tornado Winds 300 mph (480km/hr) Force of wind proportional to v2 4 times more powerful than category 5 Hurricane Ted Fujita 1970 Category F1 to F5 1% category 4,5 Source and Distribution: Source and Distribution Source of winds unknown, strongest in direction of background flow Strong tornadoes show multiple vortex Distribution Possible in any state Areas of instability, wind shear, frontal movement Tornado Alley: Tornado Alley Tornado Season: Tornado Season Follows Jet stream (source of wind shear) Minnesota- June Mississippi- Spring and Fall Could happen day or night Attraction to trailer parks? Severe Weather: Severe Weather Lightning Hail Floods Severe winds Lightning: Lightning Electrical discharge Rising and sinking air motions 85 deaths, 300 injured per year 1 in 600,000 vs 1 in 5 billion Can travel Cloud to cloud Cloud to ground Inside individual clouds Charge Separation: Charge Separation Charges distributed throughout cloud Ice particle- graupel collisions When T<-5oC Graupel-negative Ice Crystals-positive Updrafts move and separate charges Ice up Graupel down Cloud induces surface charge Ground Charge: Ground Charge Attraction to cloud High pointy metal structures Large charge separation Air acts to insulate, allows potential buildup 3000 volts/ft 9000 volts/m Lightning Formation: Lightning Formation Large charge buildup and separation Pilot leader Stepped leaders- branches act as conductive channels Spark when channel is completed to ground Electrons flow in series of flashes Lightning crashes: Lightning crashes Return stroke Current flow upward Dart leaders Negative electrons, cloud to ground Series of flashes Lightning Stroke: Lightning Stroke Flash Floods: Flash Floods Input of water faster than removal, absorption or storage Local High volume Short duration Breaking dam Influences: Influences Rainfall intensity Topography Soil conditions Ground cover Steep terrain funnels flow Extremes in soil moisture Kodak moment: Kodak moment Water Spouts: Water Spouts All hail hail: All hail hail Lumps of layered ice Formed through accretion, require super cooled drops Strong tilted updrafts Vertical Cycling Hail embryos ~1mm Hail shaft Hail: Hail Wear a helmet: Wear a helmet Is this guy for real?: Is this guy for real? Bombs away: Bombs away Blasted Hail!: Blasted Hail!