Lecture 19 Chapter 11 Thunderstorms and Tornadoes

Thunder Storms Cluster of clouds producing heavy rain, lightning, thunder, hail or tornados enormous energy Moist air, strong convection Vary in length, precipitation and windiness

Thunderstorm Requirements Warm moist air Lifting – mountains or frontal cyclones Thunderstorms often follow midlatitude storm tracks

Satellite View

Satellite View II

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 A measure of convective potential –Compares T parcel to T environment –When T p >T e, convection is possible T e -T p –-3 to -6 marginal instability –-6 to -9 moderate instability –< -9 very unstable air

Types of Thunderstorms Composed of cells –Ordinary- short lived and small –Super- large, last for hours Single Cell Multi Cell –Squall line –Mesoscale convective complex

Ordinary Single Cell Short-lived, last for ~1 hour, localized Stages –Cumulus –Mature –Dissapating

Cumulus stage Moist surface air rises and cools at dry adiabatic lapse rate 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 phase and precipitation can form

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

Dissipating stage Updraft Collapses Downdraft dominates, creates drag, snuffs updraft Moisture source lost, convection slows Dry environmental air entrains Cloud dissipates

Ordinary Single Cell

Multi Cell Systems Number of seperate individual cells at differing stages Last several hours 2 basic types –Squall line –Mesoscale convective complex (MCC)

Note how the downdrafts assist the updrafts –provide lifting

Shelf cloud above gust front

Squall line Line of storms often following or ahead of a front Boundaries of unstable air 6 to 12 hours long Long (span several states) Wind shear separates updraft, downdraft Shelf cloud above gust front

Conditions for Squall line Divergence aloft Most low level inflow Squall lines often appear ahead of cold fronts in plains and midwest

Squall Line

Squall line

Mesoscale Convective Complex Complex arrangement of individual storms 100 K Km 2 (Iowa) High pressure in upper levels Do not require high wind shear Long lived –Mature in late afternoon –Die in early morning (dawn)

MMC requirements Low level moisture source Low level jet that rises over downdrafts Jet weakens at sunrise, MMC breaks up Important source of water for US Great Plains

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

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

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 until it touches ground

Funnel Cloud

Tornado

Just the facts ~1.6 km wide Short lived <30 minutes Hard to understand due to violent nature Related to rotating super cell thunderstorms Movement with storm track, NE in US

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

Not a nice day for fishing

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!

Life Cycle Organizing Mature Shrinking Rope

Tornado Winds 300 mph (480km/hr) Force of wind proportional to v 2 4 times more powerful than category 5 Hurricane Ted Fujita –1970 –Category F1 to F5 –1% category 4,5

Source and Distribution strongest winds in direction of background flow Strong tornadoes show multiple vortices Geographical distribution –Possible in any state –Areas of instability, wind shear, frontal movement

Tornado Alley

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 Lightning Hail Floods Severe winds

Lightning Electrical discharge Rising and sinking air motions 85 deaths, 300 injured per year 1 in 600,000 Can travel –Cloud to cloud –Cloud to ground –Inside individual clouds

Charge Separation Charges distributed throughout cloud –Ice particle- graupel collisions –When T<-15 o C Graupel-negative Ice Crystals-positive –Updrafts move and separate charges Ice up Graupel down –Cloud induces surface 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 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 Return stroke Current flow upward Dart leaders –Negative electrons, cloud to ground Series of flashes

Lightning Stroke

Flash Floods Input of water faster than removal, absorption or storage Local High volume Short duration Breaking dam

Controls Rainfall intensity Topography Soil conditions Ground cover Steep terrain funnels flow Extremes in soil moisture

Kodak moment

Water Spouts

Hail Lumps of layered ice Formed through accretion, require super cooled drops Strong tilted updrafts Vertical Cycling Hail embryos ~1mm Hail shaft

Hail

Wear a helmet

Is this guy for real?

Bombs away

Blasted Hail!