Lecture 19 Chapter 11 Thunderstorms and Tornadoes Thunder

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 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 • 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 20 km 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 (480 km/hr) • Force of wind proportional to v 2 • 4 times more powerful than category 5 Hurricane • Ted Fujita – 1970 – Category F 1 to F 5 – 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 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 ~1 mm • Hail shaft

Hail

Wear a helmet

Is this guy for real?

Bombs away

Blasted Hail!