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 Km2 (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 v2

• 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<-15oC

• 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 ~1mm

• Hail shaft

Hail

Wear a helmet

Is this guy for real?

Bombs away

Blasted Hail!