Convective Storms and By-
Products:
Thunderstorms and Tornadoes

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
• 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, possible convection is
• 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

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

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
• 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 lines
– mesoscale

Shelf cloud at gust front

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
• Divergence aloft
• Most low level inflow
• Squall lines appear ahead of cold fronts

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
– Afternoon maturation
– Die in early morning (dawn)

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
• 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

There’s no place like home…

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

Tornado

Just the facts
• Less than one mile 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
• 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!!

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
• 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 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 vs 1 in 5 billion
• Can travel
– Cloud to cloud
– Cloud to ground
– Inside individual clouds

Charge Separation
• Charges distributed throughout cloud
– Ice particle- graupel collisions
– When T<-5 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

Influences
• 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

This could be you!

Bombs away

Blasted Hail!