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Convective Storms and ByProducts:
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
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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
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 Km2 (Iowa)
• High pressure in upper levels
• Do not require high wind shear
• Long lived
– Afternoon maturation
– Die in early morning (dawn)
MMC requirements
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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 v2
• 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<-5oC
• Graupel-negative
• Ice Crystals-positive
– Updrafts move and separate
charges
• Ice up
• Graupel down
– Cloud induces surface charge
Ground Charge
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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
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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!