Lecture 19
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Transcript of Lecture 19
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!