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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

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lecture 19

Lecture 19

Chapter 11 Thunderstorms and Tornadoes

thunder storms
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
Thunderstorm Requirements
  • Warm moist air
  • Lifting – mountains or frontal cyclones
  • Thunderstorms often follow midlatitude storm tracks
growth and development
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
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
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
Ordinary Single Cell
  • Short-lived, last for ~1 hour, localized
  • Stages
    • Cumulus
    • Mature
    • Dissapating
cumulus stage
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
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
Dissipating stage
  • Updraft Collapses
  • Downdraft dominates, creates drag, snuffs updraft
  • Moisture source lost, convection slows
  • Dry environmental air entrains
  • Cloud dissipates
multi cell systems
Multi Cell Systems
  • Number of seperate individual cells at differing stages
  • Last several hours
  • 2 basic types
    • Squall line
    • Mesoscale convective complex (MCC)
squall line
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
Conditions for Squall line
  • Divergence aloft
  • Most low level inflow
  • Squall lines often appear ahead of cold fronts in plains and midwest
mesoscale convective complex
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
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
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
structure of supercell
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
tornadoes
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
just the facts
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
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
a twister is born
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
life cycle
Life Cycle
  • Organizing
  • Mature
  • Shrinking
  • Rope
tornado winds
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 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 season
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
Severe Weather
  • Lightning
  • Hail
  • Floods
  • Severe winds
lightning
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
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
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
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
flash floods
Flash Floods
  • Input of water faster than removal, absorption or storage
  • Local
  • High volume
  • Short duration
  • Breaking dam
controls
Controls
  • Rainfall intensity
  • Topography
  • Soil conditions
  • Ground cover
  • Steep terrain funnels flow
  • Extremes in soil moisture
slide52
Hail
  • Lumps of layered ice
  • Formed through accretion, require super cooled drops
  • Strong tilted updrafts
  • Vertical Cycling
  • Hail embryos ~1mm
  • Hail shaft
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