Severe Weather

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Severe Weather
ATS 351 Lecture 10 November 9, 2009
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Types of Severe Weather

• Thunderstorms
• Hail
• Lightning
• Flood
• Tornado
• Severe Wind (Straight-Line Winds)?

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Thunderstorm Distribution
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Favorable Conditions

• Instability
• Fuel
• Initial Lift
• Shear
• Capping Inversion

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Instability

• Steep lapse rate
• Means warm, moist air near the surface
• Colder air above it
• Needs to be calculated from a sounding

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Fuel

• Just like any other weather phenomenon, a storm
  needs fuel to sustain itself
• The fuel for a storm is just a continued supply
  of what started it
• - Heat
• - Moisture
• - Lift
• The storm needs to remain in areas of warm, moist
  air. If storm moves into a colder region, it will
  die

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Sources of Lift

• Convective lifting
• Boundaries
• Fronts
• Drylines
• Outflow boundaries
• Orographic
• Convergence

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Shear

• Because of the way a thunderstorm works, it needs
  to be tilted to remain strong
• Therefore, winds need to change with height
• Two kinds of shear
• Speed Shear Wind is faster as you go up
• Directional Shear Wind changes direction with
  height

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Capping Inversion

• If the atmosphere is unstable all the way up, you
  get a constant updraft
• It is more effective when the energy is held back
  and released all at once
• This can happen by having a stable layer near the
  surface that suppresses convection
• As ground heats during the day, energy builds up
  until it can break the cap
• Also referred to as a capping inversion
• CIN

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Back to the Skew-T

• Meteorologists have formulated various numbers
  that can tell how favorable the weather is for a
  storm. These quantities can describe things such
  as
• Instability
• Shear
• Or a combination of both
• CAPE (Convective Available Potential Energy
• How unstable atmosphere is
• LI (Lifted Index normally at 500mb levle)?
• LI Tenvironment Tparcel

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Thunderstorm Development

• Stages
• Cumulus
• Mature
• Dissipation

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Cumulus Stage

• Warm moist air rises, condenses
• Latent heat release keeps air in cloud warmer
  than environment
• Grows to a towering Cu
• Cloud particles grow larger, begin to fall
• No precipitation at surface

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Mature Stage

• Marked by appearance of downdraft
• Falling cloud drops evaporate, cooling the air
• Storm is most intense during this stage
• Cloud begins to form anvil
• May have an overshooting top
• Lightning and thunder may be present
• Gust front forms
• Downdraft reaches the surface and spreads out in
  all directions
• Gust front forces more warm, humid air into the
  storm

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Dissipation Stage

• Usually follows mature stage by 15-30 min
• Gust front moves out away from the storm, and
  moist air is no longer lifted into the storm.
• Downdrafts become dominant
• Low level cloud drops can evaporate rapidly,
  leaving only the anvil as evidence of the storms
  existence

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Types of Thunderstorms

• Thunderstorms come in many varieties
• Likelihood of severity proportional to storm
  lifetime
• NWS definition of severe (one or more of the
  following elements)
• ¾ or larger diameter hail
• 50 kt (58 mph) or greater winds
• tornadoes

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Single Cell Thunderstorms

• Also referred to as ordinary,
• pulse, or air mass thunderstorms
• Typically do not produce severe weather
• Three stages
• Cumulus
• Mature
• Dissipating
• Life span 45-60 min.

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Multi-Cell Storms

• Cluster of storms moving as a single unit
• Stronger wind shear than the ordinary
• cell case
• More organized multi-cells
• Bow Echoes
• Squall Lines
• New cells tend to form on the upwind
• (W or SW) edge of the cluster, with mature cells
  located at center and dissipating cells found
  along the downwind (E or NE) portion of the
  cluster
• Multiple cells compete for warm, moist low-level
  air so not incredibly strong and have short life
  spans

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Multi-cell Storms

• Cell 1 dissipates while cell 2 matures and
  becomes dominant
• Cell 2 drops heaviest precipitation as cell 3
  strengthens
• Severe multicell storms typically produce a brief
  period of hail and/or downbursts during and
  immediately after the strongest updraft stage

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Updrafts and Downdrafts

• Degree of instability and moisture determine the
  strengths of updrafts and downdrafts

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Vertical Wind Shear

• Change of wind speed and/or direction with height
• Weak vertical wind shear short-lived since rainy
  downdraft quickly undercuts and chokes off the
  updraft
• Sheared environments are associated with
  organized convection

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Vertical Wind Shear
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Gust fronts

• An area of high pressure created at the surface
  by cold heavy pool of air from downdraft called a
  mesohigh
• Gust front leading edge of cold air from
  downdraft
• Passage noted by calm winds followed by gusty
  winds and a temperature drop then precipitation
• Convergence region between cold outflow and warm,
  moist inflow
• Can generate new cells
• Leads to multi-cell storms
• Production of shelf and roll clouds

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Downbursts
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Overshooting tops
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Mesoscale Convective Systems (MCSs)?

• Individual storms can grow and organize into a
  large convective system (weak upper level winds)?
• Definition 100km contiguous group of t-storms
• Range of lifetimes
• New storms grow as older ones dissipate
  (reinvigorates itself)?
• Provide widespread precipitation
• Can spawn severe weather
• Hail, high winds, flash floods, tornadoes
• Formation (in U.S.)?
• Usually during summer when a cold front stalls
  beneath an upper level ridge of high pressure
• Surface heating and moisture can generate
  thunderstorms on the cool side of the front

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Squall Lines

• Multicell storms can form as a line of storms
  extending for hundreds of km, called a squall
  line
• Squall lines often form along or just ahead of a
  cold frontal boundary (called pre-frontal squall
  lines)?
• Supercells may be embedded within prefrontal
  squall lines
• Leading line of thunderstorms may be followed by
  large region of stratiform precipitation where
  the anvil cloud trails behind the main storm.

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Bow Echoes

• Bow Echo a bowed convective line (25 150 km
  long) with a cyclonic circulation at the northern
  end and an anticyclonic circulation at the
  southern end
• Strong jet in from behind
• Can produce long swaths of damaging winds
• Form in conditions of large instability and
  strong low level shear
• Observed both as isolated convective systems or
  as substructures within much larger convective
  systems (such as a squall line)?
• May contain strong winds or tornadoes

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Supercells

• Characterized by rotating updrafts (called a
  mesocyclone)?
• Differ from multicell cluster because of rotation
  and that updraft elements merge into a main
  rotated updraft rather than developing separate
  and competing cells
• Can persist for 12 hours and travel hundreds of
  miles
• Forms in environments of strong winds aloft
• Winds veer with height from the surface
• Can be classified as either High Precipitation
  (HP) or Low Precipitation (LP)?

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Hail

• Storms contain updraft and downdraft
• Not same strength everywhere
• Hail that swept upwards in a region of lesser
  updraft
• Begins to fall, can fall into stronger updraft
• Cycling may occur
• Important contributors to creating charged
  regions in clouds

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Lightning

• Inside a cloud, updrafts and turbulence toss ice
  particles around
• Each collision creates a small amount of electric
  charge
• After a few million of those, the charge is too
  much to be held back by the air
• Discharges all at once in a flash of lightning

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Lightning

• The temperature of lightning is roughly 30,000
  degrees C
• The surface of the sun is only about 5700 degrees
  C
• One bolt of lightning carries enough electricity
  to power the entire United States for 0.1 seconds
• Lightning has been known to strike up to 15 miles
  from the actual storm

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Lightning Misconceptions

• Lightning comes down from the clouds
• It actually comes down AND goes up.
• As a bolt begins the trip down, a streamer from
  the ground shoots upward toward the oppositely
  charged cloud.
• The flash happens when they meet in the middle.
• Entire process happens in under 0.001 seconds
• Lightning always hits the tallest object
• Not true. It may seem that way, but lightning
  simply takes the path of least resistance.
• If you conduct electricity better than the 30 ft.
  tall tree next to you, you will get hit
• Lightning never hits the same place twice
• Thats just wrong.
• There are many documented cases of lightning
  hitting twice in the same spot
• Sometimes only a few seconds apart!

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Lightning Fatalities
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Thunder

• If air is heated from 75 to 90 degrees, it will
  expand
• If air is heated from 75 to 50,000 degrees, it
  will expand quickly
• Thunder is a compression wave due to this rapid
  heating
• The thunder you hear is not lightning hitting
  the ground but actually a sonic boom

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Tornadoes

• Formation
• Life Cycle
• Definition
• Types
• Damage
• EF-scale

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Wall clouds

• Lowering of cloud base
• Visible manifestation of the mesocyclone at low
  levels (contains significant rotation)?
• Develop when rain-cooled air is pulled upward,
  along with more buoyant air
• Rain-cooled air usually very humid so upon being
  lifted, will quickly saturate to form the lowered
  cloud base
• Tornado often forms from within
• wall cloud

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Formation

• Tornadogenesis is the formation of tornadoes
• We know relatively little about this process
• Basic formation steps are known
• Details are missing, but they are very crucial
  details

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• Vertical wind shear crucial

Rotation tilting

• After horizontal rotation is established, the
  storms updraft works to tilt it upright
• Now the storm has a vertically rotating component

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Mesocyclone

• The new rotating storm is called a mesocyclone
• Characterized by rotating updraft
• At this point, the rotation can be picked up on
  Doppler radar if it is strong enough

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Supercell Tornado Formation
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Funnel Cloud

• Area of rotation that does not touch the ground
• Often mistaken for a tornado

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Ground Contact - Tornado

• Once the rotation reaches the ground, the
  downward moving air will spread out
• Some will go back toward the center of the
  funnel, converging and forcing it back up
• The upward motion will begin to kick up debris

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Suction Vortices

• Many violent tornadoes contain smaller whirls
  that rotate inside them
• Rotate faster, and do a great deal of damage
• How these form is still not completely understood

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Damage

• The highest (strongest) winds on Earth are found
  inside tornadoes
• The strongest tornado ever recorded had winds
  over double that of the strongest hurricane
• Damage can be devastating

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Fujita Scale

• In 1973, Ted Fujita of the Univ. of Chicago
  devised a scale for rating the intensity of a
  tornado
• Subjective damage scale that classified a tornado
  on a scale from F0 to F5
• Assessed by going to damage sites and using a
  checklist

Enhanced Fujita Scale

• Proposed in early 2005, adopted in 2007
• Replaces Fujita Scale
• Uses more criteria to assess damage
• Has 28 damage indicators that surveyors look at

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http//www.spc.noaa.gov/efscale/ef-scale.html
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EF2 - Considerable damage
EF0 - Light damage
EF1 - Moderate damage
EF4 - Devastating damage
EF5 - Incredible damage
EF3 - Severe damage