Weather and Climate

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Weather and Climate

• Lecture 13
• Severe Weather and Hurricanes

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Thunderstorms

• A thunderstorm is simply a storm containing
  lightning and thunder.
• Thunderstorms occur all over the world

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Formation - Ingredients

• Moisture (water vapor) in the lowest levels of
  the atmosphere
• Cold, dry air aloft (2-3 miles above the surface)
• Lifting mechanism

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Air Mass (Single-Cell) Thunderstorms

• Formed by uneven heating of the earths surface
• Brief, but well-defined lifespan with three
  stages
• Cumulus Stage
• Mature Stage
• Dissipating Stage

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Three Stages
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Mature Thunderstorms

• The updraft can become so strong that it
  penetrates into a region of stable air, resulting
  in a overshooting top.

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Overshooting Tops
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Which is mature and which is dissipating?
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Lifespan of a Thunderstorm

• A single cell, consisting of an updraft and
  downdraft, lasts 20 minutes.
• But we have all observed thunderstorms that
  appear to last longer than 20 minutes.
• WHY?

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Multicell Storms

• Sometimes when the cold downdraft reaches the
  surface, it may force the warm, moist surface air
  upward
• This rising air can condense and build a new
  thunderstorm

Multicell Cluster
Multicell Line (Squall Line)
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Downdrafts
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Gust Fronts
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Gust Front
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Downbursts
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Straight-Line Winds

• Associated with a cluster of severe thunderstorms
• May exceed 90 knots
• If wind damage extends 250 miles along storm
  path, it is called a derecho.

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Tree Damage

• What does it look like from a downburst vs. a
  derecho?

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What makes a thunderstorm a severe thunderstorm?
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Severe Thunderstorm

• Hail ¾ or greater
• Winds in excess of 50 knots (57.5 mph)
• Tornado

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Severe Thunderstorms
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Supercell Thunderstorms
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Capping Inversion

• An important component of creating a severe
  thunderstorm is the strength of the boundary
  layer cap.
• Too strong? nothing develops
• Too weak? numerous cumulus clouds
• Just right?helps to funnel the air into a strong
  updraft

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Supercell Thunderstorms
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Squall Line
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Squall Line
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Dry Line
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Tornadoes
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Tornadoes
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Tornadoes
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Tornado Alley
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Tornado Wind Speed
As the tornado moves along a path, the circular
tornado winds blowing opposite the path of
movement will have less speed. For example, if
the storm rotational speed is 100 knots, and its
path is 50 knots, it will have a maximum wind of
150 knots on its forward rotation side.
Figure 15.31
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Suction Vortices

• Many violent tornadoes contain small whirls
  inside them.

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Tornadoes vs. Mesocyclone

• Development of mesocyclone
• Stretching gives rotating air a faster spin

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Supercell
View from East
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Tornadoes

• Vertical Wind Shear (pencil example)
• Horizontal roll meets updraft and is tilted
  vertically

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Rotating Clouds as Tornado Signal
The first sign that a supercell may form a
tornado is the sight of rotating clouds at the
base of the storm, which may lower and form a
wall cloud, shown in this picture.
Figure 15.41
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Tornadoes
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Position
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Fujita Scale
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Tornadoes and Corn
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Modeling Thunderstorms
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Modeling Thunderstorms
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Lightning
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Cloud to Ground
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Cloud to Cloud
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Cloud to Air
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Lightning

• The lightning we see is actually the return
  stroke, and comes from the ground.

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Lightning and Thunder
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Dust Devils
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Dust Devils
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Water Spouts
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Landspout Formation
Figure 15.43A
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Mesoscale Convective Complexes (MCC)
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Hurricanes (Tropical Cyclones)

• Sustained wind speeds in excess of 74mph
• Tropical origin
• The clear area in the center is called the eye of
  the hurricane.

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Hurricanes

• Eyewall

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Hurricanes

• The hurricane season runs from June to November.
  (sfc water is warm 80oF)
• Most Atlantic hurricanes form due to a wave
  coming off of Africa.
• Hurricanes are very efficient storm systems

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Hurricane Stages of Development
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Hurricane Movement
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Hurricane Movement
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Hurricane Dammage

• The high winds are not the most damaging part of
  hurricanes
• Most damage is from waves, high seas, and
  flooding
• About ¼ of hurricanes that hit the US coast will
  produce tornadoes

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Hurricane Kenna October 2002
Picture _at_ Luis Rico Rojo
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Hurricane Kenna October 2002
Picture _at_ Luis Rico Rojo
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Hurricane Kenna October 2002
Picture _at_ Luis Rico Rojo
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Hurricane Kenna October 2002
Picture _at_ Luis Rico Rojo
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Hurricane Kenna October 2002
Pictures _at_ Luis Rico Rojo
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Hurricane Kenna October 2002
Picture _at_ Luis Rico Rojo
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Naming Hurricanes

• 1953-1977 female names were used
• 1978 to current alternates male/female names
• A name can be retired if a hurricane is category
  3 or higher, with lots of damage

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Naming Hurricanes
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Flash Floods
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Floods and Droughts

• Large widespread floods, like the Mississippi
  River, occur when the weather pattern becomes
  locked in.
• Similarly droughts occur when a weather pattern
  becomes locked in. (i.e. domination by high
  pressure)