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Weather and Climate Lecture 13: Severe Weather and Hurricanes Thunderstorms A thunderstorm is simply a storm containing lightning and thunder. Thunderstorms occur all over the world Formation - Ingredients Moisture (water vapor) in the lowest levels of the atmosphere

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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 earth’s surface

  • Brief, but well-defined lifespan with three stages

    • Cumulus Stage

    • Mature Stage

    • Dissipating Stage



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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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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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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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Severe Thunderstorm

  • Hail ¾” or greater

  • Winds in excess of 50 knots (57.5 mph)

  • Tornado




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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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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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Lightning

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






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Landspout Formation

Figure 15.43A



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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 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 @ Luis Rico Rojo


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Hurricane Kenna October 2002

Picture @ Luis Rico Rojo


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Hurricane Kenna October 2002

Picture @ Luis Rico Rojo


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Hurricane Kenna October 2002

Picture @ Luis Rico Rojo


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Hurricane Kenna October 2002

Pictures @ Luis Rico Rojo


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Hurricane Kenna October 2002

Picture @ 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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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)