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Lecture 6

EESA09H3 Lecture Notes - Lecture 6: Outflow Boundary, Cumulus Cloud, Wind Shear


Department
Environmental Science
Course Code
EESA09H3
Professor
Tanzina Mohsin
Lecture
6

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What is thunderstorm?
- Convective storm
- Caused by surface heating
Ordinary Thunderstorm
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Stages
- Surface heating induces upward flow, cumulus cloud formation
- Mature phase - development of a downdraft with precipitation
- Gust front(Kind of sustaining the storm) develops as downdraft air spreads along
horizontal surface
- Warm air rises, cools and condenses, then forms cumulus clouds.
- When cloud formation is complete, we see precipitation this is known as the Matured
stage
- Last stage is called Dissipating.
-Gust front forces more air up into the updraft.
- Updraft and downdraft form a convective cell.
When the gust front moves past the updraft, the updraft weakens
-Rain starts to fall into the updraft, cutting off rising humid air.
- Second stage is where downdraft is formed, Gust front can sustain a storm for less than
an hour.
- You need both updraft and downdraft for the mature stage.
- At the final stage Downdraft cuts off updraft, and the storm loses its energy.
Final stage
- Downdraft cuts off updraft and storm loses energy source and dissipates
- It is relatively short lived < hour, Diameter, 1km or less
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Scientific term for Rising air is Updraft
Downdraft cuts off updraft easily
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Multicell ------------------------------------------------------------------------------------------------------------------
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Similar to ordinary except moderate wind shear
- Storm tilts
- Downdraft forms downwind of updraft
- Storm lasts longer
- Gust front of one storm initiates or induces another storm.
Differences between Multicell and Ordinary
- Moderate wind shear for multicell and little wind shear for ordinary
- Storm tilts for multicell, it’s vertical for ordinary
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- Downdraft doesn’t cut off immediately
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Super Cell
- Form with strong vertical wind shear
- Surface winds (mT air) from south/southwest
- Upper level winds (cP air) from north/northwest
- Along cold front of a midlatitude cyclone
- Tornadoes can form
- Microbursts can form
- Hail can also form
- Downdraft does not cutoff updraft
- Storm can last for several hours
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Microbursts
- Localized downdrafts
- Radial burst of surface wind
- Aviation hazard
- Airplane crash, August 1985 at Dallas
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MCC
- Mesoscale convective complexes
- Multiple thunderstorms
- Circular fashion
- Covers over 100,000 square kilometers
- 12 hours or more, self sustaining
- Heavy precipitation
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-Squall line
- String of thunderstorms along a cold front
- November 15, 1989 - The Huntsville Tornado is an example
- Hail and Tornadoes are formed
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Lightning
- Special characteristics of Thunderstorms
- Charge separation occurs in the cloud (charge is carried by the cloud droplets or ice
crystals)
- Smaller particles tend to go to top of storm with positive charge, larger ones with
negative charge to the bottom
- Acts like a magnet to attract positive charge at the surface
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