EESA09H3 Lecture Notes - Lecture 5: Mesoscale Convective Complex, Outflow Boundary, Wind Shear

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Published on 14 Aug 2012
School
UTSC
Department
Environmental Science
Course
EESA09H3
Lecture 5
- What is a thunderstorm?
- A thunderstorm is a convective storm, which is caused by surface heating, rather than
upper level flow , SURFACE HEATING AND LATENT HEAT RELEASE
- WHERE ARE THEY MOST COMMON? In Southern Ontario in the summer months
- It’s the only storm that has…. THUNDER AND LIGHTNING
- What can large thunderstorms do ? They can spawn Tornadoes
What are the four types of thunderstorms? Ordinary, Multicell, Supercell , Mesoscale
Convective Complex (MCC)
Where to ordinary thunderstorms tend to develop? IN LARGE AIR MASSES
, not necessarily near a frontal system with little vertical wind shear. Vertical wind is the change
in the speed and direction of the horizontal wind.
THE AIR MASS MUST BE: VERTICALLY UNSTABLE . A necessary condition is that the air mass is
vertically unstable. Vertical instability arises when less underlies denser air. Typically this occurs
warm air underlies colder air.
Three Stages
Three stages
Differential surface heating induces upward flow in unstable air, updraft, cumulus cloud
formation
Mature phase development of a downdraft with precipitation
Gust front develops as downdraft air spreads along horizontal surface
• 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 the rising humid air.
Final stage
Downdraft cuts off updraft and storm loses energy source and dissipates
• Relatively short-lived
< 1 hour
Diameter, 1 km or less
Multi cell thunderstorm
Similar to ordinary except moderate wind shear
Storm tilts
Downdraft forms downwind of updraft, so the storm can last longer
Storm lasts longer
Gust front of one storm initiates or induces another storm
Multicell thunderstorms are very similar to ordinary thunderstorms except there is
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a moderate vertical wind shear. This shear causes the storm to tilt and the downdraft is
formed downwind of the updraft; thus the storm can last longer. The gust front of the
downdr aft is more likely to induce another thunderstorm and a string of thunderstorms
occur, often at different stages of development. These storms tend to occur near a frontal
system of a midlatitude cyclone.
Supercell Thunderstorm
• 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, in the warm sector
• 100 – 600 m in diameter
• Tornadoes can spawn
•downdraft forms downst ream of the updraft so storms tend to last longer. Downdraft does
not cutoff updraft
Storm can last for several hours
• Hail can form
• Microbursts can also form
Superce ll thunderstorms occur with strong vertical wind shear. These storms form
in front of a cold front of a midlatitude cyclone in the cyclone‟s warm sector.
The strong wind shear results from warm, tropical air from the south to southwest pushing into
the
region above the warm sector. Above this is colder, drier air moving in from the west.
Above this air is the jet stream, often providing divergence aloft from a variety of
mechanisms. A small layer of stable air exists above the surface air, acting like a cap or
lid on the emerging updraft. Under some conditions the rising air can break through this
cap. Once this is done, the storm grows quickly if not explosively.
The strong wind shear insures that the downdraft that develops is downstream of the
updraft, thus allowing supercell thunderstorms to last longer than ordinary thunderstorms
often for more than several hours. Supercells often produce large hail with diameters of
as much as 10 cm. The strong wind shear also enables the formation of tornadoes.
Microbursts: localized downdrafts, radial burts of surface winds
- Aviation hazard
A string of thunderstorms that occur ahead of a cold front often form in a line called a squall
line.
- Occasionally multiple thunderstorms organize in a circular fashion covering over
100,000 square kilometers. These are called mesoscale convective complexes (MCCs).
These complexes are self sustaining and can last over 12 hours and produce heavy
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Document Summary

A thunderstorm is a convective storm, which is caused by surface heating, rather than upper level flow , surface heating and latent heat release. , not necessarily near a frontal system with little vertical wind shear. Vertical wind is the change in the speed and direction of the horizontal wind. The air mass must be: vertically unstable . A necessary condition is that the air mass is vertically unstable. Vertical instability arises when less underlies denser air. Typically this occurs warm air underlies colder air. Differential surface heating induces upward flow in unstable air, updraft, cumulus cloud formation. Mature phase development of a downdraft with precipitation. Downdraft cuts off updraft and storm loses energy source and dissipates: relatively short-lived. Downdraft forms downwind of updraft, so the storm can last longer. Gust front of one storm initiates or induces another storm. Multicell thunderstorms are very similar to ordinary thunderstorms except there is a moderate vertical wind shear.

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