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 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 precipitation which has resulted in flooding. Unlike supercells, MCCs can form with only
weak vertical wind shear.
- Multiple thunderstorms organize in a circular fashion and cover over 100,000 square
kilometers
- Self-sustaining and can last over 12 hours
- Produces heavy precipitation , causes flooding
- Weak vertical wind shear as opposed to a strong one (required for supercell
thunderstorms)
Lightning
A special characteristic of thunderstorms is the production of thunder and lightning.
Thunderstorms arise from surface heating and latent heat release which destabilizes the
atmosphere. These are called convective storms as convection is the primary driving
force. Thunderstorms are so named because of the thunder and lightning generated which
are not characteristic of other storms.
Although the process is not clear, in a thunderstorm hydrometeors become
charged.
- Smaller ice crystals become positively charges and tend to migrate to the top of
the thunder cloud. Larger, heavier hydrometeors tend to exist in the lower part of the
cloud and are negatively charged. The negative charged cloud base, acting like a magnet,
attracts positive surface charge which focuses on the highest surface object (such as a
building or tree).
WHERE CAN LIGHTNING TAKE PLACE? Lightning can take place either from the cloud base to
the surface or the reverse.
- Most lightning takes place at the base of the cloud (90%)
- Only in relatively infrequent cases when the base of the cloud is positively charged does
lightning take place from the surface to the cloud base. However, the appearance is
usually the opposite due to the two step process of lightning formation.
-In the first step for the first type of lightning, when the electrical potential exceeds
3 million volts, a path of 50 m or so is ionized beginning at the cloud base.
-Electrical discharge rushes from the cloud base to the surface in 50 to 100 m steps. Each step
takes
about 50 millionths of a second. This is called a step leader and is very faint; usually not
visible to the naked eye. As the step leader approaches the surface, surface positive
charge flows upward on the locally highest object. Once the connection is made the
luminous return stroke, several centimeters in diameter, occurs.
The process can be repeated (lightning can strike twice). Subsequent step leaders are called
dart leaders and
usually follow the original path. Sometimes, however, the dart leader may at some point
deviate to a different path; this produces forked lightning.
The light ning stroke heats the air to over
and sends out a shock wave which is heard as thunder. This is called a step leader and is very faint; usually not
visible to the naked eye. As the step leader approaches the surface, surface positive
charge flows upward on the locally highest object. Once the
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