EESA09H3 Lecture Notes - Sudbury Tornado, Edmonton Tornado, Lur
45 views11 pages
Lecture 5 Notes
Outline of this lecture
Part One Thunderstorm Primer
1.2 Dynamics of Thunderstorms
1.5 World Distribution of Thunderstorms
Part Two Tornado Primer
2.1 Definitions and Dynamics
2.3 Famous Canadian Tornadoes
Part Three Hail Research
Etkin and Brun (1999) Canada‟s Hail Climatology
Part I. Thunderstorm Primer
What is a thunderstorm?
A thunderstorm is a convective storm. This means that the storm is caused primarily
by surface heating (rather the upper level flow). These occurred in the summer season
(locally). This is the main feature in weather variability in Southern Ontario in the
summer months. It is the only type of storm that has thunder and lightning. Large
thunderstorms can spawn tornadoes. Toronto gets between 30 and 40 thunderstorm days
1.2 Dynamics of Thunderstorms
A storm that produces thunder and lightning is called a thunderstorm. There are four
types of thunderstorms: ordinary (single cell) thunderstorms, multicell thunderstorms,
super cell thunderstorms and mesoscale convective complexes.
Ordinary thunderstorms 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. A necessary condition is that the air mass is vertical
unstable. Vertical instability arises when less underlies denser air. Typically this occurs
when warm air underlies colder air. Differentially surface heating is often the trigger for
ordinary thunderstorms although other triggers are possible, such as topography and
surface wind convergence. All of the triggers forces air upward into a strong updraft. If
the rising air remains warmer than its immediate surroundings it continues to rise. Rising
air also cools due to expansion. Water vapor in the air will condense once saturation is
reached. The condensation process releases latent heat, providing an additional energy
source which warms the air and adds to the instability. Often thunderstorms span the
entire troposphere (8 – 13 km in height) and sometimes penetrate into the stable
stratosphere. The condensation of the water vapor creates the cumulonimbus clouds
which are a vast collection of hydrometeors, ice crystals and water droplets. When these
hydrometeors become large enough they begin to descend. This marks the beginning of
the end of an ordinary thunderstorm. As the droplets fall, some are evaporated. This uses
energy and the air becomes colder and begins to sink and a downdraft starts to form.
Since there is little vertical wind shear the downdraft forms in the same area as the
updraft, thus counteracting the updraft. As the downdraft gains strength, the updraft
dissipates. With this dissipation the fuel for the storm, latent heat release, is also cut off.
The downdraft provides the surface with a blast of cool air. This cool air upon
encountering the surface spreads horizontally. This is called the gust front. Gust fronts
are able to cause surrounding warm air to rise and thus are capable of inducing further
thunderstorms. Because of the self limiting process – the downdraft dissipating the
updraft – ordinary thunderstorms are short lived, rarely lasting an hour and typically
having a diameter of less than one kilometre.
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
downdraft 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 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.
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.
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).
Lightning can take place either from the cloud base to the surface or the reverse.
However, 90% of lightning is the first type. 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 lightning stroke heats the air to over 30,000C. The air expands explosively
and sends out a shock wave which is heard as thunder. Since sound travels more slowly
than light, thunder is heard after the lightning stroke is seen. Radio waves are also
produced. These waves, called sferics, propagate further with less dispersion. This
provides the basis for a lightning detection system.
The lightning described so far has been lightning which electrically connects a
cloud to the Earth‟s surface. However lightning can also occur within a cloud and
between clouds. This is called sheet lightning. The lightning is usually viewed through a
cloud and thus appears as a luminous sheet. Heat lightning is a term used to describe
lightning that occurs at a distance. Due to the curvature of the Earth and refraction in the
atmosphere, the thunder from this lightning is not heard, but a flash (often orange) briefly
lights the sky. It has the name „heat lightning‟ because it is often seen on hot, clear
summer evenings. Dry lightning occurs when a thunderstorm‟s precipitation is
evaporated in the downdraft and does not reach the surface.