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Lecture

Lecture #4 Notes that accompanies Lecture #4 Slides


Department
Environmental Science
Course Code
EESA09H3
Professor
Tanzina Mohsin

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EESA09H WIND
Lecture 4 Notes
Outline of this lecture
Part I Midlatitude Cyclones
What are midlatitude cyclones?
Midlatitude cyclones in the Great Lakes region
Famous Midlatitude Cyclones
Edmund Fitzgerald
Perfect Storm
1998 Ice Storm
Part II Research: Toronto Blizzard of 1999
Gough (2000)
Climatological Context of the Blizzard of 1999
Part I. Midlatitude Cyclones
a. What are midlatitude cyclones?
In Lecture 2 we examined the large scale circulation of the world. Midlatitude
cyclones occur within the moving boundary of the Ferrel and Polar cells referred to as the
Polar front. A series of low and high pressures propagate along the polar front. These
lows are called midlatitude cyclones and largely characterize the weather conditions of
the midlatitudes in the fall, winter and spring. During the summer the polar front often
lies to the north of the Great Lakes region. Storms during the summer are either mid-
latitude cyclones or convective storms arising from surface heating.
Midlatitude cyclones, commonly referred to as low pressures, are the major source of
weather variation in the midlatitudes (30 to 60). These storms occur approximately
every four to seven days. In North America, they occur at the boundary of two major air
masses, a cold, polar air mass (cP) and a moist, tropical air mass (mT). The warmth and
latent heat (in water vapour) of the mT air provide the energy for the storms. In North
America, there are four major regions of cyclone development. Two of the regions are on
the lee-side (eastern side) of the Rocky Mountains producing Alberta Clippers and
Colorado Lows. The other two regions have cyclones developing over ocean bodies. Gulf
Lows begin over the Gulf of Mexico and Hatteras Lows (Nor‟easters) off the eastern
coast of the United States.
Midlatitude cyclones are typically 100s to 1000s km in extent (larger than
hurricanes). They have less intense winds than hurricanes. They may have thunderstorms
and tornadoes associated with them (along the cold front).

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How are they formed?
Unlike hurricanes, surface conditions are not the dominant mechanism for midlatitude
cyclone formation. The upper level flow triggers storm formation, particularly the jet
stream. More details on this are provided in EESB03H Principles of Climatology.
Polar front theory and fronts
Fronts are divisions between masses of air of different characteristics in a midlatitude
cyclone. These are storms often referred to as low pressures or extratropical storms that
dominate the weather in the midlatitudes.
An air mass is a large body of air spanning up to several thousands of kilometers
in the lower troposphere. An air mass is characterized by horizontally uniform
temperature and moisture content (humidity) at any given altitude. The air masses have
relatively stable high pressure, with low pressure systems (storms or midlatitude
cyclones) forming at the boundaries between them. In this view of the atmosphere, there
are a number of relatively stable high pressure systems punctuated by smaller, energetic
low pressures, or frontal systems, occurring in battle zones between the air masses (see
Lecture 2).
Polar front theory, also called the „Norwegian cyclone model‟, describes the
growth of a kink or disturbance along the Polar Front into a frontal wave characterized by
familiar features such as the cold front and warm front. Accompanying these fronts are
characteristic bands of precipitation, the most intense occurring along the cold front.
After a few days of development the front wave dissipates and the storm ends. Four types
of fronts are identified in this framework, the stationary front, the cold front, the warm
front and the occluded front.
A stationary front exists between air masses but lacks the instability or energy for
a storm to develop.
A cold front is typically the division between cP and mT air, although it can occur
between mP and mT air. The mT air is forced over the colder air mass with a slope of
1:50; that is, for every 50 horizontal kilometres, the air is forced up 1 km vertically. This
may not seem at first to be a particularly steep slope, but it is the steepest of the all the
frontal slopes. As a result of the lifting of the mT air, cumulus and cumulonimbus clouds
form resulting in heavy precipitation. The most violent weather of a midlatitude cyclone
occurs along the cold front. In addition to the cumulus type clouds, thunderstorms and
tornadoes can spawn from the cold front.
On a weather map, the cold front is the easiest front to identify. Across the front
there is a strong temperature gradient, a natural result of the differences in the two
colliding air masses. In addition to this, there is a strong gradient in the moisture content,
also a result of the meeting of dry and moist air masses. The cold front is also denoted by
a strong shift in wind direction. Winds in the cP are typically heading south or southeast
whereas winds in the mT air mass are heading north or north east. This is due to air
masses rotating around the low pressure in a cyclonic or counterclockwise direction.
Midlatitude cyclone cloud structure when viewed via satellite is often comma shaped.
The tail of the comma occurs along the cold front, thus providing an easy way to identify
the front from a satellite picture. This cloud structure brings a variety of heavy
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precipitation at the surface depending on the storms temperature. Precipitation types
include snow, sleet (ice pellets), freezing rain and rain.
A warm front is another division between mT and mP or cP air. It is the leading
edge of a midlatitude cyclone. The mT air overrides the cold air mass to the north and
east. The frontal slope is shallower than the cold front, having a slope of 1:150 to 1:300.
As a result the mT air is more gently lifted above the surface. This results in the
formation of stratus and nimbostratus clouds in front of the surface warm front. These are
warm clouds that typically produce drizzle (light rain or snow). As the air is pushed
further aloft in advance (east and north of the front), other higher cloud types form, such
as altostratus, altocumulus and cirrus type clouds. Since midlatitude cyclones propagate
generally from west to east, the appearance of these high clouds in the western sky is
often a harbinger of a midlatitude cyclone.
Lying between the cold front and the warm front is a body of mT air called the
warm sector. The warm sector is roughly triangular with the apex at the low center where
the warm and cold sectors meet. The warm sector has the characteristics of mT air, warm
and moist. Thus, after the drizzle that precedes the warm front, surface observers can
experience relatively warm and humid conditions sometimes oppressively so before
the arrival of the cold front.
An occluded front is a front that is not directly observable at the surface. In a
frontal system, the cold front is more dynamic and propagates faster than the warm front.
Therefore, the cold front will eventually catch up to the warm front, beginning at the apex
of the storm. As this occurs the apex will no longer reside at the center of the low
pressure of the midlatitude cyclone but is shifted to the south at the junction of the fronts.
mT air in this area is entirely pushed above the surface. An occluded front is the line
linking the center of the low pressure to the apex where the cold and warm fronts are now
meeting.
This squeezing process of the mT air is not only the most intense phase of the
midlatitude cyclone, but also the beginning of the end. By cutting off the warm sector
from the low pressure centre, the mT air becomes less available to provide heat and latent
heat energy to the evolving midlatitude cyclone. As the cold front continues to catch up
with the warm front the occluded front becomes larger and the dynamic cold front is
further displaced from the center of the low, which gradually weakens and dissipates.
In summary, according to polar front theory, a midlatitude cyclone begins as a
stationary front between mT air and colder air such as cP or mP. A disturbance occurs
along the front. The disturbance is unstable and grows rapidly fueled by the warmth and
latent heat transported to the upper troposphere. Clearly defined warm and cold fronts
develop. The warm front is the leading edge of the storm and moves east or northeast.
The apex where the warm and cold are connected is the low pressure center. The cold
front follows the warm front. Between the two is a warm sector. The cold front moves
quickly than the warm front and an occluded front forms from the low center to the apex
where warm and cold fronts meet. This process squeezes the warm sector and forces air
aloft. The cold front becomes increasingly disconnected from the low center and thus
removes energy source of the storm, the mT air from the storm center. The storm
gradually dissipates and a stationary front reforms. This entire process takes about four to
seven days.
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