Download Weather Systems Atmospheric Moisture and Precipitation Air Masses

Weather Systems
Chapter
7
Atmospheric Moisture and
Precipitation
Some patterns of wind circulation occur regularly and give
rise to recurring weather conditions.
For example, travelling lowpressure centres often
bi
bringclouds
l d and
d precipitation;
i it ti
outbreaks
tb k off polar
l air
i
typically give cold, clear weather in winter.
These types of recurring circulation patterns and the
conditions associated with them are called weather
systems.
Air Masses
An air mass is a large body of air with fairly uniform temperature
and moisture characteristics.
Air masses can be several thousand kilometres across and
extend to the top
p of the troposphere.
p p
Each air mass is characterized by a distinctive combination of
conditions at the surface, particularly temperature, environmental
temperature lapse rate, and humidity.
These properties are acquired in source regions over which the
air moves slowly or stagnates.
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Air Masses
Five types of air masses are distinguished with respect to
latitude. These are referred to as arctic (A), antarctic (AA),
polar (P), tropical (T), and equatorial (E)
For the type of underlying surface,
surface two subdivisions are
used: maritime (m) and continental (c).
Six important air mass types can be identified from these
descriptive labels: cA, cP, cT, mP, mE, mT.
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Air Masses
North American Air Masses
Continental polar (cP) and continental arctic (cA) air
masses are the dominant types affecting North America.
Maritime polar air masses originate over the North Pacific
and Bering Strait, in the region of the persistent Aleutian
low pressure centre.
Air Masses
North American Air Masses
Maritime tropical air masses from the Gulf of Mexico most
commonly affect the central and eastern United States; mT
air is the main opponent of cA and cP over North America.
The properties of air masses are modified as they move
away from their source regions (degree of modification
depends on their speed and also the general routes they
follow).
Air Masses
North American Air Masses
An air mass usually has a well-defined boundary between
itself and a neighbouring air mass; these transitional zones
are termed fronts.
In North America, the boundary between polar and tropical
air masses produces the well-defined polar front that
is located below the axis of the polar front jet stream.
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Poleward Transport of Heat and
Moisture
Because solar energy does not warm the Earth’s surface
uniformly, the atmosphere and oceans exhibit a complex
circulation pattern of air and water flows, which acts to
redistribute absorbed solar radiation more evenly by way of
various convection loops (poleward heat transport).
Lower Latitudes: through thermally direct Hadley Cells.
Mid, High Latitudes: through Rossby Wave mechanism.
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Travelling Cyclones and Anticyclones
Air masses are set in motion by the pressure gradients
(between regions of high and low pressure) created by
uneven heating or cooling of the atmosphere.
Then driven along by the general global wind system and
by regional pressure gradients associated with centres of
high and low pressure.
Travelling Cyclones and Anticyclones
Because pressure varies regionally over great distances,
air masses are moved far their source regions.
This type of movement is a characteristic feature of the
midlatitude atmosphere
atmosphere, where travelling cyclones and
travelling anticyclones bring changing weather conditions
as they pass across a region.
Travelling Cyclones and Anticyclones
In a travelling cyclonic system, convergence and upward
motion cause air to rise and be cooled adiabatically.
If the air is moist, condensation or sublimation can occur;
this may lead to cyclonic precipitation
precipitation.
When pressure gradients are steep and the air is laden
with water vapour, strong winds and heavy rain or snow
can accompany them (producing a cyclonic storm).
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Travelling Cyclones and Anticyclones
Travelling cyclones fall into three types distinguished
primarily by their intensity and geographic location:
1)
Midlatitudes, arctic and Antarctic - wave cyclones or
depressions
2)
Tropical and subtropical – tropical cyclones (which can
intensify to hurricanes and typhoons)
3)
tornado
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Travelling Cyclones and Anticyclones
Wave Cyclone
In mid- and high latitudes, the dominant form of weather system
is the wave cyclone, a large mass of inward-spiralling air that
repeatedly forms
forms, intensifies
intensifies, and dissolves along the polar front
front.
A situation favourable to the formation of a wave cyclone occurs
when two large anticyclones lie on either side of the polar front.
The boundary between the two high-pressure cells is necessarily
a region of lower pressure and so forms a low-pressure trough.
Travelling Cyclones and Anticyclones
Wave Cyclone
A warm front is where warm air is moving over a region of
colder air.
The cold air mass remains in contact with the ground
because it is denser, so consequently the moving mass of
warm air is forced to rise over it.
The gradient on a warm front is generally about 1:300.
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Travelling Cyclones and Anticyclones
Wave Cyclone
A cold front develops when a cold air mass advances into the
zone occupied by the warm air mass.
Because the colder air mass is denser, it remains in contact with
the ground.
The gradient of a cold front is about 1:50, so it is much steeper
than a warm front.
In this case, the front represents the leading edge of the
approaching cold air.
Travelling Cyclones and Anticyclones
Wave Cyclone
Cold fronts normally move across a region at a faster rate
than warm fronts, and where they are in close proximity, a
cold
ld ffrontt can eventually
t ll overtake
t k a warm ffront.
t
This produces the occluded stage of the wave cyclone.
The colder air of the faster-moving cold front remains next
to the ground, forcing the warm air ahead of it to rise along
the occluded front.
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Travelling Cyclones and Anticyclones
Wave Cyclone
Two types of occluded fronts:
A warm occlusion develops when the leading mass of cold air is
colder and denser than the trailing mass of cold air (the trailing
cold air mass will rise up the warm front and lift the warm air off
of the surface).
In a cold occlusion, the trailing mass of cold air is denser than
the cold air ahead of the warm front (the warm sector air is lifted
off of the surface as the trailing cold air catches up with and
pushes under the leading mass of cold air).
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Travelling Cyclones and Anticyclones
Cyclone Tracks and Cyclone Families
Wave cyclones tend to form in certain areas and follow similar routes until they
dissolve.
In the northern hemisphere,
p
, wave cyclones
y
are heavilyy concentrated in the
neighbourhood of the Aleutian and Icelandic Lows.
Three or four commonly form in succession, then travel as a chain across the
North Pacific and North Atlantic oceans
The general eastward movement of wave cyclones is largely controlled by the
paths of the Rossby waves and their relationship to the polar front jet streams.
The curving paths of Rossby waves is a prime factor in wave cyclone
development.
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Tropical and Equatorial Weather
Systems
Tropical and equatorial zone weather systems show some basic
differences from those of the midlatitudes.
Upper-air winds are often weak, so air mass movement is slow and
gradual.
Tropical and equatorial air masses tend to be warm and moist
moist, and
consequently clearly defined fronts and large, intense wave cyclones
do not develop.
On the other hand, strong convectional activity occurs because of the
high moisture content of low-latitude maritime air masses.
With these conditions, even slight convergence and uplift can be
enough to trigger abundant precipitation.
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Tropical and Equatorial Weather
Systems
Easterly Waves and Weak Equatorial Lows
One of the simplest forms of tropical weather systems is an easterly
wave, a slowly moving, convectively active trough of low pressure that
is driven westward by the trade winds.
The easterly waves that cross the Atlantic Ocean are initiated by
instability over northwestern Africa and are associated with the low
latitude easterly jet.
Another related weather system is the weak equatorial low,
a disturbance that forms near the centre of the equatorial trough.
Moist equatorial air masses converge on the centre of the low, causing
numerous convectional storms and heavy rainfall.
Tropical and Equatorial Weather
Systems
Polar Outbreaks
Another distinctive feature of low-latitude weather is the
occasional penetration of powerful tongues of cold polar air from
the midlatitudes into very low latitudes.
The leading edge of these polar outbreaks is marked by squalls
along the cold front, and is followed by unusually cool, clear
weather with strong, steady Winds (best developed in the
Americas).
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Tropical and Equatorial Weather
Systems
Tropical Cyclones
Tropical weather disturbances range from weak tropical lows to
powerful and destructive tropical cyclones (“a storm of tropical origin of
small diameter with a minimum surface pressure below 95 kPa, with
greater than 125 kph,
p and accompanied
p
by
y torrential
violent winds g
rain.”)
A tropical cyclone is known variously as a hurricane in the western
hemisphere, a typhoon in the western Pacific, and a cyclone in the
Indian Ocean.
These storms revolve around a central “eye,” which is about 30 to 50
km in diameter and characterized by light winds and clear skies.
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Tropical and Equatorial Weather
Systems
Tropical Cyclones
Tropical cyclones develop over oceans at 7° to 15° N and S
latitudes.
The poleward limit is determined by the need for sea-surface
temperatures that are high enough to ensure sufficient
evaporation and release of latent heat to maintain the storm
(sea-surface temperatures of over 27°C).
Although sea-surface temperatures exceed 27°C near the
equator, at latitudes below 7° the Coriolis effect is too weak to
initiate the storm’s circular motion.
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Tropical and Equatorial Weather
Systems
Tropical Cyclones
A characteristic feature of the tropical cyclone is its central
eye, in which clear skies and calm winds prevail.
The eye is a cloud-free vortex produced by the intense
spiralling of the storm, and usually develops when
sustained wind speeds exceed 120 kph.
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Tropical and Equatorial Weather
Systems
Tropical Cyclones
The Saffir-Simpson scale rates the intensity of tropical
cyclones.
The rating is based on the central pressure of the storm,
mean wind speed, and height of accompanying storm
surge, which can raise sea level several metres above
normal.
Category 1 storms are weak, while Category 5 storms are
devastating.
Tropical and Equatorial Weather
Systems
Tropical Cyclones
Weather forecasters tracking tropical cyclones give them
names (alternate male and female names in an
p
sequence,
q
, with the exception
p
of ones
alphabetical
beginning with Q, U, X, Y, and Z).
Each region has its own set of names; in North America
one set is used for hurricanes in the Atlantic, and another
for typhoons of the Pacific.
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Tropical and Equatorial Weather
Systems
Impacts of Tropical Cyclones
Tropical cyclones can be tremendously destructive. Islands and coasts
feel the full force of the high winds and flooding as tropical cyclones
move onshore.
Densely populated low-lying areas are particularly vulnerable to tropical
cyclones
Deadliest tropical cyclone on record struck Bangladesh on
November 13, 1970, with at least 500,000 deaths reported.
In terms of cost, Hurricane Katrina, in August 2005, caused an
estimated US$200 billion in damage.
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Tornado
A tornado is a small but intense cyclonic vortex in which air
spirals at tremendous speed.
Tornadoes generally occur in the midlatitudes between
about 30° and 50°.
Nowhere are they as frequent, or as violent, as in the
United States, where 1,000 or more strike annually.
In Canada, about 80 tornadoes are reported each year
(tornado season in Canada typically extends from May to
September, with most occurring in June or July).
Tornado
Tornadoes form most often in association with intense supercell
thunderstorms, with the first stage of tornadogenesis indicated
by the development of a mesocyclone due to rotation in the
updraft core induced by strong wind shear.
Initial dust whirl stage: a tornado usually takes the form of a
swirling
i li d
dustt cloud
l d th
thatt extends
t d ffrom th
the ground
d tto a short,
h t
pendant funnel cloud hanging below the base of the
mesocyclone cloud mass.
Funnel clouds form by condensation of water vapour when the
air is cooled adiabatically through expansion as it is taken aloft in
the low-pressure vortex.
Tornado
The organizing stage: often marked by the rapid lowering of
the base of the thunderstorm as a rotating wall cloud
develops beneath the mesocyclone (many tornadoes never
develop beyond the organizing stage, but simply lose
energy and disappear).
The mature stage: in which the vortex becomes nearly
vertical and is darkened with dust and debris (the tornado
is generally in the order of 100 to 500 m in diameter with
wind speeds ranging from 250 to 350 kph).
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Tornado Development
in the United States
Tornado
In 1971, the Fujita (F) scale was devised to assess the intensity
of North American tornadoes on the basis of the damage they
caused (F1 to F5).
The Fujita scale has been criticized since it is biased toward the
worst damage, even though no consideration is given for
building
b
ildi ttype or method
th d off construction
t ti ((resulting
lti iin
overestimated wind speeds in some cases).
As result, in Canada and the United States, the F-scale was
replaced in 2007 by the Enhanced Fujita (EF) scale.
(incorporates 28 damage indicators, including different types of
buildings and trees for which varying degrees of damage are
assessed).
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A Look Ahead
Annual cycles of temperature and precipitation in most
regions are quite predictable given the changes in wind
patterns, air mass flows, and weather systems that occur
with the seasons.
This creates spatially variable weather patterns and
provides the basis for climate and climate classification
(Chapters 8 to 10).
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