Transcript 10_WeatherSystems

NAS 125: Meteorology
Weather Systems of Middle Latitudes
Defoe and Franklin
• Daniel Defoe was the first to propose that storms in
the middle latitudes usually tracked from west to east.
– Defoe based his conclusions on study of a storm that struck
the British Isles on 7-8 December 1703.
• He had heard that a similar storm had run up the East Coast of
North America a few days earlier.
– Benjamin Franklin came to a similar conclusion after a
storm in Philadelphia caused him to miss a 21 October
1743 lunar eclipse.
• His brother (in Boston) told him the eclipse was visible, but that it
was stormy in Boston the next day.
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Storms and the landscape, part 1
• Storms are phenomena that are more limited than the
broad-scale wind and pressure systems.
– They are transient and temporary.
• Storms involve the flow of air masses as well as a
variety of atmospheric disturbances.
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Storms and the landscape, part 2
• They have short-run and long-run impacts.
– In some parts of world, have major influence on weather,
some on climate.
– Long-run includes both positive and negative impacts on
landscape.
• Positive: promote diversity in vegetative cover, increase size of
lakes and ponds, and stimulate plant growth.
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Air masses, part 1
• An air mass is a large parcel of air that has relatively
uniform properties in the horizontal dimension and
moves as an entity. Such extensive bodies are distinct
from one another and compose the troposphere.
• Characteristics
– An air mass must meet three requirements:
• It must be large (horizontal and vertical).
• Its horizontal dimension must have uniform properties
(temperature, humidity, and stability).
• It must be distinct from surrounding air, and when it moves, it must
retain that distinction (not be torn apart.
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Air masses, part 2
• Origin
– Air mass formation occurs if air remains over a uniform
land or sea surface long enough to acquire uniform
properties.
• Source regions are parts of Earth’s surface that are particularly
suited to generate air masses because they are
– Extensive
– Physically uniform
– Associated with air that is stationary or anticyclonic.
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Air masses, part 3
• Classification
– Because the source region determines properties of air
masses, it is the basis for classifying them.
– Classifications use a one- or two-letter code.
– The following table provides a simplified classification of
air masses, along with the properties associated with each.
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Air masses, part 4
• Movement and modification
– Some air masses remain in source region indefinitely.
– Movement prompts structural change:
• Thermal modification – heating or cooling from below;
• Dynamic modification – uplift, subsidence, convergence,
turbulence;
• Moisture modification – addition or subtraction of moisture.
– A moving air mass modifies the weather of the region it
moves through.
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Air masses, part 5
• North American air masses
– The physical geography of U.S. landscape plays a critical
role in air-mass interaction.
• There are no east-west mountains to block polar and tropical air
flows, so they affect U.S. weather and climate.
• North-south mountain ranges in the west modify the movement,
therefore the characteristics, of Pacific air masses.
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Air masses, part 6
• North American air masses (continued)
– Maritime tropical (mT) air from the Atlantic,
Caribbean/Gulf of Mexico strongly influences climate east
of the Rockies in the United States, southern Canada, and
much of Mexico.
• Primary source of precipitation. Also brings periods of
uncomfortable humid heat in summer.
– Continental tropical (cT) air has insignificant influence on
North America, except for bringing occasional heat waves
and drought conditions to the southern Great Plains
– Equatorial (E) air affects North America only through
hurricanes.
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Fronts, part 1
• Fronts are zones of discontinuity between unlike air
masses where properties of air change rapidly.
– They are narrow but three-dimensional.
– The yare typically several kilometers wide (even tens of
kilometers wide).
– Fronts function as a barrier between two air masses,
preventing their mingling except in this narrow transition
zone.
– Frontogenesis occurs when density contrast between air
masses strengthen.
– Frontolysis occurs when density differences weaken.
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Fronts, part 2
• Fronts (continued)
– Though all primary physical properties are involved in a
front, temperature provides the most conspicuous
difference.
– Fronts lean, which allows air masses to be uplifted and
adiabatic cooling to take place.
• Fronts lean so much, closer to horizontal than vertical.
• Fronts always slope so that warmer air overlies cooler air.
– Fronts move in association with the direction of the more
active air mass, which displaces the less active.
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Fronts, part 3
• Warm fronts
– A warm front is the leading edge of an advancing warm air
mass.
• It brings warm air.
• It results in clouds and precipitation, usually broad, protracted, and
gentle, without much convective activity.
• Unstable rising air can result in showery and even violent
precipitation.
• Weather maps show ground-level position of warm front;
precipitation usually falls ahead of this position.
• Frontal fogs may develop ahead of the front.
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Fronts, part 4
• Cold fronts
– A cold front is the leading edge of a cool air mass actively
displacing warm air mass.
• It brings cold air.
• It leads to rapid lifting of warm air, which makes it unstable and
thus results in blustery and violent weather along the front; if the
front is fast moving, a squall line may form.
• Weather maps show ground-level position of cold front (usually has
a protruding “nose”); clouds and precipitation tend to be
concentrated along and immediately behind the ground-level
position.
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Fronts, part 5
• Stationary fronts
– A stationary front is the common “boundary” between two
air masses in a situation in which neither air mass displaces
the other.
• Precipitation and clouds along the front may form a broad or
narrow band.
• Warm air blowing up and over stationary fronts cools by expansion,
leading to condensation and possibly precipitation; this is called
overrunning.
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Fronts, part 6
• Occluded front
– An occluded front is a complex front formed when a cold
front overtakes a warm front.
• A cold-type occlusion occurs when the air behind the advancing
cold front is colder than the cool air ahead.
• A warm-type occlusion occurs when the air behind the advancing
cold front is warmer than the air ahead.
• A neutral occlusion features insignificant temperature differences
between the air behind the advancing cold front and the air ahead.
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Fronts, part 7
• Occluded front
– An occluded front is a complex front formed when a cold
front overtakes a warm front.
• A cold-type occlusion occurs when the air behind the advancing
cold front is colder than the cool air ahead.
• A warm-type occlusion occurs when the air behind the advancing
cold front is warmer than the air ahead.
• A neutral occlusion features insignificant temperature differences
between the air behind the advancing cold front and the air ahead.
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Midlatitude cyclones, part 1
• A midlatitude cyclone is a large migratory lowpressure system that occurs within the middle
latitudes and moves generally with the westerlies;
midlatitude cyclones are also called lows or wave
cyclones, depressions.
– Midlatitude cyclones are probably most significant of all
atmospheric disturbances.
– They are basically responsible for most day-to-day weather
changes.
– They bring precipitation to much of the world’s populated
regions.
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Midlatitude cyclones, part 2
• Characteristics
– Typical mature midlatitude cyclone is 1,600 kilometers
(1,000 miles) in diameter; it has an oval shape.
– Patterns of isobars, fronts, and wind flow in the Southern
Hemisphere are mirror images of those in the Northern
Hemisphere.
– Northern Hemisphere patterns:
• Circulation pattern converges counterclockwise;
• Wind-flow pattern attracts cool air from north and warm air from
south; creates two fronts.
• These two fronts divide the cyclone into a cool sector north and
west of center and a warm sector south and east.
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Midlatitude cyclones, part 3
• Characteristics (continued)
– Northern Hemisphere patterns (continued)
• Circulation pattern converges counterclockwise;
• Wind-flow pattern attracts cool air from north and warm air from
south; creates two fronts.
• These two fronts divide the cyclone into a cool sector north and
west of center and a warm sector south and east.
• Size of sectors varies with location: on ground, cool sector is larger,
but in atmosphere, warm sector is more extensive.
• Warm air rises along both fronts, causing cloudiness and
precipitation, which follows patterns of cold and warm fronts.
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Midlatitude cyclones, part 4
• Characteristics (continued)
– Northern Hemisphere patterns (continued)
• Much of cool sector is typified by clear, cold, stable air, while air of
warm sector is often moist and tending toward instability, so may
have sporadic thunderstorms. May have squall fronts of intense
thunderstorms.
• Movements
– Midlatitude cyclones move throughout their existence.
• Movement is typically from West to East.
• Cold front moves faster than warm front.
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Midlatitude cyclones, part 5
• Lifecycle
– Origin to maturity typically takes 3 to 6 days, then another
3 to 6 days to dissipate.
– Cyclogenesis is the birth of cyclones.
– Most common cause believed to be upper-air conditions in
the vicinity of the polar-front jet stream.
– Most begin as waves along the polar front.
– Cyclogenesis can also occur on the leeward side of
mountains.
– Often bring heavy rain or snowstorms to the northeastern
United States and southeastern Canada.
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Midlatitude cyclones, part 6
• Lifecycle (continued)
– A comma cloud may be apparent at the peak of the storm.
– After cyclonic circulation is well developed, occlusion
begins.
– After occluded front is fully developed, cyclone dissipates.
• Occurrence and Distribution
– Occur at scattered but irregular intervals throughout the
zone of the westerlies.
– Route of cyclone is likely to be undulating and erratic, but
it generally moves west to east.
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Midlatitude cyclones, part 7
• Bomb cyclogenesis is the rapid development of a
cyclone.
– Bombs are defined as storms whose central pressure drops
by 24 mb in 24 hours.
– Most form in winter over warm water, such as over the
Kuroshio Current off Japan and the Gulf Stream off the
United States.
• The “Perfect Storm” of 1991 is a classic example of a bomb.
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Midlatitude cyclones, part 8
• Cyclone models
– In the Norwegian cyclone model, cyclones can form and
intensify with adequate upper-air support.
• Cyclones form along the polar front under an area of strong
horizontal divergence in the upper troposphere.
• If divergence aloft removes more air than is replaced by
convergence at the surface, surface air pressure drops, a horizontal
pressure gradient develops, and cyclonic circulation begins.
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Midlatitude cyclones, part 9
• Cyclone models (continued):
– In the conveyor belt model, cyclogenesis is driven by the
three intersecting air streams, or conveyor belts.
• The first two conveyors form in the rising air to the east of the
cyclone, helping form the characteristic comma clouds.
– A warm conveyor belt originates in the cyclones warm sector and
follows the warm side of the cold front near the surface, to the south
and east of the storm center, riding as it blows toward the west.
– A cold conveyor forms in the easterly and northeasterly winds
flowing ahead of the warm front to the east of the storm center, rising
as if blows toward the west.
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Midlatitude cyclones, part 10
• Cyclone models (continued):
– Conveyor belt model (continued):
• A dry airstream is the third conveyor belt, which forms aloft and
descends along the west side of the cyclone.
– One branch flows toward the southward behind the cold front,
bringing clear skies.
– The other branch moves northward and toward the storm center,
which ascends east of the upper-level trough and forms the cry slot
that separates the head and tail of the comma.
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Midlatitude cyclones, part 11
• Cold- and warm-core systems
– A cold-core system is one in which the lowest temperatures
occur throughout the column of air above the low-pressure
center.
• In an occluded cold-core system, isobars dip downward in the
center of the cyclone.
• In a non-occluded cold-core system, the lowest air temperatures are
northwest of the storm center and the highest air temperatures are
southeast of the center, such that , the column of cold air is tilted
toward the cold air.
– A warm-core system is stationary, without fronts, and is
caused by intense heating of the ground.
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Midlatitude cyclones, part 12
• Cold- and warm-core systems (continued):
– A warm-core system, caused by intense solar heating of the
ground, forms over arid or semiarid landscapes.
• The heating of the land warms the air above, which in turn lowers
the density of the air and leads to formation of a synoptic-scale low.
• Warm-core systems are stationary, without fronts.
• They are shallow, with their circulation weakening rapidly with
altitude.
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Midlatitude anticyclones, part 1
• A midlatitude anticyclone is an extensive migratory
high-pressure cell of the midlatitudes that moves
generally with the westerlies.
– They are typically larger than a midlatitude cyclone, but
also moves west to east.
– They travel at the same rate, or little slower, than
midlatitude cyclone.
• Anticyclones are prone to stagnate or remain over same region
(while cyclones do not).
• They can cause concentration of air pollutants.
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Midlatitude anticyclones, part 2
• Cold- and warm-core systems
– A cold-core anticyclone is a dome of cold air.
• In the Northern Hemisphere, continental polar (cP) or Arctic (A) air
masses are cold-core systems.
– cP air masses form from the polar high.
– A air masses form from the arctic high.
• Cold-core anticyclones are the most intense (with highest
pressures) in the winter.
– Powerful systems can bring freezing weather to South Florida.
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Midlatitude anticyclones, part 3
• Cold- and warm-core systems (continued):
– A warm-core anticyclone forms south of the polar front and
consists of subsiding warm, dry air.
• Warm-core anticyclones bring intense heat waves and droughts.
• The subtropical highs, such as the Bermuda High, are warm-core
anticyclones, but others may form over the interior of continents.
• Cyclones and anticyclones alternate with one another
in an irregular sequence.
– There is often a functional relationship between the two.
• An anticyclone can be visualized as a polar air mass with the cold
front of cyclone as its leading edge.
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Localized wind systems, part 1
• Lesser winds have a considerable effect on weather
and climate on a localized scale.
• Sea and land breezes are a common local wind
system along coastlines.
– They are essentially a convectional circulation caused by
differential heating of land and water surfaces.
– A land breeze is a local wind blowing from land to water,
usually at night (and normally considerably weaker flow
than that of sea breeze).
– A sea breeze is a local wind blowing from sea toward the
land, usually during the day.
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Localized wind systems, part 2
• Valley and mountain breezes are a daily cycle of
airflow occurs with valley and mountain breezes.
– The are convectional circulation caused by differential
heating of higher versus lower elevations.
– Mountain air cools quickly at night, allowing cooler air to
drain down the slope in the evening. Conversely, valley air
heats more rapidly during the day, allowing warm air to
move upslope during the day.
• A valley breeze is upslope flow during the day.
• A mountain breeze is downslope flow during the night.
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Localized wind systems, part 3
• Valley and mountain breezes (continued)
– Air drainage is the sliding of cold air downslope to collect
in the lowest spots, usually at night; it is a modified form of
mountain breeze common in winter.
• Katabatic winds originate in cold upland areas and
cascades toward lower elevations under the influence
of gravity.
– The air is cold and dense, and usually colder than the air it
displaces in its downslope flow.
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Localized wind systems, part 4
• Katabatic winds (continued):
– A mistral is a cold, high-velocity wind that sometimes
surges down France’s Rhone Valley, from the Alps to the
Mediterranean Sea. The mistral has considerable
destructive power.
– Similar winds are called bora in Adriatic region and taku in
Alaska.
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Localized wind systems, part 5
• Chinook winds are localized downslope winds of
relatively dry and warm air, which is further warmed
adiabatically as it moves down the leeward slope of
the Rocky Mountains.
– Similar winds are called foehns in Europe.
– Santa Anas are similar to chinooks/foehns. They have high
speed, high temperature, and extreme dryness. They may
prompt wildfires.
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Localized wind systems, part 5
• Desert winds form because of the intense solar
heating of the ground.
– Most of the absorbed radiation goes into sensible heating
because of the lack of water.
– In some cases, surface temperatures may exceed 55 °C,
which in turn may generate a superadiabatic lapse rate
(greater than 9.8 °C).
• Superadiabatic lapse rates are associated with great instability,
intense convection, and gusty winds at the surface.
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Localized wind systems, part 6
• Desert winds (continued):
– Dust devils are whirling masses of dust-laden air,
commonly seen over flat, dry terrain.
• They are microscale systems.
– Dust devils rarely exceed 300 m in diameter and last more than 20
minutes or more.
» Large dust devils may be seen to extend to 900 m in altitude, but
the part not seen may extend to 4,500 m.
– Most are less than 1 m. across and last less than a minute.
• They develop in response to uneven heating of a dry, but
heterogeneous landscape.
• Air is forced to rise over the hot spot by converging surface winds.
• Wind shear sets the column spinning.
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Localized wind systems, part 7
• Desert winds (continued):
– Surface winds generated by thunderstorms or migrating
cyclones can trigger dust storms or sandstorms.
• The difference between the two is in the size of the particles carried
by the winds.
– Dust: less than 0.06 mm
– Sand: from 0.06 mm to 2.0 mm
• Haboobs are spectacular dust storms formed by powerful
downdrafts that spread out as the downdraft reaches the ground.
– Common throughout northern and central Sudan and the deserts of the
American Southwest.
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