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AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 10
Weather Systems of Middle
 Extra-tropical cyclones are major weather
makers in middle and high latitudes
 In 1703, Daniel Defoe was the first to
propose that storms generally track from
west to east in middle latitudes
 In 1743, Benjamin Franklin was the first
American to discover that storms usually
move in an easterly or northeasterly
– Based on observations, he concluded that wind
direction in a storm was not an indication of the
storm’s direction of movement
Driving Question
 What systems shape the weather of the
middle latitudes?
– Middle latitudes extend from Tropics of Cancer
and Capricorn, poleward to the Arctic/Antarctic
– Weather is particularly dynamic because of the
migration of cyclones and anticyclones
embedded in the prevailing westerlies
– This chapter examines:
 Air masses, fronts, cyclones, and anticyclones
 Local and regional circulation systems
Air Masses
 An air mass is a huge expanse of air covering
thousands of square kilometers, and is relatively
uniform horizontally in temperature and water
vapor concentration (humidity)
 Abbreviations for air mass types:
– Cold (polar or P) or warm (tropical or T)
– Dry (continental or c) or humid (maritime or m)
– Arctic (A) air
 Air mass source regions have nearly
homogeneous surface characteristics over a broad
area with little topographic relief
– The air mass stays over the source region for an
extended period, and takes on the characteristics of the
source region
North American Types
and Source Regions
North American Air Masses
Air Masses
 Air Mass Modification
– Air masses eventually move out of their source
– As they move, their properties are modified by
the surface they pass over
– Air mass modification occurs from:
 Exchange of heat or moisture, or both, with the
surface over which the air mass travels
 Radiational heating or cooling
 Adiabatic heating or cooling associated with largescale vertical motion
Air Masses
 Air Mass Modification
– In winter, as a cP air mass travels southeastward from
Canada into the lower 48-states, its temperature usually
modifies rapidly
 By the time it arrives in the southern states, temperatures will not
usually drop much below freezing
 The sun warms snow-free ground, and the warmer ground heats
the bottom of the air mass. Heat is then distributed vertically.
– A similar process of heating and destabilization occurs
when a cP air mass crosses the East Coast and moves
over the western Atlantic. Evaporation from the sea
surface leads to extensive areas of low clouds and fog
– cP traveling over snow-covered ground experiences less
 Much of the incoming solar radiation is reflected rather than being
Air Masses
 Modification of Air Masses, continued
– Tropical air masses modify less than polar masses
They are often warmer than the ground they travel over
The bottom of the air mass cools, and stabilizes
Convective currents are suppressed
If a tropical air mass moves over a warmer surface, the air mass
can become even warmer
– Air masses undergo significant modification through
orographic uplifting (e.g., mP air mass sweeping inland
from the Pacific Ocean)
 Rising air cools adiabatically, condensation/deposition occur,
and precipitation is triggered on the windward slopes
 Descent on the leeward side leads to adiabatic warming and
cloud dissipation
 Air mass emerges considerably milder and drier (e.g., modified
Pacific air)
Frontal Weather
 A front is a narrow zone of transition between air masses that
differ in density
– Density differences are usually due to temperature contrasts, hence the
names cold fronts and warm fronts
– Density differences may also be caused by humidity contrasts
– A front’s transition zone may be 100+ km and a line representing a front
on a weather map is drawn along the warm edge of the zone
 A front is also associated with a trough in the sea-level pressure
pattern, a corresponding wind shift, and converging winds
 When air masses meet at fronts, the colder, denser air forces
the warmer, less dense air to rise
– This induces adiabatic cooling and often cloud/precipitation development
 The slope of the front influences the types of clouds that form
– Cold and warm fronts have different slopes associated with them
Frontal Weather
 Stationary Front
– A front that exhibits essentially no lateral motion
– This often happens along the Front Range of the Rocky
Mountains when a shallow pool of polar air surges south
over the plains and the leading edge is too shallow to
cross the mountains. Milder air remains in the Great
Basin to the west of the Rockies.
– May also develop when a preexisting front becomes
parallel to the upper-level flow pattern or along a
boundary in the surface temperature pattern
– Typical front
Slopes from Earth’s surface towards denser air
Lies in a trough in the pressure pattern
Wind changes direction rather abruptly across the front
May have broad region of clouds and precipitation (e.g.,
Stationary Front
Frontal Weather
 Warm Front
– Warm air advances while cold air retreats
– Overall characteristics very similar to a stationary front
– As a warm front approaches:
 Clouds thicken and become lower in altitude
 Sequence is cirrus, cirrostratus, altostratus, nimbostratus,
and stratus when the advancing warm air is relatively
 Initial cirrus appearance may be more than 1000 km (620
mi) ahead of the front
 Just ahead of the front, steady precipitation usually gives
way to drizzle and sometimes frontal fog
 If advancing warm air is unstable, more vigorous uplift
can occur with thunderstorms embedded in the
overrunning zone
Warm Front
Cirrus clouds
Frontal Weather
 Cold Front
– Colder air displaces warmer air
– In North America in winter, the temperature contrast
along a cold front is usually greater than across a warm
or stationary front
– In summer, temperatures on either side of the front may
be essentially the same
 Density contrasts arise because of humidity differences
– The slope on a cold front is much steeper than the
slope on a warm front
 Uplift is confined to a narrow area at or near the cold
front’s leading edge
 If the warm air is unstable, thunderstorms may form and
a squall line can develop
 If the warm air is relatively stable, nimbostratus and
altostratus may form
Cold Front
Advancing Back-Door Cold Front
A cold front generally trails south or southwestward from the center of
an extra-tropical cyclone. Back-door cold fronts move south along the
eastern side of the Appalachian Mountains.
Frontal Weather
 Occluded Fronts
– Typically form late in a cyclone’s life cycle as it moves into
colder air
– Faster moving cold front catches up with the warm front
– There are 3 types of occlusions, distinguished by the
temperature contrast between the air behind the cold front
and ahead of the warm front
 Cold occlusion
– Air behind cold front colder than cool air ahead of warm front
– Like a cold front at the surface but, with less air mass contrast
 Warm occlusion
– Air behind cold front is not as cold as the air ahead of the warm front
– Like a warm front at the surface
 Neutral occlusion
– Little difference between air masses
– Marked by a trough, wind shift line, band of cloudiness & precipitation
Cold-Type Occlusion
 Air behind advancing
cold front colder than
cool air ahead of warm
 More common in
eastern North America,
where the colder air
follows behind the front
on northwest winds
Warm-Type Occlusion
 Air behind the advancing
cold front is not as cold as
the air ahead of the warm
 Occurs in northerly
portions of western coasts,
such as in Europe or the
Pacific Northwest, where
mP air is behind the cold
Air Masses
 Summary
– Fronts are characterized based on the movement of the
cold air mass
– Clouds and precipitation may develop along fronts when
there is a significant density contrast between air
masses and there is an adequate supply of water vapor
– Properties that define a front are differences in
temperature and humidity, wind shift, convergence, and
a pressure trough
– Frontogensis: front forms or grows stronger
– Frontolysis: front weakens
Extra-tropical Cyclones
The extra-tropical cyclone (also called a low-pressure system or low), is a major weather
maker of middle and high latitudes. Surface winds blow counterclockwise and inward.
Surface winds converge, air rises, expands, and cools, resulting in clouds and precipitation.
Extra-tropical Cyclones
The comma-shaped cloud pattern is
characteristic of a well-developed extratropical cyclone.
 Life Cycle
– Norwegian cyclone model: conceptual model originally
developed around WWI still closely approximates our
current understanding
– (A) Incipient cyclone: Cyclogenesis (birth of a cyclone)
usually takes place along the polar front directly under
an area of strong horizontal divergence in the upper
 Air pressure at the bottom of the air column falls, a horizontal air
pressure gradient develops, and cyclonic circulation begins
 Westerlies aloft steer and support the cyclone as it progresses
through its life cycle
 West of the low center, the polar front pushes southeast as a
cold front. East of the low, the polar front advances north as a
warm front.
warm sector
 Life Cycle
– (B) Wave cyclone: In this stage, the central pressure
continues to drop and winds strengthen due to an
increased pressure gradient. The upper-level trough
deepens while remaining west of the surface low center.
 Warm sector becomes better defined
 Fronts form a pronounced wave pattern and comma cloud is
seen in satellite images
 Extensive stratiform cloudiness appears north of the warm front
 Cyclone moves eastward or northeastward at 40-55 km per hr
(25-35 mph)
 Life Cycle
– (C) Beginning of occlusion
 Faster moving cold front advances on the warm front
 Warm sector area diminishes and occluded front begins to form
 Upper level pattern shows closed circulation and is directly over
the surface low (vertically stacked)
 Dry slot separates the cold front cloud band from the comma
 Cyclone moves slower at approximately 30 km per hr (20 mph)
triple point
 Life Cycle
– (D) Bent-back occlusion
 Surface low may become detached from the westerly
steering flow and the occluded front is drawn around
the low center
 Warm sector is detached from cyclone center
 Triple point favors development of a secondary
 Eventually the cyclone weakens (cyclolysis)
Extra-tropical Cyclones
 Entire cycle can occur over several days, or a
much shorter period
– Speed of development depends on upper air support
 Weak divergence aloft will cause poorly defined systems
– Sometimes cloudiness and precipitation occur with an
upper-level or surface trough, which is not associated
with a closed surface cyclonic circulation
 When upper-level conditions are ideal, the entire
life cycle can occur in less than 36 hours
Extra-tropical Cyclones
 Cyclone Bomb
– This is the term applied to a rapidly intensifying cyclone,
and is defined as a central pressure drop of at least 24
mb in 24 hours
– Few cyclones meet this criteria, and most that do occur
in winter over a warm ocean surface current (e.g., Gulf
 Conveyor Belt Model
– This is an alternate, 3-D model to the steps discussed
previously (Norwegian cyclone model)
– Combines horizontal and vertical air motions
– Depicts the circulation in a mature cyclone in terms of
three broad interacting systems called conveyor belts,
which transport air with certain properties from one
location to another
 Belts are (1) warm and humid, (2) cold, and (3) dry
Extra-tropical Cyclones
 Conveyor Belt Model, continued
– (1) Warm and humid conveyor belt originates in the
cyclone’s warm sector
 Ascends slightly as it flows northward in the warm sector at low levels
and then ascends more rapidly over the warm front
 Helps explain the broad region of clouds/precipitation north of the
warm front
– (2) Cold conveyor belt originates north of the warm front
 Ascends as it progresses toward the west
 Forms the comma cloud and produces precipitation
 Turns clockwise at upper levels and follows westerly flow aloft
– (3) Dry conveyor belt
 This air originates high in the troposphere and low stratosphere
upstream of the upper-level trough
 One branch descends southward behind the cold front; the other
forms the dry slot that separates the head & tail of the comma cloud
Conveyor Belt Model
Extra-tropical Cyclones
Cyclone Weather
Figure represents an intensifying
cyclone in the Upper Midwest
Four sectors about the low center:
– Strong cold air advection,
stratiform clouds, and nonconvective precipitation
northwest of the low
– Cold front south of low is
accompanied by convective
precipitation. Sinking air and
mostly clear skies characterize
the southwest sector behind the
cold front.
– The mildest air is in the
southeast (warm) sector of the
– An extensive overrunning zone
is found to the northeast of the
low center
Principal Cyclone Tracks
 As a general rule, the cyclone center moves forward in the same direction
and at about one-half the speed of the 500-mb winds
 Principal storm tracks tend to converge toward the northeast
 Storm tracks appear to originate just east of the Rocky Mountains, but
actually form over the Pacific Ocean near Alaska
– As a cyclone travels over the mountains, it often loses its identity, but reforms
over the Great Plains
 Nor’easters often intensify off the North Carolina coast and track toward
the northeast along the East Coast
– 2 motions exist
 Movement of the cyclone along the coast
 Counterclockwise flow of winds around the storm center; winds in northeast sector
of the cyclone blow from the northeast (gives the name “nor-easter”)
– Some may become powerful systems; drawing copious amounts of water
vapor from the ocean and producing large amounts of precipitation over a
broad area
 Generally, cyclones that form in the south yield more precipitation
because they have access to greater amounts of mT air
 Cyclogenesis is more frequent in the winter when the mean position of the
polar front and jet stream shift southward
Principal Cyclone Tracks
Extra-tropical Cyclones
 Cold Side/Warm Side
– Storm track determines
weather at points on the
– Track A puts Chicago on the
warm side with passage of the
warm and cold fronts
– Track B puts Chicago on the
cold side with no frontal
– Table summarizes the general
sequence of weather
conditions at Chicago
Extra-tropical Cyclones
 Winter Storms
– An extra-tropical cyclone that produces any combination
of frozen or freezing precipitation
– An associated hazard is a cold wave, which often
follows a winter storm
– Necessary ingredients include cold air (typically brought
in by a sprawling cold high to the north), a moisture
supply, and uplift mechanisms
– A major storm requires warm and humid air brought
– A storm moving to the northeast produces heaviest
snow to the north and west of the low center
– Blizzard: a severe storm characterized by high winds
and reduced visibility due to falling or blowing snow
Colorado-track Winter Storm System
Extra-tropical Cyclones
Cold and Warm Core Systems
 An occluded cyclone is a
cold-core system
– Lowest temperatures occur in a
column just above the surface
– Depth of low increases with
– Cyclonic circulation prevails
throughout the troposphere and
is most intense at high altitudes
– The requirement that thickness
(mean temperature) be lowest
at the low center produces the
classic isobar pattern
Extra-tropical Cyclones
Cold- and Warm-Core Systems
 A non-occluded cyclone
is a warm-core system
– Lowest temperatures are
northwest of the cyclone’s
center, and highest
temperatures are to the
– Low aloft is displaced to
cold side of the storm
– The system tilts with
– Upper-level low lags
behind surface low
Vertical cross-section of a low from
northwest (cold) to southeast (warm)
Extra-tropical Cyclones
Cold and Warm Core Systems
 Warm-core cyclone (thermal low)
– Stationary, have no fronts, and are generally
associated with fair weather
– From over a broad expanse of arid/semiarid
land in response to intense solar heating of the
 Hot surface heats the overlying air and lowers the
density of the air column enough for a low to form
– Usually very shallow
– Anticyclone aloft overlies low
 In anticyclones, subsiding air and diverging surface winds favor
formation of a uniform air mass, no fronts, and generally fair skies
 Arctic and Polar Highs (cold-core anticylone)
– Labeled either a polar high (cP air) or arctic high (A air) and
are products of extreme radiational cooling, often over snowcovered land
– Clockwise circulation weakens with altitude, and may reverse
– Usually has a cold trough overlying it
– These exert the highest pressure
in winter
– They are extremely stable, with an
inversion in the lower km or so
– Interact with the circulation of an extra-tropical cyclone by
helping to maintain and strengthen the temperature contrast
along the cyclone’s cold front
 Warm High (warm-core anticyclone)
– Forms south of the polar front and consists of
extensive areas of subsiding warm and dry air
– These strengthen with altitude
– Examples are Bermuda-Azores high and
systems that may develop over the interior of
North America, especially in summer
– The greater mass of air over the anticyclone
center (related to a higher tropopause) is
responsible for the high surface pressure
– A cold-core anticyclone can become warm-core
© AMS as it moves south and modifies
 Anticyclone Weather
– Fair weather system because surface winds blowing in
a clockwise and outward direction (Northern
Hemisphere) induce subsidence over a broad area
– Arctic highs produce the lowest temperatures of winter
– A stalling warm anticyclone can lead to drought and
excessive summer heat
– A weak horizontal air pressure gradient near the center
leads to intense nighttime radiational cooling
– Ahead of an anticyclone, there may be strong northwest
winds ushering in polar or arctic air
 May bring heavy lake-effect snows to the lee side of the lakes
 In the summer, the most noticeable effect is not a lowering of
temperatures, but a lowering of humidity
– Highs may become entrenched east of the Rockies in
summer, and form blocking highs
 High temperatures and eventually drought result
Local and Regional Circulation Systems
Land and Sea (or Lake) Breezes
 Sea Breeze
– Under exposure to the same intensity
of solar radiation, the land surface
warms more than the water surface
– Highest pressure over water, and
cool breeze sweeps inland
– Shallow circulation has maximum
strength in mid-afternoon
– Uplift may lead to thunderstorms
 Land Breeze
– By late evening, winds blow offshore
due to a reversal in the heat
differential between land and water
– Obtains maximum strength around
sunrise but is weaker than a sea
Local and Regional Circulation Systems
Mountain or Valley Breezes
 Valley Breeze
– Bare valley walls absorb solar radiation
and heat the surrounding air
– Cooler, denser air over the valley sinks
and air adjacent to the valley walls
blows upslope
– Cumulus clouds may form near summit
– Best developed between late morning
and sunset
 Mountain Breeze
– Bare valley walls are chilled by
radiational cooling and cool the
surrounding air
– Colder, denser air near the valley walls
sinks and gusty breeze blows
– Fog or low stratus clouds may form in
the valley
Local and Regional Circulation Systems
Chinook Winds
 A relatively warm and dry wind
that develops when air
descending the leeward slopes
of a mountain range is
adiabatically compressed
 Strong winds cause stable air in
the lower troposphere to ascend
on the windward side
 On the leeward side, stable air
descends to the original altitude,
and the larger scale of
circulation causes further
 Called Santa Ana winds in
southern California
– The figure is a schematic
representation of the surface
weather pattern that favors
© AMS development
Local and Regional Circulation Systems
Chinook Winds
 Boulder, CO, situated in
the foothills of the Rocky
Mountains, experiences
particularly strong and
destructive downslope
winds, sometimes gusting
to 160 km per hr (100
 On average, the
community sustains about
$1 million in property
damage each year due to
these winds
Local and Regional Circulation Systems
Katabatic Winds
 Driven by gravity
– Shallow layer of cold, dense air flows downhill
– Usually originates in winter over an extensive snowcovered plateau or highland
– Best known katabatic winds are the mistral and bora
 Mistral: descends from Alps down the Rhone River Valley of
France into the Gulf of Lyons
 Bora: originates in high plateau region of Croatia and cascades
onto coastal plain along the Adriatic Sea
– Usually weak with speeds less than 10 km per hr
Local and Regional Circulation Systems
Desert Winds
 Hot surfaces (i.e., deserts) may develop superadiabatic
lapse rates in the lowest levels of the atmosphere
– These are highly unstable, and generate vigorous upwelling and
gusty surface winds, but very few clouds form
 A dust devil is a whirling mass of dust-laden air formed by
localized hot spot
– Air is heated, and rises rapidly
– Cooler surface winds converge on the hot spot
– Horizontal wind shear causes the column of rising hot air to spin
about a nearly vertical axis
– Dust and debris are picked up, making these visible to altitudes
topping 900 m (3000 ft)
– May cause damage, as some have winds as higher than 75 km per
hr (45 mph)
 Strong thunderstorm downdrafts may generate dust storms
known as a haboob
Local and Regional Circulation Systems
Desert Winds
Dust storm
Dust devil