Atmospheric Circulations

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Chapter 7

• Atmospheric Circulations

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Atmospheric Circulations

• Scales of motion
• Eddies
• Local winds
• Thermal circulations
• Sea and land breezes
• Seasonally changing winds
• Mountain and valley breezes
• Katabatic winds
• Chinook (Foehn) winds
• Santa Ana winds
• Desert winds
• Global winds
• Three cell model
• Average surface winds and pressure

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Scales of Atmospheric Motion
The tiny microscale motions... constitute a part
of the larger mesoscale motions... which, in
turn, are part of the much larger synoptic scale.
Notice that as the scale becomes larger, motions
observed at the smaller scale are no longer
visible.
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Scales of Atmospheric Motion

• The hierarchy of motion from tiny gusts to giant
  storms
• Microscale smallest scale of motion- tiny
  eddies within smoke Typical size 2 m
• Mesoscale (middle scale) the circulation of
  city air range in size from a few km to about
  100 km. (Local winds, thunderstorms, tornadoes)
  Typical size 20 km
• Synoptic Scale typical weather map scale that
  shows features such as high/low pressure areas,
  fronts etc. Typical size 2000 km
• Planetary scale largest scale of motion
  sometimes called global scale (Longwaves in the
  atmosphere) Typical size 5000 km

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Eddies

• When the wind encounters a solid object, a whirl
  of air or eddy forms on the objects downwind
  side. The size and shape of the eddy often
  depends upon the size and shape of the obstacle
  and the speed of the wind.

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Winds flowing past an obstacle.(a) In stable
air, light winds produce small eddies and little
vertical mixing.
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Greater winds in unstable air create deep,
vertically mixing eddies that produce strong,
gusty surface winds.
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As the wind blows over the mountain peak, the
direction of air flow is disturbed and circular
eddies form on the mountain's leeward (downwind)
side. The shallow cloud pattern indicates that
some eddies are circulating clockwise while
others are circulating counterclockwise. Eddies
such as these are commonly called von Karman
vortices
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Mountain Waves and Rotors
Under stable conditions, air flowing past a
mountain range can create eddies many kilometers
downwind from the mountain itself.
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Thermal Circulations

• A thermal circulation is produced by the heating
  and cooling of the atmosphere near the ground.
  The lines represent surfaces of constant pressure
  (isobaric surfaces).  In this example, the
  isobars are parallel to the earths surface-
  there is no horizontal variation in pressure or
  temperature- no PGF and therefore no wind

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Thermal Circulations

• A thermal circulation produced by the heating and
  cooling of the atmosphere near the ground. The
  H's and L's refer to atmospheric pressure. The
  lines represent surfaces of constant pressure
  (isobaric surfaces). Suppose the air is cooled
  north and warmed south. PGF causes air to move
  from High to Low pressure

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Thermal Circulations

• A thermal circulation produced by the heating and
  cooling of the atmosphere near the ground. The
  H's and L's refer to atmospheric pressure. The
  lines represent surfaces of constant pressure
  (isobaric surfaces). Air aloft moves from south
  to north, air leaves the southern area and piles
  up above northern area. PGF is established at
  surface and winds flow from north to south at the
  surface. We now have a thermal circulation- air
  flow resulting primarily from the uneven heating
  and cooling of air.

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Sea Breeze

• Development of a sea breeze and a land
  breeze.At the surface, a sea breeze blows from
  the water onto the land...

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Land Breeze

• the land breeze blows from the land out over the
  water. Notice that the pressure at the surface
  changes more rapidly with the sea breeze. This
  situation indicates a stronger pressure gradient
  force and higher winds with a sea breeze.

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Sea Breeze Front

• Leading edge of the sea breeze
• Produces a rapid drop in temperature just behind
  it (as much as 9F)
• Cumulus clouds often form along this front
• Main cause of Floridas abundant rainfall

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Seasonally Changing Winds

• Monsoon Wind System changes directions
  seasonally blows from one direction in summer
  and the opposite direction in the winter.
• Especially well-developed in eastern and southern
  Asia
• During winter, air over the continent becomes
  much colder than air over ocean. High pressure
  sets up over Siberia and air flows from land to
  the ocean.

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Changing annual wind flow patterns associated
with the winter Asian monsoon. Clear skies and
winds blow from land to sea
Changing annual wind flow patterns associated
with the summer Asian monsoon. Warm humid air
blows up from equator bringing rainy weather.
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Mountain and Valley Breezes
Valley breezes blow uphill during the day
mountain breezes blow downhill at night. (The L's
and H's represent pressure, whereas the purple
lines represent surfaces of constant pressure.)
Mountain breezes are also called gravity or
nocturnal drainage winds.
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As mountain slopes warm during the day, air rises
and often condenses into cumuliform clouds, such
as these.
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Katabatic Winds

• Any downslope wind
• Usually reserved for downslope winds that are
  much stronger than mountain breezes.
• Katabatic (or fall) winds can rush down slopes at
  hurricane speeds, but most are not that intense
  and many are on the order of 10 kts or less.
• Bora a cold gusty northeasterly wind with
  speeds sometimes in excess of 100 kts. (Northern
  Adriatic)
• Mistral cold (less violent wind) that descends
  the western mountains and into the Rhone Valley
  of France

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Chinook (Foehn) Winds

• Warm dry wind that descends the eastern slope of
  the Rocky Mountains.
• Region of influence extends from NE New Mexico
  into Canada. (Similar winds occur along the
  leeward slope of the Alps)
• Snow Eater
• Occur when strong westerly winds aloft flow over
  a N-S trending mountain range producing low
  pressure on the eastern side of the mountains.
  This trough of low pressure forces air downslope.
  As the air descends it is compressed and warms.
  (Compressional heating)

See pg 177 of book
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Conditions that may enhance a chinook.
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A chinook wall cloud forming over the Colorado
Rockies (viewed from the plains)
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Santa Ana Winds

• Warm dry wind that blows from the east or
  northeast into Southern California
• Air descends from the elevated desert plateau, it
  funnels through mountain canyons in the San
  Gabriel and San Bernardino Mountains, finally
  spreading over the Los Angeles Basin and San
  Fernando Valley
• It lifts dust and sand, dries out vegetation,
  sets stage for serious brush fires, especially in
  Fall. (1961 Bel Air Fire burned for 3 days,
  destroyed 484 homes and caused 25 million in
  damage.

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Santa Ana Winds

• Surface weather map showing Santa Ana conditions
  in January. Maximum temperatures for this
  particular day are given in F. Observe that the
  downslope winds blowing into Southern California
  raised temperatures into the upper 80's, while
  elsewhere temperature readings were much lower.

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Desert Winds

• Sandstorms
• Haboob
• Dust devils (whirlwinds)

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Desert Winds
A haboob approaching Phoenix, Arizona. The dust
cloud is rising to a height of about 450 m (1475
ft) above the valley floor. Haboob forms as
cold downdrafts along leading edge of a
thunderstorm lift dust or sand into huge dark
cloud that may cover over a hundred kilometers
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Dust Devil

• The formation of a dust devil. On a hot, dry day,
  the atmosphere next to the ground becomes
  unstable. As the heated air rises, wind blowing
  past an obstruction twists the rising air,
  forming a rotating column, or dust devil. Air
  from the sides rushes into the rising column,
  lifting sand, dust, leaves, or any other loose
  material from the surface.

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Dust Devil

• A dust devil forming on a clear, hot summer day
  just south of Phoenix, Arizona.
• Dust devils are not tornadoes.

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General Circulation of the Atmosphere

• Represents the average air flow around the world
• Actual winds at any one place may vary
  considerably from average
• Model for how heat is transported from the
  equator to the poles
• Underlying cause is the unequal heating of
  earths surface

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Global Winds - Single Cell ModelHadley Cell

• The general circulation of air on a non-rotating
  earth uniformly covered with water and with the
  sun directly above the equator. (Vertical air
  motions are highly exaggerated in the vertical.)
• Does not actually exist on earth (Coriolis
  produces zonal winds at all latitudes)

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Hadley Cell

• Diagram shows the names that apply to the
  different regions of the world and their
  approximate latitudes.

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Three Cell Model

• The idealized wind and surface-pressure
  distribution over a uniformly water-covered
  rotating earth.
• Tropical regions still receive an excess of heat
  and the poles a deficit
• Doldrums equatorial waters, air is warm,
  horizontal pressure gradients are weak, winds
  light.
• Horse latitudes ?

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Three Cell Model

• Trade winds blow from northeast in N. Hemi and
  southeast in S. Hemi
• Intertropical Convergence Zone Surface region
  of convergence between trades
• Subtropical Highs semipermanent high in the
  subtropical high pressure belt centered near 30
  latitude. Bermuda high, Pacific Ridge.
• Westerlies prevailing westerly flow

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Three Cell Model

• Polar Front A semi permanent semicontinuous
  front that separates tropical air masses from
  polar air masses
• Subpolar low Belt of low pressure between 50
  and 70 latitude. In N. Hemi this belt consists
  of the Aleutian low and the Icelandic low.
• Polar easterlies a shallow body of easterly
  winds located at the high latitudes poleward of
  the subpolar low.

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Three Cell Model (wind regimes)

• Diagram shows the names of surface winds and
  pressure systems over a uniformly water-covered
  rotating earth.

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Semi-permanent pressure systems

• Highs and Lows that move only slightly during the
  course of the year
• Bermuda High (Atlantic)
• Pacific High or Pacific Ridge
• Greenland-Icelandic Low or Icelandic Low
• Aleutian Low

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Average sea-level pressure distribution and
surface wind-flow patterns for January. The heavy
dashed line represents the position of the ITCZ.
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Average sea-level pressure distribution and
surface wind-flow patterns for July. The heavy
dashed line represents the position of the ITCZ.
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A winter weather map depicting the main features
of the general circulation over North America.
Notice that the Canadian high, polar front, and
subpolar lows have all moved southward into the
United States, and that the prevailing westerlies
exist south of the polar front. The arrows on the
map illustrate wind direction.
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Idealized View of Major Pressure Systems

• Major pressure systems and idealized air motions
  (heavy blue arrows) and precipitation patterns of
  the general circulation. (Areas shaded light blue
  represent abundant rainfall.)

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General Circulation and Precipitation

• The position of the major features of the general
  circulation and their latitudinal displacement
  strongly influence the climate of many areas.
• In general we expect abundant rainfall where air
  rises (tropics and ITCZ) very little where it
  sinks (vicinty of subtropical highs and polar
  regions).

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During the summer, the Pacific high moves
northward. Sinking air along its eastern margin
produces a strong subsidence inversion, which
causes relatively dry weather to prevail. Along
the western margin of the Bermuda high, southerly
winds bring in humid air, which rises, condenses,
and produces abundant rainfall
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Average annual precipitation for Los Angeles,
California, and Atlanta, Georgia. Note they are
on different sides of semi-permanent highs
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Westerly winds and the Jet Stream

• Relatively strong winds concentrated within a
  narrow band in the atmosphere.
• Nearly continuous
• Jet Core often exceeds 100 kts
• Found at the tropopause at elevations between
  33,000 and 46,000 ft, but may occur at higher and
  lower altitudes.

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A jet stream is a swiftly flowing current of air
that moves in a wavy west-to-east direction. In
the Northern Hemisphere, it forms along a
boundary where colder air lies to the north and
warmer air to the south. In the Southern
Hemisphere, it forms where colder air lies to the
south and warmer air to the north.
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Jet Streams

• Subtropical jet stream the jet stream typically
  found between 20 and 30 latitude and at
  altitudes between 12 and 14 km.
• Polar front jet stream (Polar jet) the jet
  stream that is associated with the polar front in
  middle and high latitudes. Usually located
  between 9 and 12 km.
• Low level jets peak winds less than 60 kts that
  form above the Central Plains during the summer-
  contributes to thunderstorm activity
• Tropical easterly jet summertime over tropics
• Stratospheric polar jet forms in the polar
  winter near the top of the stratosphere

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Average position of the polar jet stream and the
subtropical jet stream, with respect to a model
of the general circulation in winter. Both jet
streams are flowing into the page, away from the
viewer, which would be from west to east.
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Position of the polar jet stream and the
subtropical jet stream at the 300-mb level during
the morning of March 10, 1998. Solid gray lines
are lines of equal wind speed (isotachs) in
knots. Heavy lines show the position of the jet
streams. Heavy blue lines show where the jet
stream directs cold air southward, while heavy
red arrows show where the jet stream directs warm
air northward.
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Global Wind Patterns and the Oceans

• Winds blowing over the oceans cause the surface
  water to drift along.
• The general atmospheric circulation influences
  the ocean currents.
• Ocean currents move with the prevailing wind.
• Gulf stream, California current, etc.

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Average position and extent of the major surface
ocean currents. Cold currents are shown in blue
warm currents are shown in red. Names of the
ocean currents are given in Table 7.2.
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The Gulf Stream (dark red band) and its eddies
are revealed in this satellite mosaic of sea
surface temperatures of the western North
Atlantic during June, 1984. Bright red shows the
warmest water (about 27C or 80F), followed by
orange and yellow. Green, blue, and purple
represent the coldest water.
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Why is the air cool off the west coast?

• Average sea surface temperatures (F) along the
  west coast of the United States during August.

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Upwelling

• As winds blow parallel to the west coast of North
  America, surface water is transported to the
  right (out to sea). Cold water moves up from
  below (upwells) to replace the surface water.

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Under ordinary conditions, higher pressure over
the southeastern Pacific and lower pressure near
Indonesia produce easterly trade winds along the
equator. These winds promote upwelling and cooler
ocean water in the eastern Pacific, while warmer
water prevails in the western Pacific. When the
trades are exceptionally strong, water along the
equator in the eastern Pacific becomes quite
cool. This cool event is called La Niña.
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During El Niño conditions, atmospheric pressure
decreases over the eastern Pacific and rises over
the western Pacific. This change in pressure
causes the trades to weaken or reverse direction.
This situation enhances the countercurrent that
carries warm water from the west over a vast
region of the eastern tropical Pacific. The
thermocline, which separates the warm water of
the upper ocean from the cold water below,
changes as the ocean conditions change from
non-El Niño to El Niño.
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Average sea surface temperature departures from
normal as measured by satellite.(a) During El
Niño conditions, upwelling is greatly diminished
and warmer than normal water (deep red color),
extends from the coast of South America westward,
across the Pacific.
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During La Niña conditions, strong trade winds
promote upwelling, and cooler than normal water
(dark blue color) extends over the eastern and
central Pacific.
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Typical winter weather patterns across North
America during an El Niño warm event.
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Typical winter weather patterns across North
America during a La Niña cold event.
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Regions of climatic abnormalities associated with
El Niño-Southern Oscillation conditions. A strong
ENSO event may trigger a response in nearly all
indicated areas, whereas a weak event will likely
play a role in only some areas. Note that the
months in black type indicate months during the
same years the major warming began months in red
type indicate the following year. (After NOAA
Climatic Prediction Center.)
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