Atmospheric Circulations

1
Atmospheric Circulations

• Chapter 7

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A Fluid Atmosphere

• Easier to envision atmospheric motions if we
  think of our atmosphere as a fluid
• Thats what it really is
• Air motions are similar to those in any fluid
• Oceans, rivers, etc.
• All kinds of circulations ranging from
  microscopic whirls to large storms (low pressure
  systems) to global wind patterns (jet streams)

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Water vs Air
Eddies
Gulf Stream
4
Scales of Motion

• In meteorology, scales of motion describe the
  size and duration of circulations
• Smallest scale were concerned with is the
  microscale, largest is the global scale
• And everything in between

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Scales of Motion

• Global
• Synoptic
• Meso
• Micro

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Scales of Motion
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Scales of Motion

• Often, its hard to tell what kind of motions are
  going on since you cant see air
• Cant see turbulence in a plane right?

8
Lenticular cloud
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Scales of Motion

• Sometimes you can see whats going on when
  motions are visible in clouds

10
Local Wind Systems

• Interesting topic because they exist almost
  everywhere
• Coastal regions - Seabreezes
• Mountains - Chinook winds, mountain/valley
  breezes
• Often due to 1) terrain or 2) temperature
  differences from one place to another
• If due to 2) - we call it a thermal circulation

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Thermal Circulations

• These circulations are due to differences in
  temperature
• Like between air over land vs air over water
• The temp differences cause something to happen to
  the pressure
• And what happens when the pressure is different
  from place to place??
• WIND

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Thermal Circulations

• Initially, if temps are uniform, so are pressure
  surfaces
• Going upward, pressure surfaces are horizontal
• But what if air to the south heats up?

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Thermal Circulations

• Remember from earlier, if a column of air warms,
  it stretches vertically
• So, the pressure surfaces are now higher in the
  warmer air causing a pressure gradient force -
  wind

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Thermal Circulations

• To the south, a thermal low pressure develops
  at the surface
• Due to air moving toward north aloft
• To the north, high pressure develops at surface
• Due to air piling up aloft

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Thermal Circulations

• So, air begins to move from high to low at both
  the surface and aloft
• Air at the surface warms near the low and rises
• Air aloft cools over the surface high and sinks

16
Sea Breezes

• Sea breezes are a good example of a thermal
  circulation
• Form in tropical/sub-tropical regions in the warm
  season
• People from coastal areas of FL know about these
  - nice at the beach but reeeaaall dark inland
• Can also have lake breezes and river breezes
  which form in the same way

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Sea Breezes

• Same as before except now the water/land is
  causing the contrast in temp/pressure
• Thermal low develops over land and wind at the
  surface moves from water to land

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Sea Breezes

• As before, air rises in the vicinity of the
  thermal low and often produces thunderstorms
  inland
• Thats why the summer weather at the coast is
  usually better than weather inland (in terms of
  sunshine)

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Sea Breezes-lightning
T-storms
East Coast Sea Breeze
Sea Breeze in panhandle
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Sea Breezes

• How would you know that a sea breeze has passed?
• Temp ?
• Decreases (air off of the water is cooler)
• Humidity ?
• Increases (air off of the water is more moist)
• Wind ?
• Shifts to onshore and increases in speed
• Sky ?
• Clear after passage, cloudy before

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Sea Breezes

• Florida gets hit often by thunderstorms due to
  sea breezes
• Sometimes sea breezes from both coasts converge
  over the central peninsula (Orlando in particular)

22
T-storm frequency
23
Sea Breezes

• Which raises an interesting question

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Land Breezes

• Remember from several weeks ago we said that land
  heats up and cools down quicker than water?
• Happens at night causing a reversal of the sea
  breeze
• Called a land breeze

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Land Breezes

• Air over land cools quickly at night
• So now air over water is warmer than air over
  land
• Winds switch to offshore
• generally weaker than the sea breeze

26
Monsoon

• What kind of image does that word conjure up?
• Lots of heavy rain?
• Floods?
• Actually it means
• A seasonal shift in wind patterns
• So it doesnt directly address rain at all
• The monsoon is a thermal circulation just like
  the sea breeze but it occurs as a seasonal cycle
  - not a daily cycle

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Asian Monsoon

• Asian monsoon is the most well developed one
• Weaker monsoon over the desert southwest of N.
  America
• Very cold air over the continent during winter
• Causes high pressure and flow toward the water
• This also keeps moisture offshore
• So, very dry over land in the winter

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Asian Monsoon

• Things (winds) change during the summer as the
  land heats up and low pressure develops
• Now wind moves onshore and brings with it very
  moist air
• This region, particularly central India, gets
  absolutely hammered w/ rain
• Some places over 400 inches a year! (most of it
  coming in 6-7 months!)

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Asian Monsoon

• Why so much rain (other than the obvious)?
• 1) Convergence of air enhances upward motion
• 2) Terrain

Lots of mountains
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Chinook Winds

• Snow eaters
• Warm, dry winds on the eastern side of the Rocky
  Mountains
• Due to compressional heating as air descends
• Enhanced if clouds form on the western side
• Can cause warming of gt40ºF in 1 hour!

31
Chinook Winds

• Chinook wall cloud - precursor of Chinook winds
• 20ºF when this pic was taken - 60ºF same time
  next day

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Chinook Winds

• How they work
• Any air which descends will warm by compression -
  compressional heating
• main source of warmth for Chinooks
• But if clouds form on the western side of the
  mountains, the Chinook wind will be even warmer
  and drier than w/o clouds
• Why?
• Have to go back to the moist/dry adiabatic stuff
• Remember moist rate6ºC/1000m, dry
  rate10ºC/1000m

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Chinook Winds

• Air which starts at 10ºC on the western side,
  ends up at 18ºC on the eastern side.
• Due to cooling at the moist rate starting at 1000
  m..but warming at the dry rate going down
• So, clouds enhance the Chinook winds

Cooling at moist rate
Warming at dry rate
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Chinook Winds

• Very localized - just near the mountain base
• As warm air hits the cold air below, the boundary
  can oscillate
• Cities near the boundary can experience rapid
  back-and-forth changes in temperature

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Chinook Winds

• One case in South Dakota
• 530 a.m. -4ºF
• 940 a.m. 54ºF
• 1030 a.m. 11ºF
• 1045 a.m. 55ºF

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Santa Ana Winds

• Named after the Santa Ana Canyon in S.California
  where they are often funneled through
• Like Chinooks - compressional heating causes them
• Air descends from high plateaus
• Hot and dry
• Major problem associated with them are forest
  fires
• Winds dry things out AND fan flames

37
General Circulation

• From earlier, there are all kinds of local wind
  systems - many more than I went over
• All kinds of smaller circulations too
• But what about the global wind patterns?
• If we average winds over a long period of time,
  the small stuff disappears and we are left with
  the global scale winds or general circulation
  of the atmosphere
• Keep in mind - just an average
• Can be different at any particular instant

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General Circulation

• A look at the average global winds can give us
  insight into
• 1) Driving mechanisms behind the winds
• 2) Model of how heat is transferred from the
  tropics to the poles
• Already established the fact that uneven heating
  of the earth is what causes weather
• Including wind
• In a nutshell tropics get excess energy while
  polar regions have an energy deficit
• Wind tries to balance things

39
Single Cell Model

• Very simplified model but still gives us some
  insight. 3 Key assumptions
• 1) All water over the earth
• 2) Sun always over equator
• 3) No rotation
• Result
• High pressure at the poles
• Low pressure at the equator
• 1 huge convective cell in each hemisphere
• Called Hadley Cells after the guy who
  postulated the idea.

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Single Cell Model

• Cold air flows toward tropics from poles (H to L)
  at the surface
• Warm air flows toward poles from tropics aloft
• Just like a sea breeze circulation
• So, some of the excess energy at the tropics is
  transferred to the poles
• What if we get rid of assumption 3 and let the
  earth rotate?

41
Three Cell Model

• Now we have 3 cells in each hemisphere rather
  than one.
• More realistic
• Still have H pressure at poles and L pressure at
  equator
• Also now, H and L pressure at 30º and 60º
  respectively

H
L
H
L
H
L
H
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Three Cell Model

• Look at the vertical motion where the cells meet
• Equator Rising air
• Lots of rain here
• 30º Sinking air
• Most of the worlds deserts lie near this
  latitude
• 60º Rising air
• This is where most big mid-latitude storms form
• So this model does a better job with surface
  winds

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Three Cell Model

• Not too bad huh?
• Everything is shifted to the south due to seasons
• Cloud bands where the model says low pressure
  should be
• Rising air

Today
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Surface Winds

• Intertropical Convergence Zone (ITCZ)
• near the equator
• tropical systems form here
• Trade winds
• named such because sailing vessels used these
  winds to travel west
• not much wind poleward of the trades (horse
  latitudes)
• Westerlies
• where we live
• Polar Easterlies

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The Real World

• Still two assumptions in the three cell model
• all water/no land and sun always over equator
• These factors change things a bit and give us the
  real pressure/wind fields.

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The Real World

• Semi-permanent features
• Lows near 60º
• Highs near 30º

January
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The Real World

• Strength and position of these features changes
  with the seasons
• The Highs on either side of the U.S. have a great
  impact on weather at each coast

July
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Semi-permanent Highs

• Generally sinking air on the east side of highs
  and rising air on the west side
• Summers are dry on the west coast and wet on the
  east coast

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Semi-permanent Highs

• LA (west coast) gets almost no rain during the
  summer
• Atlanta (east) gets a great deal of rain during
  the summer months

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Jet Streams

• Fast flowing rivers of air at upper levels
• 33,000-46,000 ft usually
• On average 100s - 1000s miles long, 100 miles
  wide, 1 mile deep
• Existence first confirmed by military pilots
  during WWII
• Suspected beforehand due to fast moving cirrus
  clouds
• Important because they can show us where cold air
  is and where storms will track
• Also if storms will intensify

51
Jet Streams

• Like all winds, jets occur due to horizontal
  differences in pressure
• Sharp north/south temperature/pressure
  differences aloft
• The temperature contrast is at a maximum during
  the winter
• Jet streams are strongest during winter season

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Jet Streams

• Cold air to the north side
• Warm air to the south
• So, we get our coldest weather when the jet
  stream dips to the south over the SE U.S.
• Like last week

Cold
Warm
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Jet Streams

• Cold air heading into the East
• Strong N-S component (like now), the flow is
  called meridional
• Active weather patterns
• Flow more E-W, called zonal
• Cold air pretty much stays to the north

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Jet Streams

• Jet stream as simulated with a mesoscale model
• Japan Jet
• Strongest jet stream (on average) in the world
• Orange indicates wind speeds of near 200 mph!
• Jet Streak - highest wind speeds w/in jets

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Atmosphere/Ocean Interaction

• Water temps along the West Coast are cold!
• Along the East Coast, temps are much warmer
• Why?

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Atmosphere/Ocean Interaction

• Winds are from the north along the coast causing
  the current to flow toward the south
• The Coriolis force deflects the surface water to
  the right
• Now, cold water from below moves up to replace it
• Called upwelling

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ENSO -El Niño Southern Oscillation

• In the news a lot several years ago
• 2-7 year cycle
• Weakening or reversal of the Pacific trade winds

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ENSO

• Normally, low pressure exists over the western
  Pacific and high pressure dominates over the
  eastern Pacific
• So, winds blow from east to west at the surface

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ENSO

• The wind causes warm water to move west and
  upwelling occurs near the coast of S. America
  (cold water up to the surface)

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ENSO

• The upwelling along the coast is essential for
  the fishing (anchovies) industry
• Cold water from below reaches the surface and is
  rich in nutrients (fish like it)

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ENSO

• Non-El Niño sea surface temperatures

Cold
Warm
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ENSO

• Every few years, the surface pressure pattern
  breaks down causing a weakening or even reversal
  of the trade winds
• Now warm water is pushed toward S. American coast

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ENSO

• Lasts a year or two on average
• Devastates the fishing industry in this region
• Changes weather patterns worldwide

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ENSO

• El Niño sea surface temperatures

Warm
Cold
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ENSO

• Warm water spreading east toward the S. American
  coast

March 17
March 27
April 6
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ENSO Worldwide

• So does it just affect weather in the Pacific
  where it occurs?
• No way