The General Circulation of the Atmosphere

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The General Circulation of the Atmosphere
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RECAP
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What we need to know for today

• Pressure gradient force from High to Low
  pressure
• Coriolis force (effect)
• Results from the rotation of the planet.
• Maximum at the poles and no effect at the
  equator.
• Acts perpendicular to the direction of motion
  changes the direction of the wind but not the
  wind magnitude.
• In the NH deflects the wind to the right.
• In the SH deflects the wind to the left.
• Winds aloft
• Balance between the pressure force and the
  Coriolis force.
• The wind is parallel to the isobars.
• Surface winds
• Balance between the pressure gradient force, the
  Coriolis force and the air friction.
• The wind crosses the isobars (from High to Low
  pressure).

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Surface Winds-a balance of three forces

• In the boundary layer (1km thick) friction is
  important!
• Friction is acting opposite the direction of the
  velocity -gt friction reduces the wind speed -gt
  the Coriolis force becomes weaker -gt it cannot
  balance the pressure force.
• The wind starts to blow across the isobars
  towards the low pressure
• The angle between the direction of the wind and
  the isobars is on average 30 deg (Buys-Ballots
  law). It depends on the topography.

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Average Wind Structure

• The direction and the magnitude of the winds at a
  given location can vary significantly during the
  day, and from day to day.
• The general circulation (GC) refers to the
  average (the prevailing) winds on a global scale
  (around the world).
• The GC of the atmosphere is the result of the
  uneven heating of the Earths surface.
• It is impacted by the Earths rotation.
• The GC transports and redistributes energy from
  one region to another (warm air towards the poles
  and cold air towards the equator).

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The Single Cell Model

• This is a very simplified model based on the
  following three assumptions
• The Earths surface is uniformly covered with
  water (no differential heating of the land and
  the oceans)
• The sun is always directly over the equator (no
  seasonal variations of the winds).
• The Earth does not rotate.
• No Coriolis effect.
• The only active force is the pressure gradient
  force.

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Thermal circulations

• Due to uneven heating of the surface. Example
• South area heats up, North area cools
• Warmer southern air aloft moves north towards low
  pressure
• It then cools and sinks
• Surface pressure to the North increases
• Surface wind from N to S
• The surface air warms up and rises.
• The process continues

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The Hadley Cell

• It is driven by the uneven heating of the Earths
  surface by the sun - thermally direct cell warm
  air rises, cold air sinks.
• One Hadley cell in each hemisphere.
• The equator is warmer than the poles.
• Warm moist air at the equator rises upwards
• It expands, cools, and saturates, the water
  vapor condenses and forms clouds.
• It creates low surface pressure
• in the tropics.
• At the poles we have cool, dry,
• sinking air that creates high surface
• pressure in the polar region.
• The PGF (pressure gradient force) drives
• the surface winds from the poles towards
• the equator.
• The winds aloft close the cell by blowing
• from the equator towards the poles.

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The one cell model does not work!

• It is obviously wrong predicts northern
  prevailing winds everywhere in the NH
• What is wrong with the model? It is too simple!
• The rotation of the Earth will deflect the winds
  to the right in the Northern hemisphere and to
  the left in the Southern hemisphere.
• This will result in surface winds blowing
• From the East (easterlies) in the NH
• From the East (easterlies) in the SH
• This will result in winds aloft blowing
• From the West (westerlies) in the NH
• From the West (westerlies) in the SH

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Intertropical convergence zone
Observing global winds from space
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Winds Aloft

• Warm air above the equator and cold air above the
  polar regions
• Higher pressure at the equator, lower pressure
  both to the north and to the south of the equator
• The pressure gradient force is towards the poles,
  sets the air
• in motion
• The Coriolis force
• NH to the right
• SH to the left
• The wind turns right in the NH and left in the
  SH, becomes parallel to the isobars
• Westerly winds aloft in both the NH and SH.
• Easterly winds at the surface in both the NH and
  SH.

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The Three Cell Model

• Keep two of the assumptions, relax the third
• The Earth is covered with a continuous ocean
• The sun is always directly over the equator
• The Earth rotates -gt Coriolis force!

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Three cell model the Hadley cell (0-30 deg)

• Thermally direct cell warm air rises, cool air
  sinks
• Intertropical Convergence Zone (ITCZ)
• A.k.a. equatorial doldrums
• Warm air, weak PGF, light winds, cumulus clouds
  and thunderstorms
• Air rises up to the tropopause, then laterally
  toward the poles
• Deflected east due to the CF
• Winds aloft in NH from southwest
• Subtropical highs (anticyclones)
• Equatorial air cools, sinks, warms up, clear
  skies -gt major deserts
• Air converges (follow the meridians on a globe)
  high surface pressure
• Horse latitudes small PG, weak horizontal winds
  -gt sailors get stuck
• Surface winds in NH from the northeast (Trade
  winds)

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Three cell model the Ferrel cell (30-60 deg)

• Thermally indirect cell cool air rises and warm
  air sinks
• Some of the sinking air in the horse latitudes
  heads toward the pole
• Deflected east by the CF
• Surface winds in NH from the southwest
  (westerlies)
• At the polar front the westerlies encounter cold
  air moving down from the poles
• Air is forced to rise, some of it returns to the
  horse latitudes, completing the Ferrel cell, the
  rest heads for the pole
• Upper air winds in the Ferrel cell from the
  northeast.

William Ferrel
William Ferrell
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Three cell model the polar cell (60-90 deg)

• It is a Hadley type of circulation.
• Surface winds from the north east (polar
  easterlies)
• Upper winds in NH from the southwest
• Summary two major areas of Low pressure (ITCZ
  and subpolar low), and two of High pressure
  (poles and subtropical highs)

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The converging/diverging regions

• ITCZ (Intertropical Convergence Zone ) - Equator
• Low surface pressure with small PG and weak
  horizontal winds.
• Upward motion of warm moist air. Results in
  convective cloud towers

• Subtropical highs (the horse latitudes) 30N
  30S
• High surface pressure
• The upper air is sinking, warms up and the
  relative humidity is very low.
• Weak winds, clear sky, dry climate large
  deserts at these latitudes.
• Subpolar lows (polar front) 60N, 60S
• A converging zone at the surface. Air moves up
  and results in strong storms.
• Weak winds
• Polar highs 90N, 90S

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Winds and pressure in the real world

• Semi-permanent highs and lows persist throughout
  the year, correspond to converging/diverging
  upper air masses.
• Bermuda, Pacific highs Icelandic, Aleutian lows
• Seasonal highs and lows (continents heat/cool
  faster)
• Winter Siberian high, Canadian high
• Summer (thermal lows) Southwest US, Iran

July
January
Subtropical highs
Subtropical highs
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The General Circulation and Precipitation Paterns

• Converging surface flows
• Low surface pressure
• Uprising air
• Heavy precipitation
• Diverging surface flows
• High surface pressure
• Sinking air
• Dry climate

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Winds and Pressure Systems Aloft

• The wind system aloft differs from the surface
  wind system. It is close to a geostrophic flow.
• There is no significant friction with the ground.
• The three cell model does not work that well in
  the middle latitudes.
• The winds aloft are stronger than on the ground.
• In the winter the gradients are bigger -gt the
  winds are stronger.

July
January