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The General Circulation of the Atmosphere. Weather with different scales. What we need to know for today. Pressure gradient force: from H igh to L ow pressure Coriolis force (effect): Results from the rotation of the planet. Maximum at the poles and no effect at the equator.

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what we need to know for today
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).
average wind structure
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 Earth’s 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).
the single cell model
The Single Cell Model
  • This is a very simplified model based on the following three assumptions:
  • The Earth’s 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.
thermal circulations
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
the hadley cell
The Hadley Cell
  • It is driven by the uneven heating of the Earth’s 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.

the one cell model does not work
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

Intertropical convergence zone

Observing global winds

from space

winds aloft
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.
the three cell model
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 -> Coriolis force!
three cell model the hadley cell 0 30 deg
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 -> major deserts
    • Air converges (follow the meridians on a globe) – high surface pressure
    • Horse latitudes: small PG, weak horizontal winds -> sailors get stuck
    • Surface winds in NH: from the northeast (Trade winds)
three cell model the ferrel cell 30 60 deg
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

three cell model the polar cell 60 90 deg
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)
the converging diverging regions
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
winds and pressure in the real world
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



Subtropical highs

Subtropical highs

the general circulation and precipitation paterns
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
winds and pressure systems aloft
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 -> the winds are stronger.