The General Circulation of the
Atmosphere
Weather with different scales
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
The direction and the magnitude of the winds at a given
location can vary significantly during the day, and from
day to day.
•
• The GC of the atmosphere is the result of the uneven
•
•
The general circulation (GC) refers to the average (the
prevailing) winds on a global scale (around the world).
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
This is a very simplified model
based on the following three
assumptions:
1.
The Earth’s surface is uniformly
covered with water (no differential
heating of the land and the oceans)
2.
The sun is always directly over
the equator (no seasonal variations
of the winds).
3.
The Earth does not rotate.
♦ No Coriolis effect.
♦ The only active force is the
pressure gradient force.
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
• Itthermally
is driven by the uneven heating of the Earth’s surface by the sun 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!
• Itin the
is obviously wrong: predicts northern prevailing winds everywhere
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
Observing global winds
from space
Intertropical convergence zone
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
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)
• 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)
• 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)
• 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
• 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
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
January
July
Subtropical highs
Subtropical highs
•
•
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
• 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.
July
January