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Week 4 - Causes of General Circulation of the Atmosphere - Coggle Diagram
Week 4 - Causes of General Circulation of the Atmosphere
GLOBAL CIRCULATION
Local wealthier doesnt come from all directions equally. 2. Everyone’s weather is part of the global circulation pattern
GLOBAL CIRCULATION MODELS
The only way forward for understanding our climate is to set up computational general circulation models.
Global circulation represents the average winds around the world.
THE HADLEY CELL - spherical earth, no difference between land and sea, sun over the equator, no influence of Coriolis force.
Hot equator and cold poles create a pressure difference that drives surface air from pole to equator
Wind aloft flow in the opposite direction, to complete the cell, as expected.
This model is too simple to predict the observed circulation
Verticle motion associated with (mass) divergence and convergence in troposphere (Barry and Chorley 2003)
SIX CELL MODEL - earths rotation makes a big difference to the very simple model - equatorial low, subtropical high, subpolar low, polar high.
CORIOLIS (Barry and Chorley 2003)
The Coriolis forces arises from the fact that the movement of masses over the earths surface is referenced to a moving co-ordinate system.
Influence of Coriolis Force - the upper level circulation from the warm equator is expected to be polewards.
Coriolis force bends the flow right (N Hems) and left (S Hems)
A geostrophic balance occurs around latitudes 30º, limiting the Hadley cell.
If the Earth rotated faster, or the insulation was less, this limit would be nearer the equator.
FRICTIONAL DRAG (Barry and Chorley 2003)
Frictional drag near surface (esp over rough land and urban areas) slows wind below geostrophic value and bends round left in N Hems. Wind attains geostrophic velocity between 500 and 1000m altitude in mid-high lats.
GLOBAL WINDS
3 cells per hemisphere define a broad picture of winds in moderately good agreement with observed surface winds.
Importance of Coriolis force in determining direction of winds.
The ITCZ - intertropical convergence zone - is where the trades run together into the doldrums,
The ITCZ is characterised by warm, rising air, plenty of clouds and rain.
The trade winds belts are comparatively cloud free.
The ITCZ is clearly seen on geostationary satellite images.
Winds from satellite measurements: Winds at different heights can be derived from observing the speed at which features seen in different wavelength bands move.
Real World Winds - the interaction of land and sea introduces semi-permanent high and lows around the world.
WIND ALOFT - a lot of weather tends to come down from above.
When an anticyclone dominates, the air within it is sinking
When a depression comes over, we experience a sequence of clouds of decreasing height.
Winds aloft tend to determine the direction that storms move and how deeply they intensify.
GLOBAL PRECIPITATION
Precipitation is associated with warm, rising air fed with ocean moisture.
The general circulation features move about 10º to 15º in latitude following the suns movement.
JET STREAMS
The polar front jet stream has an important influence on our weather.
Jet stream circles Earth in a huge wavy line a few hundred km wide, a few hundred km thick, around height of tropopause, with central speeds typically 200km hr-1 in direction of West to East.
Jet streams forms at the junction of cold, polar air to north and warm subtropical air to south.
There is also a subtropical jet stream where the Hadley and Ferrel cells meet.
ORIGIN OF JET STREAM - polar jet stream occurs at the junction of warm and cold air, where there is a sharp pressure change and a strong pressure gradient.
The faster winds occur near the highest altitude in the troposphere.
ROLE OF ANGULAR MOMENTUM
Air moving round the earth with speed v and towards the pole must increase its speed to preserve its angular momentum, since its distance from the Earths rotation axis is less.
There are other jet streams.