Tuesday, 21 July 2015

Climate Change - Atmospheric Air Circulation, Convection Cells, Coriolis force, Jet Strems

Atmospheric circulation is the large-scale movement of air, and the means (together with the smaller ocean circulation) by which thermal energy is distributed on the surface of the Earth
Essentially, the Earth gets all its energy from the Sun with much more of the energy arriving in the equatorial regions - the higher latitudes receive much less of the Sun's energy.  This increases the temperature of the sea water and land in the tropics near the Equator relative to the polar regions. (see the following 6 slides).  The heat is transferred from hotter to colder regions by ocean currents and by the movement of the atmospheric air.
SLIDE 7:  The air near the surface absorbs heat, becomes less dense and rises. Cooler air moves in to fill the space. The hot air cools as it rises. Colder air is more dense and sinks to the surface and absorbs more heat again.  This completes a closed convection cell.
This is exactly the science behind air circulation.  If the Earth did not rotate then air circulation will look as in slides 8 and 10.  Slides 9 and 10 show that the wind direction at the surface will be from the Poles to the Equator. But our planet rotates and the rotation exerts an additional force, called the Coriolis Force on the moving air masses.  Coriolis force is described by slightly more complicated physics - basically, it is
(i) always perpendicular to the direction of motion of the air mass, and
(ii) is proportional to the speed of the air mass
Slide 11 shows how the Coriolis force will cause eastward deflection of air that is moving towards the North Pole and the deflection will be westwards if the air is moving from the Pole towards the Equator - in the Northern Hemisphere. In the Southern Hemisphere, the effect is reversed
It is because of the Coriolis force that a low pressure system in the northern hemisphere rotates counter-clockwise - explained in Slide 12.

In a rotating earth due to Coriolis force the convection cell does not extend from the equator to the Pole.  As the air mass is moving north, Coriolis force pushes it eastward and after some distance the air is actually moving from west to east and not as originally from south to north.  By the time air reaches 30 degree N, it is no longer experiencing the pressure gradient and sinks down completing a smaller convection cell - this is the Hadley Cell.
But heat still needs to be transported to the poles.  this happens with two additional cells - the Ferrel and the Polar Cells - in each hemisphere.
Jet streams are fast flowing, narrow air currents.   The main jet streams are located near the altitude of the tropopause. The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape.  The last three sildes explain the characteristics of jet streams.
Slide 7:  A Convection Cell
Slide 11:  Coriolis Force

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