Lecture 4: Global atmospheric circulation 2 Flashcards
GEOSTROPHIC WIND BALANCE
- At high altitudes (no influence of friction from the ground) the PGF will result in wind motion until a speed is reached where PGF and Coriolis Force balance each other out.
- As a consequence the wind will flow parallel to the isobars and will not further accelerate.
- This wind where PGF and CF are in balance is called geostrophic wind.
THIS RESULTS IN CIRCULAR MOTION (CENTRIPETAL FORCE)
- Combination of PGF and Coriolis Force results in a Centripetal Force component which forces the wind in a circular motion.
- Consequently wind motion is anticlockwise around a low-pressure system (cyclone) and clockwise around a high-pressure system (anticyclone) in the Northern hemisphere.
NORTHERN VS SOUTHERN HEMISPHERE
• As the Coriolis force acts in the opposite direction the Southern Hemisphere a low-pressure system here results in a clockwise air flow.
FRICTION IN THE PLANETARY BOUNDARY LAYER
- Between the surface and approximately 1000 m altitude (variable!) the atmospheric flow is influenced by friction with the ground.
- This lower part of the atmosphere is called Planetary Boundary Layer (PBL).
- The upper part of the troposphere – between the top of the PBL and the tropopause – is called free troposphere and the wind here is not influenced by friction with the ground.
FRICTION IN THE PLANETARY BOUNDARY LAYER- more
- Friction reduces the wind speed, which in turn reduces the Coriolis force no longer balance between CF and PGF, wind blows across isobars.
- At stable stratification and smooth terrain (a) the effect of surface friction only extends a relatively short distance upwards above the ground.
- When the air is unstable and the terrain is rough (b) vertical mixing the effect of surface friction extends far further upwards.
GLOBAL CIRCULATION: TRADE WINDS
- Intense heating at the equator warms up air near the surface causing it to rise high into the atmosphere (why?)
- Rising air creates a space and the equatorial low pressure zone, which sucks air in from higher latitudes, forming the trade winds
- This produces the trade winds which meet at the Intertropical Convergence Zone (ITCZ)
ATMOSPHERIC VS OCEAN HEAT TRANSPORT
Trouble with the trade winds blowing towards the ITCZ is that the climate system is desperately trying to export heat away from the equator, the in-blowing winds do nothing to help this.
- In the tropics, it is the surface currents of the oceans that transport most of the heat (next week’s lectures)
- Examples include the Gulf Stream, Kuroshiro current in WNP, Brazilian current in WSA, and the East Australian current in the WSP.
GLOBAL CIRCULATION: HADLEY CELL
- As the hot air that rose over the equator moves towards the poles, it slowly cools, both due to its rise (adiabatic cooling) and its movement towards the poles (advective cooling)
- At about 30° north and south, it sinks (WHY?), forming the sub-tropical high pressure zone
- The sinking air has lost most of its moisture and therefore dries out the land it sinks onto
- Some of this sinking air is pulled back towards the equator to complete the Hadley cell
GLOBAL CIRCULATION: WESTERLIES
- Some of the sinking air at 30° N flows towards the poles and forms the Westerlies
- It is from here that the atmosphere takes over as the main poleward transporter of heat
- The poleward movement of warm sub-tropical air only ceases once it meets the Polar air mass at the Polar front
GLOBAL CIRCULATION: FERREL CELL AND POLAR CELL
- Intense cold at the poles causes the air to become super-chilled and sink, causing high pressure over the poles (the Polar high) and “blow-out” winds.
- When this cold air mass meets the warmer Westerlies at the Polar front, moisture contained in the Westerlies is forced to condense and precipitate
- It also forces the warm sub-tropical air to rise, as the cold Polar air is much more dense
- This rising air completes the other two cells, the Ferrel cell and Polar cell
GLOBAL CIRCULATION: JET STREAMS
Important component of the three cells is the high altitude, fast flowing, narrow air currents called jet streams
• Occur at the tropopause, driven by the spin of the Earth and strong differences in temperature
• Strongest jet is the polar jet, the subtropical jet is slightly weaker
GLOBAL CIRCULATION: JET STREAMS 2
• Typically flow from west to east
• May start, stop, split, flow in the opposite direction to the rest of the
jet (kinks)
• As the jet stream meanders, it can form Rossby waves (large kinks in the westerly flow).
EARTH’S MAJOR AIR MASSES
An Air Mass is an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given altitude.
Depending on their origin and properties air masses are labelled either Arctic, Polar, Tropical, or Equatorial as well as maritime or continental.
