General Circulation

1
General Circulation

• Remember HOMEWORK 2 DUE NEXT TUESDAY
• Review Session Feb. 24th
• First Exam March 1st
• Topics and Groups Due March 3rd

2
Last Time

• Latent heat of fusion going to solid to liquid
  or liquid to solid
• Snow melting takes energy from atmosphere
  (cooling), snow on ground takes heat from atmos
  and surface
• Freezing opposite

3
General Circ and Climate

• Two ideas that are closely related
• General Circulation features of the general
  circulation are statistical entities, long-term
  statistical analysis of the atmosphere (no
  specific time scale)
• Gen Circ has a specific time and space scale
  unlike climate which is just a time
• Gen Circ- large time and large space (global)
  scale

4
Time Scales

• For the most part you cannot see general
  circulation patterns on a weather map
• Long-term for gen circ refers to
• Monthly
• Seasonally
• Annually
• These mean features can show through in as little
  as 15 days

5
General Circulation Pattern
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3 Belts

• 3 cell model (or 3 belts) of the earths
  atmosphere, Coriolis gives rise to this (earths
  rotation)
• See figure
• 2 Questions to Answer
• Why westerlies in midlatitudes, why easterlies in
  tropics?
• What gives rise to the gen circ?
• Answer Temp and Momentum Transport

7
Important Third Question

• Why are these temperature and momentum transports
  necessary?
• Good Exam Question
• We will get at this answer in the next two
  lectures
• Good detailed answer coming up in the future

8
Other Planets

• Can almost see the banded structure on Earth
• Not as evident as Jupiter, but banded nonetheless
• If we double the rotation of earth, we would have
  nine bands
• Jupiter is about double our rotationhence has
  about nine belts (cells)

9
Sir George Hadley

• If warm air rises and cold air sinks, there has
  to be this rising air at the equator and sinking
  at the pole
• Had right idea but did not to take in account
  earths rotation
• Earned him the naming rights of the tropical
  cells (Hadley cells on diagram).
• Mid-latitude cells are called the Ferrel cell,
  and we have the nameless Polar Cells

10
Features of the Gen Circ

• Existing characteristics
• Gen Circ Features migrate with the seasons
• System sloshes northward during summer
• Well start at the equator and move to higher
  latitudes

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ITCZ

• Inter-tropical convergence zone
• Belt of low pressure
• Rising motions
• Is a place where the horizontal winds are weak
  (Doldrums)
• Rising motions give way to strong convection
  (thunderstorms)
• Can see this in satellite images as bands of
  convective areas near the equator

12
ITCZ

• Starting point for the energy (temp and momentum)
  transport toward the North begins
• ITCZ will be located as far north as the
  mid-latitudes in the summer (India and Florida)
• Does not progress too far south of the equator
  due to the large amount of ocean (weak temp
  gradients)
• Meeting ground of the two belts of the trade
  winds
• Height of tropopause (16-17km) greatest
  herewarmest temps

13
Subtropical Highs

• Located right around 30N/S
• Highs tend to be stronger over the ocean
• Associated with anticyclonic and downward motion
• Correspond with the majority of the worlds
  deserts (Great Basin, Sahara, Middle East,
  Kalahari, Great Sandy Desert)
• Circulations give rise to the trade winds (NE
  wind in N.H., SE wind in S.H.)
• Names Bermuda and Azores High (Atlantic), SE
  Pacific High

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Mid-latitudes

• 30N/S-60N/S
• Battle zone of air masses
• Winds are generally westerly aloft and at the
  surface
• Balance between (PGF-Coriolis)
• We find jet streams in this region

15
Jet Streams and Polar Front

• Were predicted in 1910s before upper air
  measurements, all based on math, jet stream was
  discovered during WWII over the Pacific during
  bombing runs
• Is a gen circ feature (large time and spatial
  scales)
• Reflection of the polar frontboundary between
  low polar temps and moderate continental temps
• Jet stream located above the sfc polar front in
  general

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Jet Stream (cont.)

• Jet stream and polar front are not constantjet
  stream is strongest off Asia (200 mph in
  winter)polar front has large impacts over N.
  America.
• During winter jet is strongest
• Jet and Polar Front are the tracks that storms
  take
• Ridge in Jet Streamwarmer temps
• Trough in Jet Streammore polar air mass

17
Polar Regions

• Find two low pressure regions that are located
  around 60N
• Where cyclones go to die
• Aleutian Low
• Icelandic Low