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Chapter 15 Global Circulation: Big Picture: Idealized View
Polar cell
Ferrel cell
Ferrel cell
Fig 1. Earth’s radiation budget
What do ‘surplus’ and ‘deficit’ refer to?
What wavelength ranges are for shortwave and longwave? What are their sources?
What is shortwave in?
Why are shortwave absorbed not symmetrical with latitude?
At what latitude does most heat and mass transfer occur?
Fig 2. 1735, Hadley’s circulation cell idea developed.
Ideas: Latent heat release drives convection. Trade winds are Easterlies due to the rotation
of the Earth, and he realized it 100 years before Coriolis wrote the theory for motion in a
uniformly rotating coordinate system. Main problems: No westerlies anywhere to conserve
angular momentum, and the Equator to pole temperature difference is too large.
Fig. 3. Rossby theory of circulation, 1930s
clouds
polar front
Warm air rides up
the colder air
deserts
doldrums
clouds
William Ferrel’s cell idea in 1856: Rossby clarified and explained more completely. Prevailing
westerlies at mid latitudes are a wavy river of air, Rossby waves, where the Coriolis force is the
restoring force, and strong horizontal gradients of pressure and temperature (baroclinic
instabilities – density depends on pressure and temperature). This conceptual model captures
many features of the Earth’s circulation.
Jet Streams (pink) between warm (orange) and cold
(blue) regions
Jet streams (shown in pink) are well-known
examples of thermal wind. They arise from the
horizontal temperature gradient from the
warm tropics to cold polar regions.
From Wikipedia
Geostrophic Wind: due to pressure gradient along x direction
Thermal wind
T(z)
T(z)
y
y
y
Barotropic atmosphere
(density depends only on pressure)
xy
Baroclinic atmosphere
(density depends on pressure and
temperature) size of temperature
symbol represents value .
The vertical variation of geostrophic wind in a barotropic atmosphere (a) and in a baroclinic
atmosphere (b). The blue portion of the surface denotes a cold region while the orange portion
denotes a warm region. Temperature difference is restricted to the boundary in (a) and extends
through the region in (b). The dotted lines enclose isobaric surfaces which remain at constant
slope with increasing height in (a) and increase in slope with height in (b). This causes thermal
wind to occur only in a baroclinic atmosphere. (from Wikipedia)
Fig 4a Longitudinally averaged winds (zonal mean) for
20 years on the equinox March 21st. Westerlies are
solid lines. Note jet stream maxima at 200 hPA.
Thermal Wind Estimation (will do derivation later)
Fig 4b Zonal mean average
circulation
Note the location of the ITCZ
What latitude is cloudiest?
What latitude has clear conditions?
Dec Jan Feb
June July August
Fig 4c Zonal mean temperature for 20 years at
equinox on March 21st.
Note the strong latitudinal gradient.
Fig 7 Lorenz theory of Atmosphere Energy Cycle
Energy density (units 105 Jm-2)
Energy flows (Wm-2)
Key points: Terminology and mechanisms at play. What are APE, KE, and eddies?
Barotropic and Baroclinic Atmosphere
Barotropic
Baroclinic
Baroclinic fluid is ‘full of life as it converts thermal energy into kinetic energy’
From Marshall and Plumb