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3 cell model and the Jet Streams
(based on C. D. Ahrens’, “Meteorology today”, Chapter 11)
1. Cell Models
Single cell model
When we look for a simple model describing the general circulation ion the
atmosphere, the single cell model is the most intuitive one. We assume that
Earth is covered uniformly with water so that we may neglect differential heating
between land and sea.
The sun is always above the equator.
We neglect earth’s rotation.
These three assumptions allow for a single cell
(northern hemisphere and southern hemisphere
cell) model where heat is convected to the poles
from the equator and cold air travels to the
equator in a closed loop.
This model was first proposed by George
Hadley.
The model, however, is too simplistic. It breaks
down when we incorporate earth’s rotation. The
Coriolis effect would cause air from the north
pole,
traveling south to feel an easterly force, causing
surface winds to move in a direction counter to
Figure 1.1
the rotation of the earth. Over time, this
mechanism would act as a negative feedback and
would halt the rotation of the earth. As this is not the case, this argument alone suffices to
demerit a single cell model.
3 cell model
When we allow the earth to spin, the single cell model breaks into 3 separate cells
(2 cells are mechanically possible, but would generate a Low Pressure peaks at the poles
which, we know do not exist.). The cell closest to the equator is a Hadley confined to the
0-30 latitides. The Ferrel (William Ferrel) cell
is less pronounced than the Hadley cell. In fact,
the 3 cell model suggests easterly upper winds in
the Ferrel cell while measurements show that the
mid latitudes exhibit westerly winds. On the
other hand, the model is quite consistent with
recorded surface wind patterns.
Figure 1.1
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Figure 1.3
Figure 1.4
Doldrums: Low pressure, light winds area near equatorial waters. The monotony of the
whether has given rise to the expression “down in the doldrums”. The hot air rising over
these waters produces large Cumulus clouds and thunderstorms, releasing a lot of latent
heat and driving the Hadley cell. The tropopause acts as a ceiling where the rising air
deflects to the poles.
Subtropical Highs: sinking air at the 30s (anticyclones). As the air descends, it warms
by compression. Subtropical areas are generally warm and dry, with clear skies and weak
winds. The major deserts on earth are along the 30s latitudes. The name “Horse
latitudes” is derived from the sad fact that many horses were thrown overboard along
these latitudes as ships were stuck for weeks at the becalmed waters.
Trade winds: air from the horse latitudes flows back towards the equator, twisting to the
west due to the coriolis force.
ITCZ(InterTropical Convergence Zone): along the equator, the northeast and south east
trade winds converge and rise again with the continuing circulation of the Hadley cell.
Westerlies: at the 30s, some of the descending air moves poleward, forming the
westerlies (again, resulting from the coriolis force).
Polar Front: as the westerlies move poleward, they encounter the polar front (cold air
moving from the poles). The two masses of air differ in temperature and do not readily
mix. At the Subpolar Low air rises and storms form. Some of the rising air moves back to
the horse latitudes, completing the Ferrel cell and the rest move poleward, completing the
Polar cell (also the weakest cell).
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2. Layers of the Atmosphere
Troposphere: where most air mass and wind
action take place.
Tropopause: where air stops cooling as it rises.
Stratosphere: air heats with height. This might
be due to the ozone (absorbs UV) maximum
being just about at the height where the heating
begins (30km). The shift from cooling to
heating with increasing height is called
Temperature Inversion.
Mesosphere: air density and ozone density in
particular is very thin. The air looses energy
faster than it gains energy, hence cooling with
increasing height resumes.
Thermosphere: oxygen molecules (O2)
dominate at this height. The ability of O2 to
absorb heat resumes heating with height.
Exosphere: atoms are loosely bounded to earth
and easily bounce off to space. (mean free path
of the order of a few km).
Homosphere / Heterosphere: below the thermosphere the air is dense enough to mix
(hence, homogeneous). Above the mesosphere, light and heavy molecules settle at
different layers.
Ionosphere: 70-80km and up, we have large concentrations of ions and free electrons.
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3. Jet Streams
“swiftly flowing air currents, thousands of km long, a few hundred km wide and only a
few km thick. Wind speeds at the core of a jet stream often exceed 100 knots (1knot =
1.9km/h) and occasionally exceed 200 knots.”
Jet streams are often found at the tropopause, at elevations of 10-15km.
Figure 3.1
The subtropical jet stream is the dominant of the two. Averaging over many years raises
doubts as to whether we may even regard the polar jet as a definable entity (N. P.). The
jet streams meander in broad loops in a general westerly direction.
Formation of the polar and subtropical jets
The formation of the jet streams is attributed to two major mechanisms. First, the
jets form along fronts of steep pressure gradients caused by the counter motion of hot and
cold air masses. The steep pressure gradients intensify the winds and causes the jet
streams.
Secondly, both jet streams are westerly. The westerly motion of the subtropical jet
stream is consistent with the conservation of angular momentum. If we look back at
figure 1.1, we see that in the northern hemisphere, winds traveling equatorward,
experience an easterly coriolis force and winds traveling poleward experience a westerly
coriolis force. At the 30s, according to the 3 cell model, hot Hadley cell winds from the
equator meet cold Ferrel winds from the poles. The coriolis bending of the two air masses
are opposite. However, the air coming from the equator has greater angular momentum to
begin with and larger mass. Thus, we would expect a westerly jet at the 30s.