Bài giảng khí hậu học chương 7

G304 – Physical Meteorology and Climatology

Chapter 7
Atmospheric circulation

By Vu Thanh Hang, Department of Meteorology, HUS


7.1 Single-cell Model
- A simple circulation pattern called
the single-cell model to describe the
general movement of the atmosphere.
- In the single-cell model, air expands
upward, diverges toward the poles,
descends, and flows back toward the
equator near the surface.
- Winds blowing east-to-west or westto-east are referred to as zonal winds;
those moving north-to-south or southto-north are called meridional winds.


7.1 Single-cell Model (cont.)
• Hadley’s

idealized scheme assumed a planet covered by a
single ocean and warmed by a fixed Sun that remained
overhead at the equator.
• Hadley’s main contributions were to show that differences in
heating give rise to persistent large-scale motions (called
thermally direct circulations) and that zonal winds can result
from deflection of meridional winds.
Æ Not so realistic

7.2 Three-cell model
- The three-cell model divides the circulation of each hemisphere into three
distinct cells: the heat-driven Hadley cell that circulates air between the
Tropics and subtropics, a Ferrel cell in the middle latitudes, and a polar cell.
-

Each cell consists of one belt
of rising air with low surface air
pressure, a zone of sinking air
with surface high pressure, a
surface wind zone with air
flowing generally from the
high-pressure belt to the lowpressure belt, and an air flow
in the upper atmosphere from
the belt of rising air to the belt
of sinking air.


7.2 Three-cell model (cont.)
• The Hadley cell:
- Along the equator, strong solar heating causes air to
expand upward and diverge toward the poles, creating a
zone of low pressure at the equator called the equatorial low
or the Intertropical Convergence Zone (ITCZ).
- The ITCZ is the rainiest latitude zone in the world and is
observable as the band of convective clouds and showers
extending from northern South America into the Pacific on
the satellite image.

- The ITCZ is sometimes called the doldrums.


7.2 Three-cell model (cont.)

ITCZ on satellite images


7.2 Three-cell model (cont.)
• The Hadley cell (cont.):
- At about 20° to 30° latitude, air in the Hadley cell sinks
toward the surface to form the subtropical highs, large bands
of high surface pressure Æ Cloud formation is greatly
suppressed and desert conditions are common in the
subtropics.
- In the NH, as the pressure gradient force directs surface air
from the subtropical highs to the ITCZ, the weak Coriolis
force deflects the air slightly to the right to form the northeast
trade winds.
- In the SH, the northward-moving air from the subtropical
high is deflected to the left to create the southeast trade
winds.


7.2 Three-cell model (cont.)
• The Ferrel and polar cells:
- Immediately flanking the Hadley cell in each hemisphere is
the Ferrel cell, which circulates air between the subtropical
highs and the subpolar lows.
- On the equatorial side of the Ferrel cell, air flowing

poleward away from the NH subtropical high undergoes a
substantial deflection to the right, creating a wind belt called
the westerlies.
- In the SH, the pressure gradient force propels the air
southward, but the Coriolis force deflects it to the left,
producing a zone of westerlies in that hemisphere as well.


7.2 Three-cell model (cont.)
• The Ferrel and polar cells (cont.):
- In the polar cells of the three-cell model, surface air
moves from the polar highs to the subpolar lows.
- Very cold conditions at the poles create high surface
pressure and low-level motion toward the equator. In
both hemispheres, the Coriolis force turns the air to
form a zone of polar easterlies in the lower atmosphere.


7.2 Three-cell model (cont.)


7.3 Semipermanent pressure cells
• The real world is not covered by a series of belts that
completely encircle the globe Æ a number of alternating semipermanent cells of high and low pressure
• They are called semipermanent because they undergo
seasonal changes in position and intensity over the course of
the year.
• Some of these cells result from temperature differences and
others from dynamical processes.

7.3 Semipermanent pressure cells (cont.)
January


7.3 Semipermanent pressure cells (cont.)
July


7.3 Semipermanent pressure cells (cont.)

The Sahel is a region of Africa bordering the southern Sahara
Desert. During the summer (left), the ITCZ usually shifts
northward and brings rain to the region. For much of the year,
the ITCZ is located south of the Sahel, and the region
receives little or no precipitation (right).


7.4 The upper troposphere
• Upper tropospheric heights decrease poleward from
lower latitudes due to the increased density of colder air

Decreasing heights
with latitude


7.4 The upper troposphere (cont.)
• Westerly winds in the upper atmosphere:
- Height differences correspond to pressure differences Æ
when the 500mb surface slopes steeply Æ exists a strong

pressure gradient force.
- On 500mb map, there is always a PGF across the middle
latitudes trying to push the air toward the poles.
- In the absence of friction, the wind do not blow poleward,
but rather blow parallel to the height contours, from W to E.
- PGF is strongest in winter Æ upper level westerlies are
strongest in winter Æ affect aviation.


7.4 The upper troposphere (cont.)
• Westerly winds in the upper atmosphere (cont.):
- Wind speeds generally increase with height between the
surface and the tropopauseÆ because of decreasing of
friction and PGF is stronger at high altitudes.
- The surfaces representing the 900, 800, and 700 mb
levels all slant downward to the north, but not by the same
amount.
- Higher surfaces slope more steeply, which means that the
pressure gradient force is greater.


7.4 The upper troposphere (cont.)
• Westerly winds in the upper atmosphere (cont.):

The difference in heights between successive surfaces continues to
increase upward, leading to stronger winds.


7.4 The upper troposphere (cont.)
• The polar front and jet streams:

- The polar front is a strongly sloping boundary between
warm mid-latitude air and cold polar air.
- Within the front, the slope of the pressure surfaces
increases greatly because of the abrupt horizontal change
in temperature.
- With steeply sloping pressure surfaces there is a strong
PGF, resulting in the polar jet stream situated above the
polar front near the tropopause Æ affecting daily weather in
mid-latitudes.
- The jet stream as a consequence of the polar front, arising
because of the strong temperature gradient (9-12km above
sea level).


7.4 The upper troposphere (cont.)
• The polar front and jet streams (cont.):
- Wind speeds average about 180km/hr in winter and about
half that in summer, peak winds can exceed twice these
values.
- Near the equator is the subtropical jet stream, associated
with the Hadley cell, can bring with it warm, humid
conditions.


7.4 The upper troposphere (cont.)
• Troughs and ridges:
- The 500 mb surface reveals
that heights decrease from south
to north but also rise and fall
through the ridges and troughs.

- Height contous are displaced
toward the equator in troughs
and toward the pole in ridges.
- Air flows poleward around
ridges and equatoward around
troughs.


7.4 The upper troposphere (cont.)
• Rossby waves:
- Ridges and troughs give rise to wavelike flow in the upper
atmosphere of the mid-latitudes.
- The largest of these are called long waves or Rossby
waves.
- There are from three to seven Rossby waves circling the
globe, each with a particular wavelength and amplitude.
- Rossby waves often remain in fixed positions, but also
migrate west to east Æ transporting warm air from
subtropical regions to high latitudes, or cold polar air to low
latitudes.


7.4 The upper troposphere (cont.)
• Rossby waves (cont.):
- Changes from summer
to winter Æ fewer in
number,
have
longer
wavelength,

strongest
winds in winter.


7.4 The upper troposphere (cont.)
• Rossby waves (cont.):


7.5 The Oceans
• Ocean currents:
- Ocean currents are horizontal movements of surface water
that have an impact on the exchange of energy and moisture
between the oceans and the lower atmosphere.