Download AIRMASSES The Planetary Circulation Idealized planetary circulations

AIRMASSES
•  An airmass is a huge volume of air (several
thousand sq. km) that is “relatively uniform”
horizontally in temperature and water vapor
content.
•  Airmasses take on the character of their source
region- that is, the region of the planet’s surface
within which they are in contact for several days
to weeks. For example: warm ocean, snowcovered prairie, etc.
•  For our purposes we can distinguish air masses as
either:
•  1) warm or cold (tropical or polar), and as
•  2) moist or dry (maritime or continental)
Typical configuration of air masses
Note BOTH Arctic front and Polar Front
CASE OF NO ROTATION
This movement of air in the upper
atmosphere would tend to
converge near the poles causing
subsidence (sinking) in polar
regions
This cooled air would
then return equatorward
near the surface.
In this simple conceptual model,
the circulation occurs as
hemispheric HADLEY CELLS
For our purposes we can
distinguish air masses as
either:
1) warm or cold (tropical or
polar), and as
2) moist or dry (maritime or
continental)
giving us continental
tropical cT (dry and hot),
continental polar cP (dry
and cold) maritime
tropical mT (moist and
warm), and maritime polar
mP (moist and cool).
We will also add arctic cA
(very dry and very cold)
The Planetary Circulation
Recall from our earlier discussion that “warm air
rises” and “cold air sinks”.
•  While this is an over-simplification it is a useful
starting point. Consider an idealized non-rotating
planet covered by ocean:
•  The intense insolation near the equator would
likely create regions of intense convection
(thunderstorms).
•  Since thunderstorms have strong updrafts =>
move air upward, lots of air. This air near the
tropopause would tend to move poleward, cooling
as it does so.
Idealized planetary circulations
•  If we now add rotation, the flows at the
surface would feel the coriolis force.
•  In the N. Hemisphere, surface flow would
turn towards the right (west) against the
sense of Earth’s rotation, creating a drag on
the planet.
•  HOWEVER, with rotation our simple
model breaks down anyway and the real
solution is one where the flow tends to
occur in three belts of latitude.
Thus, we have a big convective current.
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Hadley Cell
let’s add some rotation to our simple model
The Conceptual picture from before still works, but
the subsidence occurs at about 30° N and 30° S in
“symmetric” belts.
30 N
EQ
30 S
This gives us:
The Inter-Tropical Convergence Zone (ITCZ), the band of convection
near the equator where air rises (surface low pressure)
This “subtropical Hadley Cell
Model” also gives us:
•  The “Horse Latitudes” near 30° N and S
where air sinks (surface high pressure)
•  As surface air flows south in N.H. it is
deflected by coriolis force to right (west)
giving northeasterly TRADE WINDS.
•  Subsidence in Horse Latitudes is
responsible for most of world’s great
deserts. (why?)
HIGH LATITUDE WINDS
•  The poles, again in agreement with the
simple conceptual model are typically
regions of subsidence and high surface
pressure, especially during the winter season.
•  Again, (cold) surface air flows equatorward,
in N.H. is deflected to right, again giving us
surface north easterly winds.
•  This southward flow of cold air helps nature
“mix out” the pole-to-equator temp gradient.
Midlatitude Winds
More on Midlatitude Winds
•  It is in the mid-latitudes that our conceptual
model really breaks down.
•  Some of the descending subtropical air
(Horse Latitudes) moves north rather than
completing the Hadley Cell.
•  This air is again deflected to right by
coriolis and gives the surface westerlies we
are so familiar with between 25-35° and
55-65°, the midlatitudes.
•  In the midlatitudes, winds are typically westerly
through-out the depth of the troposphere. In other
words, no “Hadley Cell” type of phenomena
typically occurs.
•  Note that where the midlatitude westerlies (which
on average have a slight southerly component)
meet the cold polar northeasterlies, convergence
occurs.
•  This has 2 consequences
2) strong temp. gradients
–  1) forced upward motion
2
The previous results give us
convenient bands of latitude
to refer to when talking
about global circulation and
global climate
Of course these are
idealizations of how
climate works, Actually
the boundaries btwn bands
are not constant but change
with time
The “cells” and surface
wind regimes
Figure 11.2a
The world’s major regions
of cloudiness occur in areas of
surface convergence
Figure 11.2b
Figure 11.5
REALITY BITES!
•  So much for conceptual models, both rotating and
non-rotating... What about the Real Earth!...
•  Well, reality always rears it’s ugly head.
•  The main effects of land masses are:
–  1) thermal inertia. Continents are warmer in summer
and cooler in winter than oceans
–  2) Orography (mountains.) Mountains provide a
substantial “aerodynamic drag” on the winds and also
act as a barrier to air mass motion, especially near the
surface.
Semipermanent features in N. HemisphereWinter
3
N hemisphere summer
4