Atmospheric Circulation

1
Chapter 4 Atmospheric Circulation
2
Regions near the equator receive light at
90o High latitudes receive light at low angles
Earth
3
Regions near the equator receive light at
90o High latitudes receive light at low
angles Light energy is more concentrated near
the equator. In other words, there is a greater
flux per unit area (W/m2)
Earth
4
Solar energy is concentrated near the equator
Image Netherlands Center for Climate Research
5
absorbed solar energy
Energy
90 45 0 45 90 Latitude
6
absorbed solar energy
Emitted IR energy
Energy
90 45 0 45 90 Latitude
7
More energy is absorbed near the equator than
emitted And more energy is emitted near the poles
than is absorbed.
absorbed solar energy
Emitted IR energy
Energy
90 45 0 45 90 Latitude
8
net radiation surplus
Energy
90 45 0 45 90 Latitude
9
Excess energy at the equator is transferred
towards the poles by convection cells
net radiation surplus
Energy
net radiation deficit
90 45 0 45 90 Latitude
10
Solar energy received is greatest near the
equator. Energy is moved from the equator to
the poles.
11
Solar energy received is greatest near the
equator. Energy is moved from the equator to
the poles. Energy is transferred by wind and
ocean currents
12
Air near the equator is warmed, and rises
solar radiation
13
The rising air creates a circulation cell, called
a Hadley Cell
H
L
solar radiation
H
Rising air ? low pressure Sinking air ? high
pressure
14
Hadley Circulation Cell
Rising air is replaced
Warm air rises
15
Hadley Circulation Cell
Air cools, sinks
Rising air is replaced
Warm air rises
16
Hadley Circulation Cell
Air cools, sinks
Rising air is replaced
Warm air rises
HIGH
HIGH
LOW
17
The Earth would have two large Hadley cells, if
it did not rotate. --This is exactly what we
think occurs on Venus (which rotates very
slowly)! Rotation of the Earth leads to the
Coriolis Effect This causes winds (and all
moving objects) to be deflected to the right
in the Northern Hemisphere to the left in the
Southern Hemisphere
18
The Coriolis Effect Based on conservation of
angular momentum We experience linear momentum
when we are in a car that is traveling fast and
then stops suddenly.
19
Planet Earth rotates once per day. Objects near
the poles travel slower than those near the
equator.
20
Angular Momentum
v
r
m
L mvr
Angular momentum is conserved unless some force
(a torque) is applied
21
Objects near the poles have less angular momentum
than those near the equator. When objects move
poleward, their angular momentum causes them to
go faster than the surrounding air. Conversely,
they slow as they move towards the equator.
22
When objects move north or south, their angular
momentum causes them to appear to go slower or
faster. This is why traveling objects (or air
parcels) deflect to the right in the northern
hemisphere and to the left in the southern
hemisphere.
23
Example of Coriolis effect hurricanes
H
H
isobar (line of constant pressure)

• Hurricanes are low pressure centers
• Air moves from high pressure towards low
• pressure

24
Hurricanes Northern hemisphere
H
H

• As the air moves in, it is deflected towards the
• right in the NH
• Resulting circulation is counter-clockwise

25
The Coriolis effect causes winds to deflect as
they travel within circulation cells This breaks
up the two large Hadley cells into six smaller
cells.
26
In the tropics, surface air is moving
equatorward. It is deflected to the right in the
NH (left in the SH), giving rise to easterly flow
(the trade winds)
Easterlies
27
At midlatitudes, surface air is moving poleward.
It is deflected to the right in the NH (left in
the SH), giving rise to westerly flow (the
prevailing westerlies)
Westerlies
Westerlies
28
Credit NASA
29
Hadley Circulation Cell
Air cools, sinks
Rising air is replaced
Warm air rises
HIGH
HIGH
LOW
30
Rising air cools the airs capacity to hold
water drops. Rain!
Air cools, sinks
No rain in regions where air is descending
Rising air is replaced
Warm air rises
HIGH
HIGH
LOW
31
orbit-net.nesdis.noaa.gov/arad/
gpcp/maps/frontmap.gif
32
(No Transcript)
33
Caution Zonal weather pattern is not completely
true The pattern is disrupted by land-sea
contrasts
34
Land heats and cools rapidly Water heats and
cools slowly
35
Sea Breezes
DAY
36
Sea Breezes
Offshore wind
DAY
NIGHT
37
Tibetian Plateau--Monsoon Circulation