Download Atmospheric Circulation

Atmospheric
Circulation
Chapter 7
Atmospheric circulation
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Global – Covers major portions of the earth
Synoptic – covers 100s to 1000s of km2
Mesoscale – covers 10 of km2
Microscale – small exchanges of mater and
energy
Global Scale Circulation
Single cell model
• George Hadley
The earth is warmed
at the equator.
Air expands upward
and diverges
toward the pole
At the pole air cools
and flows back
toward the equator
Single cell model
Because the earth
is rotating the
wind is shifted to
the right (left in
SH)
Single cell model
Zonal winds –
blow east to
west or west to
east
Meridional winds –
blow south to
north or north to
south
Single cell model
Not a very realistic
model
3-cell model
• Each hemisphere
is divided into 3
distinct circulation
cells
3-cell model
• Polar cell
• Ferrel cell
• Hadley cell
Each cell has
associated
pressure and
wind patterns
3-cell model
ITCZ = Intertropical
convergence zone
Creates the equatorial
low, rainiest regions
on earth
Little wind – doldrums
3-cell model
20-300 Subtropical
High – very dry
Location of many of
the earth’s deserts
Little wind – horse
latitudes
Between 0-200 NE
trade winds
3-cell model
Subpolar low –
plenty of rain
Mid latitude
westerlies
Polar high – pretty
dry
Polar easterlies
Global Atmospheric Circulation Model
3-cell model
Is OK at explaining some pressure and
wind phenomena. Ex ITCZ
The “real” world
• Not covered by distinct belts that completely
encircle the earth
• Circulation is dominated by semi-permanent
cells of low or high pressure
Winter
• Weather in the North America is dominated by
the Aleutian Low and Icelandic Low
Summer
• Weather in North
America is dominated by
the Hawaiian High and the BermudaAzores High
Seasonal Pressure and Precipitation Patterns
The jet stream
The pressure gradient force increases with
height
The
jet stream
The Polar front
is a boundary
between very cold
polar air and warmer mid-latitude air
Creates an extreme Pressure Gradient force
The jet stream
The result is a powerful flow of air, the jet stream,
located near the tropopause at the polar front
Synoptic scale circulation
Ridges and troughs
• Ridges – thicker regions of the atmosphere
with higher pressure, often associated with
surface divergence
• Troughs – thinner regions of the atmosphere
with lower pressure, often associated with
surface convergence
Ridges and troughs
Ridge
Trough
Ridges and troughs
• Can best be seen
on a 500mb map
• Troughs point
toward the equator
• Ridges point toward
the pole
500mb map
Rossby Waves
The ridges and troughs form long waves,
called Rossby waves, which circle each
hemisphere.
Rossby waves can remain stationary or move
west to east
Movement of the waves can transport vast
amounts of air
The Jet Stream and Rossby Waves
Rossby waves
Groovy web
animation
Ocean Currents
• Large scale movements of surface water that
can exchange energy and moisture with the
atmosphere
Ocean Currents
Ocean Circulation
Foehn winds
• Winds that flow down mountain slopes, warm
by compression and bring warm dry air to the
lowlands
– Chinook – formed by air descending the eastern
slopes of the Rocky Mountains
– Santa Ana – formed by high pressure over the
Rockies and Great Basin
Chinook
Santa Ana
Mesoscale circulation
Sea / land breezes
• Sea breeze
– Land warms faster in the day then water
– Causes air to expand, rise, diverge and thus
create low pressure
– Air moves from the sea to balance the pressure
• Land breeze
– At night the land cools faster than the water and
the situation is reversed
Sea / land breezes
Mountain / valley breezes
• Valley breeze
– Daytime heating of the
mountain causes air to
warm, rise and be
replaced by
– air from the valley
• Mountain breeze
– At night the mountain
cools an air sinks
down to the valley
Have a great day!