Transcript Winds and Global Circulation

Winds and Atmospheric
Circulation (I)
Winds, Cyclones and Global
Circulation Models
Introduction
Transfer of heat from the earth’s surface depends on
radiation
However:
 Air movement as winds
 Transport heat away from surplus regions
 Convection currents set up to replace warm air
with cold air
Winds
Wind is air motion with respect to the earth’s
surface
 dominantly horizontal
 occurs due to differences in pressure over
earth’s surface
 air moves from high pressure areas to low
pressure areas
Winds
When air is heated (by conduction), or rises
(by advection of cold air masses),
atmopsheric pressure is relatively low
 When air cools (by adiabatic processes or
by conduction), atmospheric pressure is
relatively high.
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Winds
Differences in pressure: sets up a pressure
gradient between areas of highs and lows
 Direction of movement: from high pressure
to low pressure
 tendency for mass movement of air referred
to as pressure gradient force
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Pressure Gradient
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The steepness of the pressure gradient is
determined by the rate of change in pressure.
The higher the rate of change, the steeper the
pressure gradient
The lower the rate of change, the less steep the
pressure gradient
Pressure gradients can be illustrated with the use
of isobars, which are lines of equal pressure
Pressure Gradient
Isobars are like
contour lines
Can be steep or
gentle
High
Affects wind
speed
Low
Direction of air flow follows Pressure
Gradient Force
Wind and Pressure Gradient Force
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Relationship can be illustrated by influence
of oceans and continents by the sea
breeze and the land breeze
Sea Breeze
Warmer conditions over
sea, low pressure, air rises
Cold, dense air moves
in to take place of
warm, rising air: sea
breeze
Land Breeze
During the night:
 more rapid radiational cooling of land
 air over land is colder than air over water
(due to diff SHC)
 over land: high pressure; over sea: low
pressure
 pressure gradient force reverses, air moves
from land to sea: sea breeze
Speed and Direction of Winds
Under an ideal situation, winds would always
flow directly with the pressure gradient
force as shown before.
However, the direction and speed of winds are
affected by
a) The Coriolis Force
b) Friction
The Coriolis Force
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causes an apparent deflection in the path of winds
as they travel across the globe
occurs due to the anti-clockwise rotation of the
earth
Ferrel’s Law: Any object or fluid moving
horizontally in the northern hemisphere tends to
be deflected to the right; in the southern
hemisphere, deflection is to the left
Regardless of direction or path of winds
The Coriolis Force
Northern Hemisphere:
Winds
Travelling from
North to South
Idealised
Path without
deflection
Deflected path
due to Coriolis
Effect
The Coriolis Force
Northern Hemisphere:
Winds
Travelling from
South to North
Idealised
Path without
deflection
Deflected path
due to Coriolis
Effect
The Coriolis Force
Other characteristics:
 Coriolis Effect is zero at the Equator
 increases with the object’s speed
 never slows or speeds up a moving object,
but changes only its direction
Friction
Air in contact with the surface experiences
frictional drag, decreases wind speed
 layer of air above this layer in turn
experiences frictional drag
 Effect of friction decreases with increasing
height
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Moving air C: slowed by friction with layer B
Moving air B: slowed by friction with layer A
Layer of moving air A: slowed by friction with surface
Surface
Friction
Because friction is greatest in the first 1.5km
of the troposphere
 wind speed for a given pressure gradient is
decreased
 dampens Coriolis Effect
 prevents surface winds from becoming
geostrophic
Anticyclones and Cyclones
Anticyclones:
 high pressure systems
 at surface, clockwise rotation in Northern
Hemisphere
 southern hemisphere, flow is anticlockwise
Anticyclones
Associated with sinking air, hence stable
conditions
 limited cloud formation if any
 usually clear skies
 dry air and low humidity
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Cyclones
Low pressure systems
 at surface, air rotates anticlockwise in the
Northern Hemisphere
 in S Hemisphere, air spirals clockwise
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Cyclones
Low pressure, hence air is rising, unstable
conditions
 tendency to form ‘thunderheads’, also
known as cumulonimbus clouds
 humid weather, mainly heavy rains in the
afternoon
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Global Air Circulation Models: 3-cell model
Hadley Cell
- strong solar heating at equator, low pressure
zone
- known as equatorial low/ITCZ
- zone of great instability, updraughts,
convectional rain
- can be seen from the space (photo)
3-cell model
- rising air at ITCZ diverges towards poles
- sinks at about 20 to 30 deg lat to form the
subtropical highs
- pressure gradient force directs air from this zone
towards to ITCZ
- Coriolis force deflects air to the right to form to
northeast tradewinds
- in S. Hemisphere, souteast tradewinds occur
3-cell model
Ferrel cell
- Ferrel cell flanks the Hadley cell
- circulates air between subtropical highs and
subpolar lows
- air flowing from subtropical high to subpolar
low is deflected to the right to form
westerlies
3-cell model
Polar cell
- air moves from the polar highs to subpolar
lows
- this flow of air is again deflected by the
Coriolis force to form a zone of polar
easterlies