Download Atmospheric Circulation

Atmospheric Circulation
F.6 Geography
1. Atmospheric circulation (大氣環流) :
 horizontal and vertical flow,
 driving forces of air movement,
 influences on surface wind system
2. Major wind systems:
patterns and characteristics of the
trades, westerlies, polar winds and
monsoons
3. Air masses:
nature and types
influences on weather and climate
4. Atmospheric disturbances
typhoons and man responses
changes of wind speed and
direction
EXPLANATORY NOTES
1. Atmospheric circulation is the
mechanism through which the
energy surpluses and deficits are
balanced, and the balance involves
air movement of different scales.
2. Students are expected to :
~ understand the flows and driving forces
of atmospheric circulation
~ relate atmospheric circulation to surface
wind systems,
~ understand the nature and
characteristics of air masses and
~ note how people response to
atmospheric disturbances, e.g.
typhoons
3.
Temporal and spatial scales for atmospheric motions
Name of Scale
1
Macroscale
Time Scale
Length
Scale
weeks –
years
Daysweeks
100040000km
1005000km
Minutesdays
1-100km
Example
Waves in Westerlies
Cyclones, anticyclones,
hurricanes
Scale
2 Mesoscale
Land-sea breeze, thunderstorms,
tornadoes
3. Microscale
Secondsminutes
< 1 km
Turbulence
I. INTRODUCTION [引言]
The atmosphere acts as heat engine in
which the difference in the temperature
between the poles and the equator
provides the energy supply to drive the
planetary atmospheric circulation.
Large-scale air circulation transport heat,
both sensible heat and latent heat present
in water vapor.
Because of the global radiation imbalance
-- a surplus in low latitudes and
a deficit in high latitudes
Atmospheric circulation must transport
heat across the latitudes from the regions
of surplus to the region of deficit.
The variable heating of different parts of
the atmosphere sets up variations in
pressure, which in turns sets the air in
motion.
Wind is air in motion and it dominantly
horizontal.
(II) ATMOSPHERIC CIRCULATION (GENERAL CIRCULATION) (大
氣環流)
HORIZONTAL AND VERTICAL AIR FLOW (橫向
及直向流動)
THE IMPORTANCE OF AIR MOVEMENT (空氣流動的重要性)
1. Thermal redistribution
2. Transfer of water vapour
1. HORIZONTAL MOVEMENT = WINDS(風)
Horizontal movement, or wind, is by far
the faster and consists of air movements
parallel to the surface.
Horizontal movements or wind is an
important climatic factor for a number of
reasons.
(1) Thermal Re-distribution (熱力再分佈)
Wind balanced warm and cold bodies of
air, thereby modifying the thermal
characteristics of places related to their
radiation regime.
Such modification may have a
considerable effect on the air temperature
of a place:
Air movement is important to weather
and climate, and human significance.
For convenience, air motion may be
resolved into two components: horizontal
and vertical
A change in wind direction may cause
changes in temperature.
For example, HK in winter was affected
by NW monsoon winds.
(2) Moisture Transfer (水份轉送)
Wind action transports water vapor. In
particular, moisture is brought from areas
where it is abundant, such as over the
oceans, to areas where it is often deficient,
such as over the interiors of continents.
e.g. Onshore winds in E and SE China,
including HK, in summer.
Example: Figure below illustrates the
significance of a seasonal reversal of wind
direction in rainfall amount for Hong
Kong. It shows the effects of monsoon
winds on rainfall.
(3) Environmental hazards (環境災害)
Air in rapid motion is, a severe
environmental hazard. On average, more
lives are lost each year as a result of
tropical storms than from the combined
effects of fire, lightning, floods, tidal
waves and earthquakes.
2. VERTICAL MOVEMENT
Vertical motions, on the other hand, involve
sinking and rising masses of air perpendicular
to the surface and are usually 100-1000 times
slower than their horizontal counterparts.
Vertical movements of air, although normally
less rapid than their horizontal counterparts,
are very important, since they strongly
influence whether the climate and weather will
be cloudy and rainy or clear and dry.
Areas where air is sinking are relatively
cloud-free and dry, e.g. TD;
whereas in areas characterized by rising
air motion the opposite weather types
tend to prevail. e.g TRF.
3. RELATIONSHIP BETWEEN AIR MOTION (BOTH HORIZONTAL
& VERTICAL) AND THE GLOBAL ENERGY BUDGET.
Air in motion, however, has an even more
fundamental function to fulfilled at a global
scale -- the transfer of heat.
It will be recalled from energy budget that
THE UNEQUAL HEATING OF THE EARTH
SURFACE BY THE SUN PRODUCES A
LATITUDINAL CONTRAST IN ENERGY
BUDGETS between about 40 N and 35 S,
where the amount of incoming radiation
exceeds that lost by the cooling of the earthatmosphere system, whereas towards the poles
the reverse applies.
Obviously, if such a situation persisted ,it
would cause the low latitudes to be very
much hotter than they are at present, and
the high latitudes to be very much more
cold.
Atmospheric movement implies the
existence of a mechanisms whereby heat
is moved from the surplus areas to the
deficit areas to compensate for the
shortfall in the energy budget of the latter.
B. DRIVING FORCES THAT CONTROL
AIR MOVEMENT
1. SPATIAL VARIATION OF
TEMPERATURE (地面溫度差異)
 The energy required to drive the
gigantic circulation of the earth surface
is provided by the temperature contrasts
between cold polar region and warm
tropical air region. Why there is
unequal heating on the earth surface ?
2. AIR PRESSURE (空氣壓力)
Although not readily noticeable, air
exerts a pressure on every surface
exposed to it.
That pressure can be considered as
resulting from the weight of overlying air
pressing down on a given area.
AIR MOTION IS A RESPONSE TO A FORCE OR
FORCES OF SOME KIND
ATMOSPHERIC MOTION IS
CONTROLLED BY THE INTERPLAY
BETWEEN 5 FORCES:
1. THE PRESSURE-GRADIENT FORCE
2. THE CORIOLIS FORCE
3. FRICTION
Air motion is initiated by a pressure gradient
between places, with initial movement
occurring from high to low pressure locations.
The air is pushed from areas of high pressure to
areas of low pressure. The air ought to move at
right angles to the isobars.
(Spatial variations of pressure are depicted on
maps by means of isobars, which are lines
connecting places having the same barometric
pressure).
1. THE PRESSURE GRADIENT FORCE (大氣梯度)
-- INFLUENCE THE DIRECTION AND
SPEED OF WIND
The gradual change of pressure between
different areas is known as pressure gradient.
Where a pressure gradient exists, air molecules
tend to drift in the same direction as that
gradient. This tendency for mass movement of
air is referred to as the pressure gradient force.
The magnitude of the pressure gradient force is
directly proportional to the steepness of the
gradient.
A simple relationship between pressuregradient and wind speed exists: the
steeper the pressure-gradient, the faster
the wind speed.
Falling pressure (low pressure) generally
generate the onset of poor unstable
weather, and a rising barometer(high
pressure) suggests a trend towards sunny
stable weather conditions.
A pressure gradient exists both vertically
and horizontally.
(a) Vertical Pressure Gradient
Pressure decreases vertically. As we
move upwards through the atmosphere,
the weight of overlying air diminishes.
Obviously, the layers closest to the
surface will have the greatest weight
overlying them and thus the pressure will
be greatest. Therefore, rapid decrease in
air pressure occurs with increasing height.
(b) Horizontal Pressure Gradient
Pressure varies laterally because of the
temperature
differences
resulting
from
differences in the intensity of solar heating of
the atmosphere.
Where solar radiation is intense, the air warms
up, expands and its density decreases. As a
result, air pressure falls.
Where cooling occurs, the air contracts, its
density increase and air pressure becomes
greater.
2. Coriolis force
 As the earth will rotates , the wind
blowing in Northern Hemisphere will
deflected to its right.
In Southern Hemisphere, it will deflected
to its left.
The force exerted greatest in pole, but
lowest in equator
3. Friction
All types of obstacles produce frictional
drag where the wind blowing through.
Frictional drag acts in a direction
opposite to the path of motion and can
cause deceleration
It also reduces the magnitude of the
Coriolis force which is dependent on
wind speed
It will disturbed the combined Coriolis
force and frictional force and cause the
wind to blow obliquely across the isobar
(c) Pressure in the Upper Atmosphere
But the pattern of air pressure close to the
surface is reverse in the upper atmosphere.
This is because as cold air contracts, the upward
decline in pressure is rapid and at any constant
height above a zone of cool air the pressure is
relatively low. (High pressure at lower
atmosphere, but low atmosphere at the upper)
Conversely, warm air expands and rises, so that
the vertical pressure gradient is less steep.
Above areas of warm air(low pressure),
therefore, the pressure tend to be relatively high
(high pressure).
Figure 4.8 Upper Westerlies
wind
Pressure
decreases
Pressure
gradient
force
Cold
North
Pole
Coriolis
force
Warm
Equator
With increasing altitude, wind tend to be
prevailing westerly
It blow at high speed , (125km hr)
Band of rapid air movement in the upper
called jet streams
At this height, the frictional effect of the
ground surface upon winds is very weak
Air flows nearly approximates to the
geostrophic winds
The lower air density at high altitude also
allows air to flow more easily
The upper air westerlies occur as wavelike forms, called Rossby Waves
This is due to the effects of land and sea
difference on the surface and relief
differences along the same latitude
Three to six Rossby waves encircle the
globe in amplitudes covering 15°to 20° of
latitude