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 typesinfluences 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

Time Scale

Length Scale

Example

1 Macroscale Scale

  weeks –

years Days-

weeks

 

1000-40000km 100-5000km

Waves in Westerlies Cyclones, anticyclones,

hurricanes

2 Mesoscale

Minutes-days

1-100km

Land-sea breeze, thunderstorms, tornadoes

3. Microscale

Seconds-minutes

< 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 heatsensible heat and latent heatlatent 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 earth-atmosphere 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 pressure-gradient 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).

Cold Warm

Pressuredecreases

Equator

Pressure gradient

force

wind

Coriolis force

North Pole

Figure 4.8 Upper Westerlies

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 wave-like 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