Bài giảng khí hậu học chương 7

- The three-cell model divides the circulation of each hemisphere into three distinct cells: the heat-driven Hadley cell that circulates air between the Tropics and subtropics, a Ferrel

Chapter 7

Atmospheric circulation

By Vu Thanh Hang, Department of Meteorology, HUS

G304 – Physical Meteorology and Climatology

7.1 Single-cell Model

- A simple circulation pattern called

the single-cell model to describe the

general movement of the atmosphere.

- In the single-cell model, air expands

upward, diverges toward the poles,

descends, and flows back toward the

equator near the surface

- Winds blowing east-to-west or

west-to-east are referred to as zonal winds;

those moving north-to-south or

south-to-north are called meridional winds.

7.1 Single-cell Model (cont.)

• Hadley’s idealized scheme assumed a planet covered by a single ocean and warmed by a fixed Sun that remained overhead at the equator

• Hadley’s main contributions were to show that differences in heating give rise to persistent large-scale motions (called

thermally direct circulations) and that zonal winds can result from deflection of meridional winds

Æ Not so realistic

- The three-cell model divides the circulation of each hemisphere into three distinct cells: the heat-driven Hadley cell that circulates air between the Tropics and subtropics, a Ferrel cell in the middle latitudes, and a polar cell.

7.2 Three-cell model

- Each cell consists of one belt

of rising air with low surface air

pressure, a zone of sinking air

with surface high pressure, a

surface wind zone with air

flowing generally from the

high-pressure belt to the

low-pressure belt, and an air flow

in the upper atmosphere from

the belt of rising air to the belt

of sinking air.

7.2 Three-cell model (cont.)

• The Hadley cell:

- Along the equator, strong solar heating causes air to expand upward and diverge toward the poles, creating a zone of low pressure at the equator called the equatorial low

or the Intertropical Convergence Zone (ITCZ)

- The ITCZ is the rainiest latitude zone in the world and is observable as the band of convective clouds and showers extending from northern South America into the Pacific on the satellite image

- The ITCZ is sometimes called the doldrums

7.2 Three-cell model (cont.)

ITCZ on satellite images

7.2 Three-cell model (cont.)

• The Hadley cell (cont.):

- At about 20° to 30° latitude, air in the Hadley cell sinks toward the surface to form the subtropical highs, large bands

of high surface pressure Æ Cloud formation is greatly suppressed and desert conditions are common in the subtropics

- In the NH, as the pressure gradient force directs surface air from the subtropical highs to the ITCZ, the weak Coriolis force deflects the air slightly to the right to form the northeast trade winds

- In the SH, the northward-moving air from the subtropical high is deflected to the left to create the southeast trade winds

7.2 Three-cell model (cont.)

• The Ferrel and polar cells:

- Immediately flanking the Hadley cell in each hemisphere is the Ferrel cell, which circulates air between the subtropical highs and the subpolar lows

- On the equatorial side of the Ferrel cell, air flowing poleward away from the NH subtropical high undergoes a substantial deflection to the right, creating a wind belt calledthe westerlies

- In the SH, the pressure gradient force propels the air southward, but the Coriolis force deflects it to the left, producing a zone of westerlies in that hemisphere as well

7.2 Three-cell model (cont.)

• The Ferrel and polar cells (cont.) :

- In the polar cells of the three-cell model, surface air moves from the polar highs to the subpolar lows

- Very cold conditions at the poles create high surface pressure and low-level motion toward the equator In both hemispheres, the Coriolis force turns the air to form a zone of polar easterlies in the lower atmosphere.

7.2 Three-cell model (cont.)

7.3 Semipermanent pressure cells

• The real world is not covered by a series of belts that completely encircle the globe Æ a number of alternating semi- permanent cells of high and low pressure

• They are called semipermanent because they undergo

seasonal changes in position and intensity over the course of the year

• Some of these cells result from temperature differences and others from dynamical processes

7.3 Semipermanent pressure cells (cont.)

January

7.3 Semipermanent pressure cells (cont.)

July

The Sahel is a region of Africa bordering the southern Sahara Desert During the summer (left), the ITCZ usually shifts northward and brings rain to the region For much of the year,

the ITCZ is located south of the Sahel, and the region

receives little or no precipitation (right).

7.3 Semipermanent pressure cells (cont.)

• Upper tropospheric heights decrease poleward from lower latitudes due to the increased density of colder air

Decreasing heights

with latitude

7.4 The upper troposphere

• Westerly winds in the upper atmosphere:

- Height differences correspond to pressure differences Æwhen the 500mb surface slopes steeply Æ exists a strong pressure gradient force

- On 500mb map, there is always a PGF across the middle latitudes trying to push the air toward the poles

- In the absence of friction, the wind do not blow poleward, but rather blow parallel to the height contours, from W to E

- PGF is strongest in winter Æ upper level westerlies are strongest in winter Æ affect aviation

7.4 The upper troposphere (cont.)

• Westerly winds in the upper atmosphere (cont.):

- Wind speeds generally increase with height between the surface and the tropopauseÆ because of decreasing of friction and PGF is stronger at high altitudes

- The surfaces representing the 900, 800, and 700 mb levels all slant downward to the north, but not by the same amount

- Higher surfaces slope more steeply, which means that the pressure gradient force is greater

7.4 The upper troposphere (cont.)

• Westerly winds in the upper atmosphere (cont.):

The difference in heights between successive surfaces continues to

increase upward, leading to stronger winds.

• The polar front and jet streams:

- The polar front is a strongly sloping boundary between warm mid-latitude air and cold polar air

- Within the front, the slope of the pressure surfaces increases greatly because of the abrupt horizontal change

in temperature

- With steeply sloping pressure surfaces there is a strong PGF, resulting in the polar jet stream situated above the polar front near the tropopause Æ affecting daily weather in mid-latitudes

- The jet stream as a consequence of the polar front, arising because of the strong temperature gradient (9-12km above sea level)

7.4 The upper troposphere (cont.)

• The polar front and jet streams (cont.):

- Wind speeds average about 180km/hr in winter and about half that in summer, peak winds can exceed twice these values

- Near the equator is the subtropical jet stream, associated with the Hadley cell, can bring with it warm, humid conditions

7.4 The upper troposphere (cont.)

• Troughs and ridges:

- The 500 mb surface reveals

that heights decrease from south

to north but also rise and fall

through the ridges and troughs.

- Height contous are displaced

toward the equator in troughs

and toward the pole in ridges.

- Air flows poleward around

ridges and equatoward around

troughs

7.4 The upper troposphere (cont.)

7.4 The upper troposphere (cont.)

• Rossby waves (cont.):

- Changes from summer

to winter Æ fewer in number, have longer wavelength, strongest winds in winter

7.4 The upper troposphere (cont.)

• Rossby waves (cont.):

7.5 The Oceans

• Ocean currents:

that have an impact on the exchange of energy and moisture between the oceans and the lower atmosphere.

7.5 The Oceans (cont.)

• Ocean currents (cont.):

- Ocean currents are driven by

winds in the lower atmosphere

that exert a drag on the water at

an angle 45° to the right (NH)

and continue to shift clockwise

as their speed decreases

- At a depth of about 100 m, the

direction of the current is in

opposite with the direction of

7.5 The Oceans (cont.)

• Upwelling:

- Strong offshore winds along a coastal region sometimes drag the warmer surface waters seaward, which draws up cooler waters from below to take their place

- This process, called upwelling, greatly influences sea surface temperatures over the eastern portions of the major oceans

• Features such as the Intertropical Convergence Zone, the westerlies, and large Rossby waves exist on a global scale.

• Smaller features, such as cyclones, anticyclones, troughs, and ridges, exist at the synoptic scale, covering hundreds or thousands of square kilometers

• Mesoscale events are on the order of tens of square kilometers and last for periods as brief as half an hour

• The smallest exchanges of mass and energy operate at the

microscale

7.6 Major wind systems

• Monsoons:

- refers to the climatic pattern in which

heavy precipitation alternates with hot, dry

conditions on an annual basis due to the

seasonal reversal in surface winds caused

by an oscillation between high- and

low-pressure cells.

- During winter (top), dry air flows

southward from the Himalayas

- When summer arrives (bottom) moist air

is drawn northward from the equatorial

oceans

- Surface heating, convergence, and a

strong orographic effect cause heavy rains

over the southern part of the continent.

7.6 Major wind systems (cont.)

• Foehn wind:

- Foehn is the generic name for synoptic scale winds that flow down mountain slopes, warm by compression, and introduce hot, dry, and clear conditions to the adjacent lowlands

7.6 Major wind systems (cont.)

Foehn effect

• Sea and land breeze:

- During the daytime, land surfaces

warm more rapidly than the adjacent

water (a) Æ the air column overlying

the land to expand and rise upward

(b)

- At a height of about 1km, the rising

air spreads outward (c)

- The air over the water moves

toward the low-pressure area over

the land, which sets up the daytime

- At night the land surface cools more

rapidly than the water The air over

the land becomes dense and

generates a land breeze.

7.6 Major wind systems (cont.)

• Valley and moutain breeze:

- A valley breeze (a) forms when

daytime heating causes the

mountain surface to become

warmer than nearby air at the

same altitude

- The air expands upward and the

air flows from the valley to replace

it

- Nocturnal cooling makes the air

dense over the mountain and

initiates a mountain breeze (b)

7.6 Major wind systems (cont.)

7.7 Air-Sea interactions

• ElNiño, LaNiña and the Walker circulation:

- ElNiño is a recurrent event in the tropical eastern Pacific in which sea-surface temperatures are significantly above normal

- LaNiña is the inverse event (cold sea-surface temperatures)

- The Walker circulation is an east-west circulation pattern of the Tropics, characterized by several cells of rising and sinking air connected by horizontal motions along parallel lines of latitude

- The Southern Oscillation is the reversal of surface pressure patterns over the tropical Pacific associated with ElNiñoevents

7.7 Air-Sea interactions (cont.)

• ElNino, LaNina and the Walker circulation (cont.):

7.7 Air-Sea interactions (cont.)

• ElNino, LaNina and the Walker circulation (cont.):