Slide Show Chapter 17 07-05-2012

7/28/2019 Slide Show Chapter 17 07-05-2012

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SUN, ATMOSPHERE SYSTEM, AND HEAT BALANCE

The energy that derives most of the physical and chemical processes in

the earth comes from the sun.

Some of this energy is imparted to the atmosphere, but most of it

absorbed by the surface of the earth.

The energy received from the sun is in the form of electromagnetic

radiation.


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The motions in the atmosphere are due to energy received from sun plus

rotation of the earth.

The thickness of the atmosphere (110 m) is small compared to the

radius of the earth (average 6370 km).

But it has a fair mass 5x1018 kg. This high mass shows that it can store

fair amount of heat and energy in it.


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Albedo and Angle of Incidence

Albedo

Fraction of the incoming radiation reflected or scattered back to

space

Clouds, Snow, Ice covered surfaces 0.5 0.8

Fields, Forests 0.03 0.3

Water 0.02 0.05

Average over the earth surface 0.35


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Solar radiation reaching to the surface of the earth is remarkably constant.

The sun spots and variations in solar activity effects the x-rays reaching to

earth.

Since x-ray region of the spectrum is absorbed at the very top of the

atmosphere, such activities do not effect the radiation reaching to the

surface.


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Albedo is only one of the factors affecting the solar flux.

The amount solar radiation reaching to the surface

The angle of incidence of radiation compared to the

perpendicular to the surface

Zenith angle

Also effects the solar flux at a given point.

Zenith angle


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The solar flux on a horizontal surface (Sh) is given by

Sh = S cos Z

S: flux through an area normal to the solar beam,

Z: zenith angle which is the angle between the local vertical and solar

beam.

Note thatat angles away from local vertical (large Z) the solar radiation has

to traverse greater distance to cross the atmosphere and scattered more

and absorbed more in the process.


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The earths axes is 23.5 tilted. Because of this, the north pole is tilted towardthe sun on June 22 and away from the sun at December 21

Because of this tilt, the zenith angle will be different at different latitude.

23.5 23.5
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The zenith angle (Z) is given by

Cos Z = sin sin + cos cos : Latitude (positive for northern hemisphere): Solar declination: Hour angle (15 x # of hours before local noon)


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Solar azimuth ()Angle between south and the direction toward the sun in a horizontal plane.

sin = (cos sin )/sin Z


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Since most of the surfaces receiving solar radiation are not horizontal, we

must be able to find the solar flux over an inclined surface. It is given by

Ss = S[cos Z cos I + sin Z sin I cos ( - )I: slope of the surface

: degrees from south and are negative to the east of south and positive to the west of south.


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STABILITY AND INSTABILITY

Introduction

Ventilation

Means enhancement or suppression of the vertical motion of air in the

atmosphere.

There are three mechanisms with which air moves vertically.

1. Lifting over terrain

2. Lifting over weather fronts

3. Convergence toward low-pressure centers.

Downward movement of air is generally due to make up uplifted air

masses.


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Mixing height

Plot radiosonde ascent in the morning

Plot the dry adiabat starting from surface temperature at any time of

the day.

The altitude at which these two intersect is the mixing height.

This is an approximation because temperature profile determined by

morning radiosonde is assumed to be the same through the day.


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Potential temperature

Temperature of the parcel of dry air would be if it is brought to 1000 mb

adiabatically. It is given by

= T(1000/p)0.288If potential temperature decrease with height, the atmosphere is

unstable, and if the increase with height the atmosphere is stable.


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Radiation and Nocturnal inversion

This is the inversion caused by rapid heating and cooling of the surface.

z

T

8:00

At nightUnstable

9:00

12:007:00

Radiation Inversion

z

T

8:00

At nightUnstable

9:00

12:007:00

Radiation Inversion


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Subsidence inversion

Subsidence in atmospheric jargon means descent of air masses.

Air descends in front of high-pressure systems.

As it descends it heats up adiabatically (temperature increase).

At the lower altitude where it ends up, its temperature will be higher

than the temperature based on Actual lapse rate. This generate a

stable inversion layer called subsidence inversion.


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LAWS OF MOTION

There are both horizontal and vertical motions in the atmosphere. So far we

have discussed the vertical movement of air masses.

The horizontal component of atmospheric movement is called wind.

Winds are the result of various forces in the atmosphere.

They can be treated using Newtons second law of motion (F = ma)

Newtons law does not take into account the rotation of the earth, but the

rotation do effect winds and should be taken into account in the treatment.


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There are three important forces acting on an air parcel in the atmosphere:

Pressure-gradient force

Gravity

Friction


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Pressure gradient force is the force acting on an air parcel due to existenceof pressure difference in the atmosphere.

There are horizontal pressure gradients in the atmosphere (due to

horizontal temperature and density differences).

Exerts a force on an air parcel. This force is called pressure gradient force

and given by:

Ph = p/xp is the horizontal p difference at distance x.


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Direction of pressure gradient force is perpendicular to the lines of equal

pressure (isobars).

The direction is from high to low pressure.

High-P

Center

Low-P

Center

P-gradient

force

High-P

Center

Low-P

Center

P-gradient

force


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Coriolis Force

It is an apparent force, but not a real force.

Due to the rotation of the earth. Figure

Take an air parcel in the shown wind direction if wind blows in the

indicated direction for 1 hr at a speed of 10 m s-1.

Air parcel will move 36 km. But since the earth rotates (360 in 24 hrs,or 15 per hour), when the air parcel reaches to point B, it meets withpoint C not point B, because point A moved 15 to the west.


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When you look at the movement of air parcel from space you can

see that it is flowing straight down. But when you view it from the

surface of the earth, you can say that the wind is deflected to the east

by 15.This is called coriolis force. Although it is not a true force, it should be

treated like a true force.

Most important for aircraft flying long distance Figure

It can be considered as the force required to cause this deflection.


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Coriolis force is given by:

D = vf

Where f is the coriolis parameter and has the value of

F = 2 sin angular speed of earths rotation (7.27 x 10-5 s-1)

is latitudeDeflecting force is to the right of the wind vector in the northern hemisphere,

and to the left of the wind vector in the southern hemisphere.


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Geostrophic wind

When the p-gradient, and coriolis forces are the only forces acting,

the wind direction is the vector sum of these two forces.

When coriolis force is equal in magnitude and opposite in direction to the

p-gradient force, a wind vector which is perpendicular to both vectors

occur (note that coriolis force is perpendicular to the wind vector). This

wind is called geostrophic wind.

Geostrophic wind is parallel to the isobars.

Figure


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Gradient wind is observed when isobars are curved.

It is because of the additional force, namely the centrifugal force acting on

the system.

Gradient wind is observed when the gradient force is equal in

magnitude and opposite in direction to the sum of the coriolis and

centrifugal forces.

The direction gradient wind is parallel to the isobars.


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In the treatment of winds we have ignored effect of thefriction.

Effect of friction is felt when you get close to the surface.

Above 700 m we can assume that the effect of friction is negligible.

The friction effects the winds in two ways:

It slows down the wind,

It turns the wind direction.


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Because as the wind speeddecrease the coriolis force also decrease. The

new wind will be in the direction where coriolis + friction forces are

balanced by p-gradient force. Figure

This turning of the wind with friction is called ekman spiral.


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LOCAL WIND SYSTEMS

Sea and Land Breeze

Breeze system is due to different heating rates of the land and the sea

Land heats and cools faster than the sea (because solar radiation

penetrate several meters in the sea and have to heat all the water in

that layer. But it penetrates only few cm in the land and can easily

heat up that thin layer)


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Particularly in sunny summer days lands heat up early in the morning hence

the air above the land surface warms and ascends.

Due to this ascent the P over the land is smaller than the P at the same

altitude over the sea.

The ascending air also cause a high P over approximately 200 m

from the surface.

The ascending air over the land surface is replaced by cooler air from thesea, resulting in a flow from sea to land (see breeze).

Figure

L

HL

H


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The air that moves towards the land is replaced by the even cooler air

from higher altitudes.

This descent causes a Low P system at high altitudes.

At about 200 m from the surface air flows from high P over the land to

low P over the sea.

At night the land cools faster. The system is reversed and surface air moves

from land to sea forming land breeze.

Figure


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Mountain and valley winds

Due to different heating rates of the slopes of the valley and the

center.

A slope facing to the sun heats up more than the middle of the valley

during day time.

Air adjacent to the slope warms faster and rises along the slope. This

wind is called upslope wind.


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The air that rises is replaced by the cooler air from the centre of the valleyforming a complete loop.

At night situation is reversed and air descends along the slope (because

slopes cools faster) forming downslope winds.

Figure

There are various combinations of downslope and upslope winds

depending on the orientation of the valley.

Figure


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Urban rural circulations

These type of winds are due to different thermal properties of the urban and

rural regions.

The concrete, asphalt and steel material used in urban areas can

heat up faster and can store heat more than the vegetations in the

rural area.

The net result is that the urban areas are heated more than their rural

surroundings and radiate heat most of the night.

Consequently urban areas are warmer than their surroundings throughout

the day.


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The air ascends from the warmest part of the urban centre.

This ascended air is replaced by cooler air from rural areas. Forming a

surface flow from rural to urban and high altitude flow from urban to ruralregions.

The flow generally occurs in multiple cells in several directions.

This circulation is called urban heat island.


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If regional air flow dominates upper flow, urban rural flow occur only in one

direction. This regional circulation is called urban plume.

Urban plume carries pollutants emitted in the urban area to rural

areas.


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GENERAL CIRCULATION

There is an imbalance in the heat received by earth; equator receives more

energy than mid-latitudes and polar regions, thus heated more.

Means net influx of heat in equator and net out flux of heat at poles.

Atmosphere itself is not efficient in transporting the heat from equator

to poles.

Heat is transported to/from equator to poles by so called large-scale

circulations (winds).

This transport is accomplished by three consecutive cells in each


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This transport is accomplished by three consecutive cells in each

hemisphere.

In the tropical region surface winds move toward equator. The surface

winds in this cell are called easterlies, (because they are deflected towest due to earths rotation and winds are named from where they

come) or trade winds.

Most of the surface at tropics is oceans. The surface winds pick up heat

and moisture as they move toward equator.

The region where trade winds from each hemisphere are met is called

intertropical convergence zone.

It is a low pressure zone.


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Hot and humid air ascends and as it ascends it expands and cools. Water

in it condenses. Results in heavy rainfall in tropical regions.

As the water in ascending air condenses into clouds and rainfall, it

releases heat of condensation. Hence heating the air mass. This

latent heat is transported in the upper layers toward midlattitudes.


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At 30N latitude, there is a high-pressure system. The warm air coming fromtropics at high altitude descends. While it descends it adiabatically warms

up. Thus pumping heat from equator to mid latitudes.

This first cell between the tropics and equator is called Hadley cell.

Mid-latitude winds are more transient.


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Mid latitude winds are more transient.

In this region potential energy is converted to kinetic energy.

There is mostly in the form of closed pressure systems (cyclones and

anticyclones)

These cell systems are mobile and causes frequent weather changes.

That is why the weather in mid latitudes (we are living in mid latitudes) are

more variable than the weather in tropics and in polar regions.

The general flow is toward the pole in the surface and towards the equator


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The general flow is toward the pole in the surface and towards the equator

at the top.

The last cell in each hemisphere is the one closest to the polar

regions.

In this cell air ascends in front of the polar front. Moves toward the

pole at the top and sinks over the pole, and moves toward the

equator at the surface.

It pumps heat to the polar zone with the same mechanism with the Hadley

cell.


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Air picks up moisture over the ocean.

When it ascends it condenses and release heat. This heat is

transported toward pole.

When the air mass descends over the pole it warms up adiabatically.

This compensates the cooling as it ascends in the mid latitudes.


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The overall picture is that the Hadley cell pumps heat to the mid-latitudes

by evaporating ocean in the equator. And the last cell pumps heat by

evaporating the ocean in mid-latitudes.

Figure

Warm ocean currents (such as Golfstream) also carries heat from

equator to the poles.

Cold currents (such as Labrador) brings cold water from poles to the

equator which is heated and sent again to the polar regions.

Figure


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Vertical

Ventilation

(convective)

Emissions

Horizontal

Ventilation

(winds)


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Inversion

Episode

Calm

STAGNANT


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-6 C/km

Actual lapse rate

Back

Dry adiabat

-10C/km

-3C/km+6C/km


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A

T

z

DALRALR

z1

z2

z3

T5 T6

UNSTABLE

ALRDALR

STABLE

T3 T4T1 T2 T7 T8

Back


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BackTemp

Pressure

(torr)

10

11

17

20

Phase diagram H2O


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z2

WALR

T

DALR

z

z3

z4

z1


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2

3 4

5

1


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Figure 17.4. Radiation heat balance.


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Figure 17.6. Temperature of a parcel of air forced to rise 200 m compared to the superadiabaticenvironmental lapse rate.


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Figure 17.7. Temperature of a parcel of air forced to rise 200 m compared to an inversionenvironmental lapse rate.


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Motion of the earth

1 hr laterTime 0

15

Motion of the earth

B

AA

BC

Wind blows

From point A to B

At ws = 10 m s-1For 1 hr

= 3600 s x 10 m s-1

= 36000 m = 36 km

Earth rotates

360 per 24 hr

= 15 hr-1


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Istanbul Capetovn = 4000 km

Aircraft speed = 1000 km hr-14 hours

Rotation of the earth


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564 x 15 = 60

This is where you end up

This is where you want to go


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High-P

Center

Low-PCenter

P

P-gradient

force

Geostrophic wind

P-1

P+1

Coriolis force

Geostrophic winds


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P-gradientForce

Coriolis F.

Friction F. Rotated Wind

Low

P

HighP

Wind


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HighP

Low

P

1000 m

G.LP-2

P-1

P

P-3


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1. Warm air over land rises2. P at land surface < p sea surface

3. Cumuli develop aloft and move seaward

4. Upper level return land breeze

5. Cool air aloft sinks over water

6. Sea Breeze (meso-cold) Front

L

HL

H


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1. Cool air over land sinks

2. Land Breeze moves out over water

3. Relatively warmer water heats air which then rises

4. Upper level return sea breeze

5. Cool air over land sinks


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London


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New York

London


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Slide Show Chapter 17 07-05-2012

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