Atmospheric Dynamics

Front 1

Lapse rate

Back 1

Temperature decreases with elevation

Average lapse rate – cooling at rate of 6.4°C/km

Lapse rate varies with water content in air

Front 2

Saturated Adiabatic Lapse Rate (SALR )

Back 2

Dew forms when the temperature drops to the point where water is condensing out of the air.

Condensation releases energy (think of the opposite of evaporation)

As a parcel of air rises, the (actual) temperature drops and water is condensed/precipitated.

SALR varies with temperature from 4 oC/ km to9 oC/ km

Front 3

Atmospheric pressure

Back 3

~90% of atmosphere’s mass is within 15 km of the surface the Troposphere

Basically– atmosphere is heavy and greatest “pressure” is closest to ground

Front 4

IdealGas Law

Back 4

PV = nRT

P is the pressure (Pa)

V is the volume in (m3)

n is the number of moles (mol)

1 mole = 6.022×1023molecules

R is the gas constant 8.314 (J·K−1·mol−1)

T is the temperature (K)

Front 5

Vertical and horizontal air movement

Back 5

Front 6

How are low and high atmospheric pressure created at the surface

Back 6

One column warms

Air rises leading to lower density

Lower atmospheric pressure – at surface

The other column cools

Air sinks leading to higher density

Higher atmospheric pressure – at surface

Front 7

Air movement

Back 7

Related to differences in air pressure

Wind is the horizontal movement of air from high pressure to low pressure

Greater the difference = faster the wind

Front 8

Pressure gradient

Back 8

Change in pressure measured across a given distance

lPG = change in pressure / distance

The pressure gradient results in a net force that is directed from high to low pressure and this force is called the pressure gradient force.

Wind is perpendicular to pressure gradiant

Front 9

Surface pressure chart and pressure gradient isograds (isobars)

(How to see if winds are strong on a map !)

Back 9

The closer the isobars, the greater the PGF magnitude. Larger PGF result in stronger winds, so strong winds are associated with closely spaced contour lines.

Front 10

Reasons for pressure variation

Back 10

. Pressure differences are ‘forced’ by uneven absorption of solar radiation at the Earth's surface

remember Ideal Gas Law:

PV = nRT

warmer air is less dense and typically exerts less pressure than cooler air

however, for a given surface pressure, a column of cold air will decrease in pressure with height more rapidly than will a warm column of air

. Moisture content of air also affects pressure

The greater the amount of water vapour, the less dense the air will be, and the less pressure it will exert

. Wind develops as a result of spatial (i.e. horizontal) differences in atmospheric pressure.

Wind speed tends to be at their greatest during the daytime when the greatest spatial extremes in atmospheric temperature and pressure exist.

Front 11

GlobalEnergy Budget

Back 11

Global energy budget is determined by the balance between:

- Solar energy absorbed by Earth

- Infrared radiation emitted to space

Neither is distributed uniformly

Incoming solar radiation varies with latitude and season

Outgoing radiation depends on temperature of the surface/atmosphere at a particular location

Front 12

Surface area of incoming solar radiation

Back 12

Sun's rays arrive parallel to Earth.

They are direct at the equator and oblique at higher or lower latitudes.

The equator receives 2.5 times more energy than the poles (More energy received in a smaller area, more concentrated)

It is more diffused at the poles since the same amount of energy (heat from the sun) covers a larger area due to the obliquity.

Front 13

Average distribution of absorbed solar and emitted infrared radiation with latitude

Back 13

Front 14

Global energy distribution - what creates circulation

Back 14

Distribution of global energy budget is a permanent feature of the system

If there was no atmosphere =

tropics would get warmer while poles get cooler

The latitudinal energy gradient creates atmospheric temperature and density differences forcing the atmosphere to circulate carrying warm air to poles and colder air towards tropics

Front 15

General circulation of the atmosphere

Back 15

Heat surplus in the tropics

Air warms and rises via convection

This creates a low pressure region

As air rises it must be replaced by horizontally moving air from higher pressure regions

This is known as convergence

The converging air masses that meet in the tropics are termed the intertropical convergence zone (ITCZ)

Front 16

ITCZ (intertropical convergence zone)

Back 16

Surface heating in tropics also results in evaporation

As convecting air rises, it cools, and the evaporated water (water vapour) condenses to form clouds

ITCZ is therefore characterised by extensive cloud cover and heavy rainfall

As winds come from northern and southern hemisphere they mix to create theso-called “doldrums” erratic weather patterns with stagnant calms and violent thunderstorms

Front 17

Generalcirculation of the atmosphere

Back 17

Stratosphere warmer than Troposphere

- Troposphere warmed from below

(Energy absorption, turbulence, convections etc)

- Stratosphere warmed from above

(heat comes mainly from ultra violet light interacting with ozone molecules)

As air rises in troposphere it cools

Stratosphere acts a cap to stop further rising

Front 18

General circulation of the atmosphere 2.

Back 18

Top of Troposphere: 10-15 kms

This barrier forces uplifting air to diverge outward away from convective zone

At ~30oN and ~ 30oS, the air subsides to replace the surface air moving towards equator

The air warms as it sinks preventing condensation and cloud formation

These regions of atmospheric subsidence are characterised by higher surface pressure, clear skies, low rainfall amounts (i.e. the desert regions)

Front 19

Hadley Cell Circulation

Back 19

Low pressure at the surface at the equator. Hot, wet air.

Air rises by convection. It rises, expands and cools. Creating high pressure at higher altitude.

Some heat radiates to space.

The air flows towards low-pressure area.

It becomes cool and dry and thus subsides, condenses and warms to the subtropical high-pressure area.

Reached the ground as warm bit dry hair. It flows towards the low pressure system at the equator thanks to trade winds and picks and moisture and heat.

Repeat

Front 20

What happens at 30-90° latitudes

Back 20

Low temperatures at the poles (esp. winter)

Increased density of air near surface due to subsidence

Higher pressures than at the tropics

This leads to divergence and a general horizontal movement of cool air towards the equator

This cool air will meet the warmer air coming from the tropics

This contact zone produces a steep temperature gradient called the Polar Front

These air masses do not mix well

Warm air less dense

Rises above cool air from poles

Polar front slopes poleward with elevation

Front 21

The Ferrel cell

Back 21

Subsidenceat ~30 degrees

Upliftat ~60 degrees

Inevitably, this leads to another cell: Ferrel

Front 22

The Coriolis Effect

Back 22

The tendency for a fluid (air or water) moving across the Earth’s surface to be deflected from its straight-line path due to the rotation of the Earth

There are two aspects to the Coriolis effect that we must understand

- Coriolis Force (affects winds going vertically N-S)

- Centrifugal Force (affects winds going horizontally E-W)

Front 23

The Coriolis Force

Back 23

- Viewed from Earth

in northern hemisphere

A north-to-south moving object appears to be deflected to the west

To the east for south-to-north – always to right in NH

- Viewed from Space

Same object will be seen to be moving in a straight line

Front 24

The Centrifugal force

Back 24

Whenair is moving eastwards or westwards, the Coriolis effect still comes intoplay, but for a different reason

Centrifugalforce willpush the object (air) away from the centre of rotation

If anair mass is moving eastwards faster than the Earth is rotating at thatlatitude, the air mass will be diverted away from the spin axis of the Earth

Front 25

High - Low Pressure systems & Coriolis effect

Back 25

High - Low pressure systems :

Air moves outwards from high pressure areas Air moves inwards towards low pressure areas

Coriolis Effect :

Both aspects of the Coriolis Effect deflects the wind and causes circulation around these systems

Front 26

Geostrophic wind

Back 26

A parcel of air will accelerate from high pressure to low pressure because depending on the pressure gradient force (PGF).

As the air parcel begins to move, it is deflected by the Coriolis Effect (to the right in the northern hemisphere, to the left in the southern hemisphere). Coriolis Effect always acts at 90° to the wind motion.

As the wind gains speed, the deflection increases until the Coriolis Effect equals the pressure gradient force. This balance is called geostrophic balance. The geostrophic wind blows parallel to contours. / Actual wind can cross isobars

Front 27

Friction

Back 27

The surface of the Earth exerts a frictional drag on the air blowing just above it.

This friction can act to change the wind's direction and slows it down

The difference in terrain conditions directly affects how much friction is exerted.

For example, a calm ocean surface is pretty smooth, so the wind blowing over it does not move up, down, and around any features.

By contrast, hills and forests force the wind to slow down and/or change direction much more.

Front 28

How friction affects wind direction

Back 28

Geostrophic wind:

case where pressure gradient force is equally and oppositely balanced by coriolis effect.
Resulting wind blows parallel to isobars / height contours

At the surface, friction force induces a drag and creates imbalance between PGF & CF. Resulting wind crosses isobars, diverging from high and converging into low pressure

Front 29

Important elements & cells and problems with this model

Back 29

Important Elements:
Hadley cell
Ferrel cell
Polar cell

Polar easterlies
Polar Front
Westerlies
Trade winds
Doldrums

Problems with it :
- still too simplistic
- As stated, friction is an important factor which means that this model is disrupted by the presence of land

- The convective properties of rising air is also different between land and sea

Front 30

Meridional (North ↔ South) Circulations

Back 30

Hadley cell

Polar cell

Ferrel cell

Front 31

Zonal(East ↔ West) Circulations

Back 31

Tradewind

Walkercirculation

Jets streams

Front 32

WalkerCirculation

Back 32

Longitudinally the Earth is not homogenous.

Differences cause a variation in the circulation longitudinally

Warm air rises over land (and warm seas).

Below shows the Walker circulation. This circulation is in addition to the general Easterly equatorial flow (i.e. it is superimposed).

Front 33

How do jetStreams form?

Back 33

In the upper troposphere:

-Large pressure gradient from tropics towards poles

-Greatest wind speed when pressure gradient is steepest

-South (high) to north (low) pressure gradient – wind to north [in northern hemisphere]

-Influenced by coriolis effect so wind deflect to right (in NH) – eastwards.

As there is no friction in upper troposphere, wind is geostrophic

However, these upper level winds move in a wavelike trajectory around the globe - known as Rossby waves

Front 34

Rossbywaves

Back 34

Theundulating motion of the Rossbywaves, coupled with the migratory pressure systems and storms associated withthe westerlies, give the middle latitudes more short-termweathervariability than any other place on Earth

Front 35

What are the two jet streams

Back 35

There are two cores of high-speed winds at high altitudes in the westerlies:

- Polar front jet stream

- Subtropical front jet stream

Polar Jet stronger than subtropical jet

Front 36

Characteristics of jet streams

Back 36

- are typically thousands of kilometres long, a few hundred kilometres wide, and only a few kilometres thick -usually found somewhere between 10-15 km above the earth's surface

- position of this upper-level jet stream denotes the location of the strongest SURFACE temperature contrast

- Speeds in the Jet Steams extremely fast (>200 kph)

-Huge influence on our weather

Front 37

Seasonalimplications on variability and strength of atmospheric circulation

Back 37

Thehemisphere experiencing summer has less of a temperature gradient between thetropics and the poles

Basicallylarger distance between the ITZC and poles

Thisweakens the strength of the atmospheric circulation

Oppositeis true for the winter

Front 38

Local wind circulation - sea shore

Back 38

During day, land and ocean heat up differently

By afternoon, land is warmer than ocean as it has a lower thermal heat capacity

Warm air rises creating low pressure

A local pressure gradient sets up and air moves from H to L, resulting in a sea breeze

At night, land cools down faster than ocean, reversing the pressure gradient

Wind blows off shore, resulting in land breeze

Front 39

Local wind circulation - Valley winds or breezes

Back 39

During the day the air close to the mountain sides becomes warmer than the surrounding air, thus lowering pressure relative to the cooler air at the same height over the valley.

This warm air rises up the mountains and is replaced by air from within the valley.

So during the day valley air moves up the mountain sides.

This process leads to clouds and rain in the mountains in the summer, particularly in the late afternoon

Front 40

Local wind circulation - Mountain winds or breezes

Back 40

At night the sides of the mountain cool down and cool air is pulled down by gravity.

In the morning, the coldest air is, therefore, often found within the valley – capped with valley cloud related to a temperature inversion

Front 41

Local wind circulation - Moutain ranges

Back 41

Whenair encounters mountain ranges, it rises, cools, and drops moisture on thewindward slopes resulting in a precipitation distribution called a rainshadow wherethe leeward slopes are dry.

Front 42

Local wind circulation - ColdFronts

Back 42

Cloud development because of frontal lifting of warm moist air

Front 43

Local wind circulation - WarmFront

Back 43

Cloud development because of frontal lifting of warm moist air

Front 44

Local wind circulation - OccludedFront

Back 44

When a cold front overtakes a warm front, clouds develop because of frontal lifting of warm moist air

Front 45

Whatis ENSO?

Back 45

ENSOis a “coupled” phenomenon: atmospheredrives the ocean and the ocean drives the atmosphere

An irregularlyperiodical variation in winds and sea surface temperatures over the tropicaleastern Pacific Ocean, affecting much of the tropics and subtropics

Thereare two phases to ENSO:

- Warm:El Niño

- Cool:La Niña

ENSOis pseudo-cyclic on a 2–7 (3-4) year time-scale

- ElNiño –last 12-18 months

- La Niña– last from 1-3 years

Theextremes of this climate pattern's oscillations cause extreme weather (such asfloods and droughts) in many regions of the world

Front 46

La Niña

Back 46

Cool phase

Strongeasterly winds induce upwelling of cold water in the equatorial eastern Pacific

Shallowoceanicthermocline in the east Pacific (due to upwelling)

Warm SSTsconfined to western Pacific with a deep thermocline

Low pressureand convection in west Pacific

High pressureand subsidence (clear air) in east Pacific

Front 47

El Niño

Back 47

Warm phase

Weaker easterly winds result in less upwelling of cold water

Warm SSTs “spread” to east Pacific (also solar heating not offset by upwelling)

Increase in the east Pacific thermocline

Low pressure and convection shifts to the east Pacific

High pressure and subsidence shifts to the west Pacific

Front 48

Atmosphere

Back 48

The envelope of gases surrounding the earth. Is Layered.

Front 49

Troposphere

Back 49

The lowest region of the atmosphere, extending from earths surface to a height of approx 6–10km. Where weather and climate occur. The atmosphere is heavy and greatest pressure when closest to ground

Front 50

Saturated Adiabatic Lapse Rate SALR

Back 50

Rate at which atmospheric temp decreases with an increase in altitude.

Front 51

Condensation

Back 51

The conversion of vapor or gas to liquid. Water which collects as droplets on a cold surface when humid air is in contact with it. Releases energy ( think of opposite of evaporation)

Front 52

Ideal Gas Law

Back 52

PV = nRT
P= pressure
V= volume
T= Temp
n= number of moles
R= gas constant

Front 53

Cyclone

Back 53

system of winds rotating inwards to an area of low pressure. Associated with bad weather.

Front 54

Anticyclone

Back 54

Weather system with high pressure at its centre. Associated with calm, fine weather.

Front 55

Convection

Back 55

System of winds converging on area of low pressure which rises to an area of high pressure causing a cyclone.

Front 56

Subsidence

Back 56

Anticyclone, wind at altitude at low pressure subsides, goes down, and diverges at surface where there is High pressure.

Front 57

Pressure Gradient

Back 57

Pressure gradient = change in pressure/distance

Front 58

PGF

Back 58

PG results in a net force that is directed from high to low pressure. The closer the isobars, the greater the PGF. Larger PGF results in stronger winds, so strong winds are associated with closely spaced contour lines.

Front 59

seasonality

Back 59

variations of solar radiation with latitude and season.

Front 60

Intertropical Convergence Zone ITCZ

Back 60

heat surplus in tropics. Air warms and rises via convection. this creates a low pressure region in which the air that rises must be replaced by horizontally moving air from higher pressure regions (convergence). these converging air masses that meet in the tropics are the ITCZ. characterized by extensive cloud cover and heavy rainfall.

Front 61

Polar front

Back 61

Low temp at poles. Increased density of air near surface due to subsidence. higher pressures than at the tropics. This leads to divergence and a general horizontal movement of cool air towards the equator. This cool air meets the warmer air coming from the tropics. this contact zone produces a steep temp gradient called the polar front. These air masses do not mix well. Warm air less dense rises above cool air from poles.

Front 62

Centrifugal force

Back 62

will push the object (air) away from the center of rotation. Record player analogy with penny, stays on closer to middle, on sides falls off instantly. Decreasing deflection towards equator. Largest deflection at poles

Front 63

Walker Circulation

Back 63

Longitudinal circulation. The Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms.

Front 64

Rossby Waves

Back 64

as there is no friction in upper troposphere, wind is geostrophic. However, these upper level winds move in a wavelike trajectory around the globe known as Rossby Waves.

Front 65

Sea Breeze

Back 65

by afternoon, land is warmer than ocean as it has a lower thermal heat capacity. Warm air rises creating low pressure. A local pressure gradient sets up and air moves from H to L.

Front 66

Land Breeze

Back 66

At night, land cools faster than the ocean and creates a pressure gradient. cool wind blows off of the high pressure shore onto the low pressure sea where it warms and rises.

Front 67

Orogenic effects on air

Back 67

when air meets mountain ranges, it rises , cools and drops moisture on the windward slope resulting in precipitation distribution called a rain shadow where the leeward slopes are dry

Front 68

Cold Front

Back 68

advancing mass of cool air which forces warm air up causing clouds due to front lifting of warm, moist air.

Front 69

Warm Front

Back 69

advancing warm mass of air lifts up over receding cold air causing clouds to form from lifting of warm, moist air.

Front 70

Occluded Front

Back 70

when a cold front overtakes a warm front

Front 71

El Nino Southern Oscillation ENSO

Back 71

an irregularly periodical variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean, affecting much of the tropics and subtropics.

Front 72

La Nina Cool phase

Back 72

Strong Easterly winds induce upwelling of cold water in the equatorial eastern Pacific. Shallow oceanic thermocline in the East Pacific (due to upwelling). Warm sea surface temp confined to Western Pacific with a deep thermocline. Low pressure and convection in West, high pressure and subsidence in East Pacific. lasts from 1–3 years.

Front 73

El Nino Warm phase

Back 73

Weaker easterly winds result in less upwelling of cold water. Warm SSTs spead to East pacific ( also solar heating not offset by upwelling). Increase in East Pacific thermocline. Low pressure and convection shifts to east Pacific. High Subsidence and pressure shifts to West Pacific. Lasts from 12 to 18 months.

Front 74

Global Energy Distribution

Back 74

Determined by solar energy absorbed by earth and infrared radiation emitted to space. Outgoing radiation depends on temp of surface/atmosphere. variations in solar radiation due to latitude, higher lat suns rays are oblique, larger area to cover so more diffusion of energy whereas equator is direct, more concentrated, smaller area covered.

Front 75

Water Vapours effect on pressure

Back 75

Water vapour lighter than oxygen or nitrogen so the greater amount of water vapour, the less dense the air will be, and the less pressure it will exert.

Front 76

Due to seasonality, why do the tropics not get warmer and the poles get colder?

Back 76

Would be the case without atmosphere. The latitudinal energy gradient creates atmospheric temperature and density differences forcing the atmosphere to circulate carrying warm air to poles and colder air towards tropics.