Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 5: Atmospheric Convection Chris...

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Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 5: Atmospheric Convection Chris Parkes Rm 455 Kelvin Building

Transcript of Planetary Atmospheres, the Environment and Life (ExCos2Y) Topic 5: Atmospheric Convection Chris...

Planetary Atmospheres, the Environment and Life (ExCos2Y)

Topic 5: Atmospheric Convection

Chris Parkes

Rm 455 Kelvin Building

4. Solar Radiation

• Absorption spectrum of atmosphere– Spectrum of incoming & outgoing

radiation

• Insolation – daily & annual variation

• Albedo

• Energy budget

• Greenhouse effect

Wm

-2μ

m-1

μm

Sun:Incoming

Earth:Outgoing

RadiationRevision

Convection in the Atmosphere

What drives it?

Hadley cell

- a simple model

A more realistic model of earth’s atmospheric convection

weight

Upward buoyancy

Archimedes’ Principle

Objects in fluid experience an upward (buoyancy) force equal to the weight of the displaced volume of fluid

Static balloon must have buoyancy equal to its weight

Hot air is less dense than cold air

Hot air rises

Air moves in “parcels” – like balloons but without the fabric

A parcel hotter than surroundings will experience a greater buoyancy force than its weight – net force upward – it will rise.

Upward buoyancy

weight

warmer air

parcel

cooler

surroundings

As rises:Temperature decreasesPressure decreasesVolume Increases(see lecture topic 3)

pV=T

Column of air being heated

Pressure (mb)

Height

200

500

800

Initially at same temperature as surroundings

Heating at ground level

Air volume expansion

Heating

Pressure (mb)

Height

200

500

800

Initially at same temperature as surroundings

Heating near ground level

Air volume expansion

Pressure difference at the top leads to outflow

Less weight in column

Lower pressure at surface

Inflow towards low pressure

Convective flow of air

H

L

Column of air being heated

Tropopause

South

Equator (low pressure)

Air column AB expands High pressure at B Low pressure at A (w.r.t. surroundings)

Warm air rises.

At B further convection is limited by temperature inversion at the tropopause

North

A

B

The Hadley Cell (1735)

Height

Ground

Tropopause

South

Equator (low pressure)

Convection Cell

(High pressure)

Air moves away from equator

cools gradually becoming more dense (B to C)

Air sinks back to surface (C to D)

Movement of air from high to low pressure (D to A)

North

A

BC

D

The Hadley Cell (1735)

Height

Ground

Tropopause

South

Equator (low pressure)

Convection Cell

(High pressure)

Vertical motion is on average ~10 cm/s (c.f. 10 m/s in cumulus cloud) – caused by change in density and pressure

Horizontal motion is due to pressure difference.

Changes are small ΔP = 50 mb (average P = 1000mb) 5% change

North

A

BC

D

The Hadley Cell (1735)

Height

Ground

Tropopause

South

Equator (low pressure)

Convection Cell

(High pressure)

No air is created or lost Mass moved per unit time = speed × density

Must be the same for surface and high level winds.

Density lower at higher altitude high altitude winds are fast

North

A

BC

D

The Hadley Cell (1735)

Height

Ground

Pressure “systems”

High pressure Air sinking, generally cooling Mostly over oceans

Low pressure Air rising, generally being heated Mostly over land

Here high and low pressure refer to surface pressure

i.e. top of “low” pressure region has a higher pressure than surroundings

Differential heating on Earth

Poles receive same amount of energy over larger area - less energy density on surface

Solar energy

Solar energy

North

Equator Equator receives a quantity of solar energy over a small area

Rotating an unit area by 60ºreduce incident radiation by half at 60º latitudes only get half of sun’s energy

Equator

Hadley cell

Hadley cell

Hot

Cold

Cold

All area heated, but -

More solar heating at equator creates hotter region

Air rises at equator (inter-tropical convergence zone, ITCZ)

Cooler air from poles moves towards equator to region of lower pressure

In reality on Earth:

Rotation

Day/night difference

Annual variation

Global air movement takes weeks

Hadley cells on Non-rotating planet

Venus as Hadley Cell• Venus:

– Slow rotation (Venus day is 243 Earth days)

• weak rotation effect – works like Hadley cell

– Dense atmosphere• Efficient transport of heat

– equator and poles similar temperature, despite incident radiation angle effect

• Mars: – Thin atmosphere

• Very little heat transported, poles much colder than equator

Mars

Rotation - The Coriolis effect

Apparent deflection of objects from a straight path when viewed in a rotating frame

Apparent “force” pushing outward going bodies to the right and inward going bodies to the left

Rotation of earth means:

Apparent movement to the right while moving on northern hemisphere and, to the left in the southern hemisphere

Coriolis Force

• Merry-go-round– Balls path deviates to the right

• Ball rolled inwards

• Or ball rolled outwards

– Would be reversed if anti-clockwise• coriolis force opposite in south / north hemispheres

The Coriolis effect

The Coriolis effect

In Hadley cell in northern hemisphere:

upper air moves north coriolis pushes it eastwards

lower surface air moves south coriolis pushes it westwards

Simple Hadley circulation cell model breaks down

The Coriolis effect

The Three-cell model of Earth’s atmosphere

Direct (Hadley) cell

- from equator to 30º

Indirect (Ferrel) cell

- from ~30º to 60º

Polar cell

Indirect cell driven by the other two

Surface and upper winds have east/west as well as north/south component

East/west balanced such that the whole atmosphere rotates with the globe

Better model needs to include:North/south & east/west movement

Angular momentum

Differential heating

Effect of land mass

Seasonal changes

trade winds

westerlies

easterlies

jet streams

The Three-cell model of Earth’s atmosphere

Smaller scale convection – Sea BreezesLand heats up quicker than sea

Air above land begins to rise

Sea air moves inland since rising air above land produces lower pressure

Size of effect increases throughout the day

Keep coastal regions cooler than inland

Reverse at night

Example exam questions

Q1. State what is a Hadley cell and explain how it works?

Q2. How does differential heating arise on Earth?

Q3. Sketch a diagram to explain the Coriolis effect.

Q4. Explain how sea breezes keep the coastal region cooler than inland.

Next lecture – wind

Convection … advection

– mechanism of heat transfer

Current in fluid under gravitational

field & differential heating