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