Transcript Title here - University of Manchester

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-2μm-1
Revision
Radiation
Sun:
Incoming
Earth:
Outgoing
μm
Convection in the
Atmosphere
What drives it?
Hadley cell
- a simple model
A more realistic
model of earth’s
atmospheric
convection
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
weight
Upward buoyancy
cooler
surroundings
warmer air
parcel
weight
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.
As rises:
Temperature decreases
Pressure decreases
pV=T
Volume Increases
(see lecture topic 3)
Column of air being heated
Pressure (mb)
200
Height
Heating
500
800
Initially at same temperature as surroundings
Heating at ground level
Air volume expansion
Column of air being heated
Pressure difference at
the top leads to
outflow
Pressure (mb)
200
H
Height  Less weight in
500
800
L
Initially at same temperature as surroundings
Heating near ground level
Air volume expansion
column
 Lower pressure at
surface
 Inflow towards low
pressure
 Convective flow of air
The Hadley Cell
(1735)
Tropopause
B
North
Height
South
A
Ground
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
The Hadley Cell
(1735)
Tropopause
C
North
B
Height
South
Convection Cell
D
A
(High pressure)
Equator (low 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)
Ground
The Hadley Cell
(1735)
Tropopause
C
North
B
Height
South
Convection Cell
D
A
(High pressure)
Equator (low pressure)
Ground
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
The Hadley Cell
(1735)
Tropopause
C
North
B
Height
South
Convection Cell
D
A
(High pressure)
Equator (low pressure)
Ground
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
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
North
Poles receive same amount of energy over
larger area - less energy density on surface
Solar energy
Solar energy
Equator
Equator receives a quantity of solar
energy over a small area
Rotating an unit area by 60º
reduce incident radiation by half

at 60º latitudes only get half of
sun’s energy
Hadley cells on Non-rotating planet
Cold
Hadley cell
All area heated, but More solar heating at equator
creates hotter region
Air rises at equator (intertropical convergence zone,
ITCZ)
Hot
Equator
Hadley cell
Cold
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
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:
Mars
– Thin atmosphere
• Very little heat
transported, poles much
colder than equator
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
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 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
The Three-cell model of Earth’s atmosphere
easterlies
jet streams
westerlies
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
trade winds
Better model needs to include:
North/south & east/west movement
Angular momentum
Differential heating
Effect of land mass
Seasonal changes
…
Smaller scale convection – Sea Breezes
Land 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