Transcript Next 4 weeks: Atmospheric temperature profiles Stability

L15 Atmospheric Stability
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Stability and lapse rate
Stability on skew T – ln p charts
Stable, neutral, unstable, conditionally unstable
Stable atmosphere
– Inversions – fog/air pollution
• Unstable atmosphere
– Convection
– Thunderstorm development
Static stability of dry air
1st case:
Real lapse rate, Γ
is less than the DALR, Γd
What happens to air at ‘O’
that is displaced upwards
by a disturbance?
And what happens if it is
displaced downwards?
See Wallace & Hobbs section 3.6
Static stability of dry air
2nd case:
Real lapse rate, Γ > Γd
What happens to air at ‘O’
that is displaced upwards
by a disturbance?
And what happens if it is
displaced downwards?
See Wallace & Hobbs section 3.6
Useful analogy for stability:
ball on a surface
Stable
Unstable
Unstable profiles
cannot persist, so
tend to rapidly
evolve to be ~neutral
Neutral
Conditional
instability
Stability and air pollution
Match the plume dispersion
pattern to the temperature
profiles below
Atmospheric temperature profile
is the solid line.
Dry adiabat is dashed line.
Stability and air pollution
Neutral profile – smoke
goes anywhere
Stable profile – smoke
stays at emitted height
Stable above, neutral
below – smoke mixes
down but not up
Neutral above, stable
below – smoke mixes
up but not down
Atmospheric temperature profile
is the solid line.
Dry adiabat is dashed line.
Stability on skew T-ln p diagram
Conditionally
unstable
Stable
stable
Absolutely
unstable
Absolutely unstable example
Tiny unstable layer
near ground
Extreme example of instability: a forest fire
Any form of
strong heating of
the ground can
cause instability
Inversion layer example
Very stable layer
probably defining
the top of the
boundary layer.
Very stable layers
where temperature
increases with height
are called ‘inversions’.
Top of the BL often
capped by an
inversion – tends to
trap pollution in BL
Edinburgh, c 1950
‘auld reekie’
London, 1952
Buncefield oil depot fire, 2005
Satellite image at 1045 GMT,
~5 hours after start of fire
Nottingham Skew T-log P on day of
Buncefield Fire (12Z, 11 Dec 2005)
Strong
inversion
near ground
Top of BL
South Pole:
Note it is cold!!
Surface pressure is
only ~700 hPa
Very stable thin
layer close to surface
Scott-Amundsen
South Pole station
Kuching Indonesia
Conditionally unstable - tropics
Small scale cumulus
clouds – typical of
convection and an
unstable tropical
atmosphere.
In the tropics these
clouds typically build
up during the day to
cumulonimbus
as surface heating
increases the
instability.
Douala, Cameroon
(Image from Google earth)
Summary
• Stability and lapse rate
• Stability on skew T – ln p charts
• Stable, neutral, unstable, conditionally
unstable
• Stable atmosphere
– Inversions – fog/air pollution
• Unstable atmosphere
– Convection
– Thunderstorm development – more next time