Transcript Thermal Processes - Home: Earth and Environment

Thermal Processes
ENVI 1400 : Lecture 6
Radiation Processes
reflected solar
radiation
107 W m2
Incoming solar radiation
342 W m2
Outgoing
longwave
radiation
235 W m2
40
30
Reflected by clouds,
aerosol & atmosphere
77
Absorbed by
atmosphere
165
emitted by
atmosphere
67
24
78
350
back radiation
324
Reflected
by surface
30
40
168
24
Absorbed by surface
78
thermals Evapotranspiration
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324
Surface radiation
Absorbed by
surface
2
Adiabatic Processes
• An adiabatic process is one in
which no energy enters or
leaves the system.
• Many atmospheric processes
are adiabatic (or nearly so) –
particularly those involving the
vertical movement of air.
– Air is a poor thermal conductor,
and mixing often slow enough
for a body of air to retain its
identity distinct from the
surrounding air during ascent.
• Near-surface processes are
frequently non-adiabatic.
Adiabatic Processes:
– Ascent of convective plumes
– Large scale lifting/subsidence
– Condensation/evaporation
within an airmass
Non-Adiabatic Processes:
– Radiative heating/cooling
– Surface heating/cooling
– Loss of water through
precipitation
– Addition of water from
evaporation of precipitation
falling from above
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Lapse Rate
Dry Adiabatic Lapse Rate
1km
Altitude
• Lapse Rate is the term
given to the vertical
gradient of temperature.
• The fall in temperature
with altitude of dry air that
results from the decrease
in pressure is called the
Dry Adiabatic Lapse
Rate = -9.8°C/km.
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9.8°C
Temperature
4
Altitude
• Condensation releases
latent heat, thus
saturated air cools less
with altitude than dry air.
• There is no single value
for the saturated
adiabatic lapse rate. It
increases as temperature
decreases, from as low
as 4°C/km for very warm,
tropical air, up to 9°C/km
at -40°C.
Dry Adiabatic
Lapse Rate
Saturated Adiabatic
Lapse Rate
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Temperature
5
Pressure & Temperature
P5
P5
P4
P4
P3
P3
P2
P2
P1
P1
P0
cool
z
warm
P0
• A column of air has pressure
levels P1, P2, etc.
• If the column is warmed, the air
will expand and it’s density at
any given level decrease.
• The vertical interval between
pressure levels increases, so
that at any given altitude the
pressure in the warmer column
is greater than in the cooler.
• N.B. since the total mass of air
in the column is constant, the
pressure at the surface does not
change
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Cold-core Low intensifies with height
Warm-core High intensifies with height
L
H
L
warm
H
warm
cool
Warm-core Low weakens with
height, may form a high aloft
cool
warm
cool
cold-core High weakens with height,
may form a low aloft
H
L
cool
L
warm
H
cool
warm
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cool
warm
7
• Mid-latitude low-pressure cells
have colder air to the rear.
• As a result, the axis of the low
slopes towards the colder air
Cold low
L
Warm high
Sea-level isobars
500 mb contours
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• High pressure cells slope
towards the warmest air aloft.
• The centre of the cell at 3000m
may be displaced 10-15°
towards the equator.
Cold low
H
Warm high
Sea-level isobars
500 mb contours
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The Thermal Low
• Thermal lows result from the
strong contrast in surface
heating between land and sea
• Land heats up (solar radiation)
and cools down (infra-red
radiation) much more rapidly
than ocean  large diurnal
cycle cross-coast temperature
gradient
• N.B. A thermal low results from
fine, clear, warm weather, and
thus differs from the
depressions associated with
cloud and bad weather.
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1. Start with a horizontally
uniform pressure distribution.
Solar radiation starts to warm
land. Air near surface is
warmed by land, convection
mixes warm air upwards and
whole boundary layer warms.
2. Air over land warms and
expands. Can’t expand
sideways, so column expand
upwards  produces high
pressure aloft.
N.B. Surface pressure remains
constant at this stage.
H
cool
warm
cool
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3. Horizontal pressure gradient
aloft drives a flow from over
land to over ocean.
H
cool
warm
cool
H
cool
L
warm
cool
4. Mass of air in column over land
is reduced  surface pressure
falls to produce a surface low.
High pressure aloft weakens,
but is maintained by continued
heating at surface.
Surface pressure gradient
drives flow from sea to land:
the sea breeze.
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5. When solar heating stops,
pressure driven flows act to
equalize pressure, restoring
conditions to the initial uniform
pressure field.
H
If land cools sufficiently at
night, the reverse situation can
be established.
L
L
H
warm
cool
warm
Over large land masses there
may be insufficient time over
night for the sea breeze to
reach regions far from the
coast, and a weak surface low
is maintained over night. This
then deepens during the
following days, and a heat low
may be maintained for days or
weeks, until synoptic
conditions change.
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Sea Breeze
• Formation of local thermal
low over land, results in the
formation of a sea-breeze
• In-flowing cool air from sea
forms a sea-breeze front – a
miniature cold front
• Air ahead of the front is
forced upward, contributing to
the formation of cumulus.
950 mb
975 mb
1000 mb
25C
15C
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Pressure as an indicator of
temperature
Because the depth of a
layer of air increases as its
temperature increases, we
can use the difference in
altitude between two
constant pressure levels as
an indicator of the mean
temperature of the layer.
Charts are usually produced
of the depth of the layer
between 1000 and 500 mb.
The layer depth is usually
quoted in deca-metres (10s
of metres)
A useful rule of thumb is that
for 1000-500 mb layer
depths less than 528 dm
(5280 m) any precipitation
will fall as snow rather than
rain.
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SLP (mb) & 1000-500 thickness : 48hr forecast valid 0000 040922
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SLP (mb) & 1000-500 thickness (dm) : 36hr forecast valid 0000 040930
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SLP (mb) & 1000-500 thickness (dm) : analysis valid 0000 040930
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2°C
12°C
850 mb Temperature (2°C contours), RH (%), wind (m s-1) : analysis valid 0000 040930
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Surface temperature (2°C contours) and SLP (mb)(5mb contours) : analysis valid 0600 040930
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The Thermal Wind
• It is commonly observed that
clouds at different altitudes
move in different directions 
winds are in different
directions.
• The gradient of wind velocity
(speed & direction) is called
the (vertical) wind shear.
• In the free air, away from
surface (where friction effects
complicate matters), the wind
shear depends upon the
temperature structure of the
air.
• The thermal wind is a
theoretical wind component
equal to the difference
between the actual wind at two
different altitudes.
• Any two levels can be used,
but unless otherwise stated the
altitudes of the 1000mb and
500mb levels are usually used.
• Note that the 1000mb level
might be below sea level, and
is usually within the boundary
layer and thus influenced by
friction effects at the surface.
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cold
LOW
warm
500mb
HIGH
Vg(1000)
LOW
996
1000mb
HIGH
1004
1008
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500-1000 mb thickness
5580
5640
5700
LOW
VG1000
5640
5700
VT
LOW
VG500
0
5760
60
5820
HIGH
120
Contours of
1000 mb surface 180
Contours of
500 mb surface
HIGH
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• Note that cold air is to the left
of the thermal wind vector
(looking along wind) in the
northern hemisphere, to the
right in the southern
hemisphere.
• The decrease in temperature
towards the poles results in a
westerly thermal wind in the
upper atmosphere in both
hemispheres.
• The largest meridional
temperature gradient occurs in
mid-latitudes across the polar
front.
• The thermal wind makes up a
significant component of the
jet-stream, located over the
upper part of the polar front.
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