Thermal Processes
ENVI 1400 : Lecture 6
ENVI 1400 : Meteorology and Forecasting 2
Radiation ProcessesIncoming solar radiation
342 W m2
Reflected by clouds, aerosol & atmosphere
77
168
30
Reflected by surface
Absorbed by surface
Absorbed by atmosphere
67
thermals
24
24Evapo-transpiration
78
78 390 324
324350
40
4030
Surface radiation Absorbed by surface
reflected solar radiation107 W m2
back radiation
emitted by atmosphere
165
Outgoing longwave radiation235 W m2
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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• 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.
1km
9.8°C
Temperature
Alti
tude
Dry Adiabatic Lapse Rate
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• 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.
Temperature
Alti
tude
Saturated AdiabaticLapse Rate
Dry AdiabaticLapse Rate
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Pressure & Temperature• 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
P0
P1
P2
P3
P4
P5
zcool P0
P1
P2
P3
P4
P5
warm
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H
L
coolwarm warm
cold-core High weakens with height, may form a low aloft
H
H
Warm-core High intensifies with height
cool coolwarm
L
L
Cold-core Low intensifies with height
coolwarm warm
L
H
cool coolwarm
Warm-core Low weakens with height, may form a high aloft
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• Mid-latitude low-pressure cells have colder air to the rear.
• As a result, the axis of the low slopes towards the colder air
L
Sea-level isobars500 mb contours
Cold low
Warm high
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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.
Sea-level isobars500 mb contours
Warm high
H
Cold low
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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.
warm coolcool
H
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3. Horizontal pressure gradient aloft drives a flow from over land to over ocean.
warm coolcool
H
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.warm coolcool
H
L
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H
L
5. When solar heating stops, pressure driven flows act to equalize pressure, restoring conditions to the initial uniform pressure field.
If land cools sufficiently at night, the reverse situation can be established.
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.warmcool
H
L
warm
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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.
1000 mb975 mb
950 mb
25C 15C
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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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564
546
528
SLP (mb) & 1000-500 thickness : 48hr forecast valid 0000 040922
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564
546
528
SLP (mb) & 1000-500 thickness (dm) : 36hr forecast valid 0000 040930
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564
546
SLP (mb) & 1000-500 thickness (dm) : analysis valid 0000 040930
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12°C
2°C
564
546
850 mb Temperature (2°C contours), RH (%), wind (m s-1) : analysis valid 0000 040930
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564
546
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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1000mb996
10041008
HIGH
LOW
Vg(1000)
warm
cold
500mb
LOW
HIGH
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60
0
120
180 HIGH
LOW
5760
5820
5700
5640
VG500
VT
LOW
HIGH
VG1000
5700
56405580500-1000 mb thickness
Contours of1000 mb surface
Contours of500 mb surface
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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.