thermal processes

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


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


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


• 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


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


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


• 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


• 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


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.


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


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


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


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


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.


564

546

528

SLP (mb) & 1000-500 thickness : 48hr forecast valid 0000 040922


564

546

528

SLP (mb) & 1000-500 thickness (dm) : 36hr forecast valid 0000 040930


564

546

SLP (mb) & 1000-500 thickness (dm) : analysis valid 0000 040930


12°C

2°C

564

546

850 mb Temperature (2°C contours), RH (%), wind (m s-1) : analysis valid 0000 040930


564

546

Surface temperature (2°C contours) and SLP (mb)(5mb contours) : analysis valid 0600 040930


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.


1000mb996

10041008

HIGH

LOW

Vg(1000)

warm

cold

500mb

LOW

HIGH


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


• 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.

thermal processes

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