JEE 661 ATMOSPHERIC BOUNDARY LAYER SCIENCE

Lecture 1: General Description of the Atmosphere

Contents

  1. Composition of Dry Air
  2. Vertical Structure of the Atmosphere
  3. The Planetary Boundary Layer
  4. Heat Transfers
  5. Water Vapour
  6. Vertical Motion of the Air
  7. Stability and Buoyancy
  8. Pressure and Winds
  9. Surface Winds
  10. General Circulation

Composition of Dry Air

The composition of dry air is as follows:

There are small amounts of other important gases:

Vertical Structure of the Atmosphere

Table 1. Average Values for Thailand
Height (km)Pressure (kPa)Temperature (°C)Notes
Surface 0km100.9kPa24°C7:00 a.m.
1.5km85.0kPa18°C 
3.1km70.0kPa9°C 
5.9km50.0kPa-6°C 
9.7km30.0kPa-31°C 
12.4km20.0kPa-53°C 
14.2km15.0kPa-68°C 
16.6km10.0kPa-80°CTropopause
20.6km5.0kPa-63°C 
23.8km3.0kPa-54°C 

The Planetary Boundary Layer

The planetary boundary layer is the layer in contact with the Earth's surface where friction and mixing occur. The height of the planetary boundary layer may vary from about 100m on a calm night to 2km on a hot afternoon.

The lowest level below 50m, which is strongly affected by the physical roughness and temperature of the surface, is called the surface boundary layer.

Heat Transfers

The planet Earth receives heat by solar radiation having ultraviolet, visible, and near infrared wavelengths 0.3µm to 3µm. This radiation is called shortwave radiation in the atmosphere.

Solar radiation does not heat the atmosphere directly. It is transmitted through clear air, or it is reflected by clouds. It is partly absorbed and partly reflected by the surface. The absorbed solar radiation heats the surface, and the surface heats the atmosphere by convection.

The planet Earth loses heat by terrestrial radiation having far infrared wavelengths 5µm to 50µm. This radiation is called longwave radiation in the atmosphere.

Some longwave radiation emitted by the surface at wavelengths near 10µm passes through the atmosphere when the sky is clear. The remaining longwave radiation is absorbed and re-emitted by the atmosphere. Clouds absorb and emit longwave radiation.

During the day the surface is heated by solar radiation. This produces a steep temperature lapse rate in the boundary layer. At night the surface is cooled by the emission of longwave radiation. This produces a temperature inversion in the boundary layer. (See Fig. 1.)

Fig. 1.
Fig. 1. Air temperatures in the boundary layer during the day and at night.

Water Vapour

The total pressure of the air is the sum: Pdry air + Pwater vapour, where the second term Pwater vapour is the vapour pressure. The maximum possible vapour pressure is the saturation vapour pressure. This is the vapour pressure in equilibrium with a flat liquid water surface. The saturation vapour pressure depends on temperature as shown in Table 2.

Table 2. Saturation Vapour Pressure of Water
TemperatureSaturation Vapour Pressure
0°C0.6kPa
10°C1.2kPa
20°C2.3kPa
30°C4.2kPa

The relative humidity of moist air is:

(actual vapour pressure × 100%)/(saturation vapour pressure)

the vapour pressure cannot remain greater than the saturation vapour pressure. If it becomes greater, then water condenses to liquid until the vapour pressure is reduced to the saturation vapour pressure. This occurs when saturated air is cooled. When saturated air is heated without change in the vapour pressure, the relative humidity is reduced.

Example

Saturated air at 10°C has vapour pressure 1.2kPa. If this air is heated to 20°C, the relative humidity becomes:

(1.2kPa × 100%)/(2.3kPa) = 52%.

Vertical Motion of the Air

Table 3. Processes in Rising and Falling Air
Rising AirFalling Air
  • The pressure decreases.
  • The air expands.
  • The temperature falls.
  • The pressure increases.
  • The air is compressed.
  • The temperature rises.
Rising Unsaturated AirFalling Unsaturated Air
  • The dry adiabatic lapse rate is 10°C/km.
  • The relative humidity increases.
  • The dry adiabatic lapse rate is 10°C/km.
  • The relative humidity decreases.
Rising Saturated AirFalling Saturated Air
  • Condensing water vapour makes cloud and rain.
  • Latent heat of condensation is released.
  • The wet adiabatic lapse rate is about 4°C/km.
  • Clouds evaporate.
  • Latent heat of evaporation is taken from the air.
  • The wet adiabatic lapse rate is about 4°C/km.

Note: "Adiabatic" means without heat transfer to or from the surrounding air.

Stability and Buoyancy

An air parcel which is warmer than the surrounding air is less dense than the surrounding air, and it rises. An air parcel which is cooler than the surrounding air is more dense than the surrounding air, and it sinks.

When the actual lapse rate of the surrounding air is more than the (dry or saturated) adiabatic lapse rate, rising or falling parcels of air continue to rise or fall, and the atmosphere is unstable.

When the actual lapse rate of the surrounding air is less than the (dry or saturated) adiabatic lapse rate, the atmosphere is stable, and the vertical movement of parcels of air is stopped. A temperature inversion is very stable.

A conditionally unstable lapse rate is an actual lapse rate less than the dry adiabatic lapse rate and greater than the wet adiabatic lapse rate.

The boundary layer is usually stable at night, and unstable during the day.

Pressure and Winds

Winds are caused by horizontal differences in air pressure. When a column of air is heated, the surface pressure is reduced and the upper level pressure is increased. This causes a wind out of the heated column at the upper level, and into the heated column at the surface.

Low surface pressure areas have rising air, clouds and rain. High surface pressure areas have sinking air and clear skies.

Surface Winds

Above the boundary layer the wind speed is determined by the horizontal pressure gradient. Near the surface the wind speed is reduced by surface friction. (See Fig. 2.)

Fig. 2.
Fig. 2. Variation of wind speed with height in the surface layer.
The length of the arrows indicates the wind speed.

The rotation of the Earth causes a coriolis force on air moving horizontally. In the northern hemisphere the moving air is deflected to the right by the coriolis force.

Above the boundary layer the free air moves anticlockwise around low pressure areas, and clockwise around high pressure areas. These winds are called gradient winds. They are caused by the combined effects of the horizontal pressure gradient and the coriolis force.

In the boundary layer the reduced wind speed has a reduced coriolis force, so the wind blows into low pressure areas, and out of high pressure areas. Therefore, low pressure areas are areas of convergence, and high pressure areas are areas of divergence.

General Circulation

Thailand has a tropical humid wet-and-dry climate. This climate is caused by seasonal north-south shifts in the Hadley Cell between the subtropical high pressure areas and the equatorial low pressure trough. But the average global climate pattern is modified by the huge land mass of Asia, and the high mountains of Tibet.

Winter (Sun South of the Equator)

The cooling of Asia strengthens the subtropical high pressure area. This brings dry NE winds to Thailand.

Summer (Sun North of the Equator)

The heating of Asia draws the equatorial low pressure trough northwards. This brings wet SW winds to Thailand.


By R. H. B. Exell, 2001. King Mongkut's University of Technology Thonburi.
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