Radiation balance of the Earthatmosphere system

The Earth-atmosphere system consists of the ensemble of the atmosphere, ocean, continents and ice cover. The climate of this system is controlled by the orbit and rotation of the Earth, the physical state and chemical composition of the surface (including liquid water and ice), and by the density and composition of the atmosphere. This last parameter participates mainly in the control of the radiation balance. For this reason our knowledge of the radiation balance of the Earth-atmosphere system will be summarized briefly in this section. The interested reader is referred to Paltridge and Piatt (1976) for further details.

The Sun can be considered as a black body, the temperature of which is around 5800 K. In accordance with this temperature, 90 % of the energy radiated is composed of radiation with wavelength ranging from 0.4 to 4.0 /¿m. The intensity maximum is in the visible range around 0.5 pm.

Of the radiation energy coming from the Sun, 30 "/„ is reflected by the Earth-atmosphere system. This fraction is termed the planetary albedo. The planetary albedo is regulated in a significant way by the clouds in the atmosphere. It is estimated that 20-25 % of the incoming radiation is reflected by clouds. This part of the albedo obviously varies as a function of the nature and the dimensions of the cloud cover. The rest of the planetary albedo is due to reflection from the surface and scattering by gas molecules and aerosol particles. Another 25 30 % of the incoming radiation is absorbed by gases, aerosol particles and cloud elements, while the balance of the radiation reaches the surface and is absorbed there.

Among atmospheric gases, 02, Oa and water vapour molecules are the most effective absorbers. Above 100 km, radiation with wavelength shorter than 0.18 /¿m is absorbed by molecular oxygen, which leads to the heating of this air of low density (thermosphere). A very small part of the energy absorbed is used for chemical dissociation and ionization. Radiation of wavelengths in the range of 0.18-0.29 pm is absorbed in the stratosphere, and to a lesser extent in the mesosphere, by 02 and 03 molecules. The relatively high temperature around the stratopause (see Section 1.2) is caused by this process. In the lower part of the atmosphere (troposphere), absorption by water vapour molecules is the most important.

The Earth-atmosphere system radiates energy back into space in a quantity equal to the incoming energy. The Earth's surface, like all radiating bodies, emits radiation according to its temperature. Because of the inverse relationship between temperature and the wavelength of the emitted radiation, the surface radiates in the infrared range (4-400 /jm). If the total quantity of the energy emitted at the surface left the atmosphere, the temperature of our planet would be around -20 C (effective radiation temperature). However, the atmosphere absorbs a significant part of the infrared radiation. As a result the average global temperature of surface air is 14 °C.

The infrared radiation is absorbed by atmospheric gases like water vapour, carbon dioxide and, to a lesser extent, ozone. Furthermore, in this wavelength band.

clouds are nearly perfect absorbers, while it is believed that the effect of aerosol particles is less important. Cloud elements and gas molecules absorbing the infrared radiation from the Earth are also radiation emitters. A certain part of these radiations are directed back toward the surface, which reduces the heat loss of the lower layers. Since the atmospheric emitters are cooler than the surface, they emit less energy than they absorb. The energy leaving the Earth-atmosphere system is thus less than that emitted by the surface. The Earth's surface provides a heat source for the lower atmosphere in this way. With distance away from this heat source the temperature decreases, which results in the formation of the troposphere.

It has been mentioned above that the energy leaving the Earth-atmosphere system is equal to the incoming energy. It should be emphasized, however, that this is true only for the whole of the system on a long time scale. Locally, at a given location on the Earth, the deviation from the equilibrium can be very significant. Thus, in the equatorial belt between latitudes 30 °N and 30 °S, the quantity of the energy received considerably exceeds the quantity emitted. In other words this means that the radiation balance of these areas is positive. At midlatitudes equilibrium conditions are nearly satisfied, while in polar areas the balance is negative (there is an energy deficit). This latter fact is explained by the low intensity of incoming radiation as well as by the high albedo of these areas covered by snow and ice. The latitudinal distribution of the sign and value of the radiation balance is the cause of the atmospheric circulation modified considerably by the rotation of the Earth, while horizontal and vertical motions in the atmosphere, in turn, regulate the global distribution of clouds and precipitations (subject to the availability of water vapour and condensation nuclei).

Atmospheric circulation transports heat from areas of positive radiation balance to areas where there is an energy deficit. A significant characteristic of this circulation is that a portion of the heat is transported in latent form, which means that the heat is delivered by the condensation of water vapour in the moving air. It is estimated that about one-third of the energy crossing latitude of 30° of both hemispheres is in the form of latent heat. Another one-third of the energy transport takes place in the ocean. Thus only 30 % of the heat is transported directly by the atmosphere.

The surface temperature of the ocean, controlled by the mixing in the upper layers of the water, plays an important role in regulating the heat exchange between the atmosphere and ocean. Unfortunately, these exchange processes are not sufficiently known to determine quantitatively the role of the ocean in the global heat transport. Thus, further work remains to be done to clarify this point which is of great importance for climate research.

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