Surface Receipt Of Solar Radiation And Its Effects

1 Energy transfer within the earth-atmosphere system

So far, we have described the distribution of solar radiation as if it were all available at the earth's surface. This is, of course, unrealistic because of the effect of the atmosphere on energy transfer. Heat energy can be transferred by three mechanisms:

1 Radiation: Electromagnetic waves transfer energy (both heat and light) between two bodies, without the necessary aid of an intervening material medium, at a speed of300 X 106m s-1 (i.e. the speed of light). This is so with solar energy through space, whereas the earth's atmosphere allows the passage of radiation only at certain wavelengths and restricts that at others.

Radiation entering the atmosphere may be absorbed in certain wavelengths by atmospheric gases but, as shown in Figure 3.1, most short-wave radiation is transmitted without absorption. Scattering occurs if the direction of a photon of radiation is changed by interaction with atmospheric gases and aerosols. Two types of scattering are distinguished. For gas molecules smaller than the radiation wavelength (X), Rayleigh scattering occurs in all directions (i.e. it is isotropic) and is proportional to (1/X4). As a result, the scattering of blue light (X ~ 0.4 |m) is an order of magnitude (i.e. X 10) greater than that of red light (X ~ 0.7 |im), thus creating the daytime blue sky. However, when water droplets or aerosol particles, with similar sizes (0.1-0.5 |m radius) to the radiation wavelength are present, most of the light is scattered forward. This Mie scattering gives the greyish appearance of polluted atmospheres.

Within a cloud, or between low clouds and a snow-covered surface, radiation undergoes multiple scattering. In the latter case, the 'white-out' conditions typical of polar regions in summer (and mid-latitude snowstorms) are experienced, when surface features and the horizon become indistinguishable.

2 Conduction: By this mechanism, heat passes through a substance from a warmer to a colder part through the transfer of adjacent molecular vibrations. Air is a poor conductor so this type of heat transfer is negligible in the atmosphere, but it is important in the ground. The thermal conductivity increases as the water content of a given soil increases and is greater in a frozen soil than in an unfrozen one.

3 Convection: This occurs in fluids (including gases) that are able to circulate internally and distribute heated parts of the mass. It is the chief means of atmospheric heat transfer due to the low viscosity of air and its almost continual motion. Forced convection (mechanical turbulence) occurs when eddies form in airflow over uneven surfaces. In the presence of surface heating, free (thermal) convection develops.

Convection transfers energy in two forms. The first is the sensible heat content of the air (called enthalpy by physicists), which is transferred directly by the rising and mixing of warmed air. It is defined as cpT, where T is the temperature and cp (= 1004 J kg-1 K-1) is the specific heat at constant pressure (the heat absorbed by unit mass for unit temperature increase). Sensible heat is also transferred by conduction. The second form of energy transfer by convection is indirect, involving latent heat. Here, there is a phase change but no temperature change. Whenever water is converted into water vapour by evaporation (or boiling), heat is required. This is referred to as the latent heat of vaporization (L). At 0°C, L is 2.50 X 106 J kg-1 ofwater. More generally,

where T is in °C. When water condenses in the atmosphere (see Chapter 4D), the same amount of latent heat is given off as is used for evaporation at the same temperature. Similarly, for melting ice at 0°C, the latent heat of fusion is required, which is 0.335 X 106 J kg-1. If ice evaporates without melting, the latent heat of this sublimation process is 2.83 X 106 J kg-1 at 0°C (i.e. the sum of the latent heats of melting and vaporization). In all of these phase changes of water there is an energy transfer. We discuss other aspects of these processes in Chapter 4.

2 Effect of the atmosphere

Solar radiation is virtually all in the short-wavelength range, less than 4 |m (see Figure 3.1). About 18 per cent of the incoming energy is absorbed directly by ozone and water vapour. Ozone absorption is concentrated in three solar spectral bands (0.20-0.31, 0.31-0.35 and 0.45-0.85 |m), while water vapour absorbs to a lesser degree in several bands between 0.9 and 2.1 |m (see Figure 3.1). Solar wavelengths shorter than 0.285 |m scarcely penetrate below 20 km altitude, whereas those >0.295 |m reach the surface. Thus the 3 mm (equivalent) column of stratospheric ozone attenuates ultraviolet radiation almost entirely, except for a partial window around 0.20 |m, where radiation reaches the lower stratosphere. About 30 per cent of incoming solar radiation is immediately reflected back into space from the atmosphere, clouds and the earth's surface, leaving approximately 70 per cent to heat the earth and its atmosphere. The surface absorbs almost half of the incoming energy available at the top of the atmosphere and re-radiates it outward as long (infra-red) waves of greater than 3 |m (see Figure 3.1). Much of this re-radiated long-wave energy is then absorbed by the water vapour, carbon dioxide and ozone in the atmosphere, the rest escaping through atmospheric windows back into outer space, principally between 8 and 13 |m (see Figure 3.1). This retention of energy by the atmosphere is vital to most life forms, since otherwise the average

Figure 3.5 The average annual latitudinal disposition of solar radiation in W m-2. Of 100 per cent radiation entering the top of the atmosphere, about 20 per cent is reflected back to space by clouds, 3 per cent by air (plus dust and water vapour), and 8 per cent by the earth's surface. Three per cent is absorbed by clouds, 18 per cent by the air, and 48 per cent by the earth.

Source: After Sellers (1965).

Figure 3.5 The average annual latitudinal disposition of solar radiation in W m-2. Of 100 per cent radiation entering the top of the atmosphere, about 20 per cent is reflected back to space by clouds, 3 per cent by air (plus dust and water vapour), and 8 per cent by the earth's surface. Three per cent is absorbed by clouds, 18 per cent by the air, and 48 per cent by the earth.

Source: After Sellers (1965).

temperature of the earth's surface would fall by some 40°C!

The atmospheric scattering, noted above, gives rise to diffuse (or sky) radiation and this is sometimes measured separately from the direct beam radiation. On average, under cloud-free conditions the ratio of diffuse to total (or global) solar radiation is about 0.15-0.20 at the surface. For average cloudiness, the ratio is about 0.5 at the surface, decreasing to around 0.1 at 4 km, as a result of the decrease in cloud droplets and aerosols with altitude. During a total solar eclipse, experienced over much of western Europe in August 1999, the elimination of direct beam radiation caused diffuse radiation to drop from 680 W m-2 at 10.30 a.m. to only 14 W m-2 at 11.00 a.m. at Bracknell in southern England.

Figure 3.5 illustrates the relative roles of the atmosphere, clouds and the earth's surface in reflecting and absorbing solar radiation at different latitudes. (A more complete analysis of the heat budget of the earth-atmosphere system is given in D, this chapter.)

3 Effect of cloud cover

Thick and continuous cloud cover forms a significant barrier to the penetration of radiation. The drop in surface temperature often experienced on a sunny day when a cloud temporarily cuts off the direct solar radiation illustrates our reliance upon the sun's radiant energy. How much radiation is actually reflected by clouds depends on the amount of cloud cover and its thickness (Figure 3.6). The proportion of incident radiation that is reflected is termed the albedo, or reflection coefficient (expressed as a fraction or percentage). Cloud type affects the albedo. Aircraft measurements show that the albedo of a complete overcast ranges from 44 to 50 per cent for cirrostratus to 90 per cent for cumulonimbus. Average albedo values, as determined by satellites, aircraft and surface measurements, are summarized in Table 3.2 (see Note 2).

The total (or global) solar radiation received at the surface on cloudy days is

where S

= global solar radiation for clear skies; = cloudiness (fraction of sky covered); = a coefficient depending on cloud type and thickness; and the depth of atmosphere through which the radiation must pass.

For mean monthly values for the United States, b ~ 0.35, so that

Table 3.2 The average (integrated) albedo of various surfaces (0.3-0.4 jUm).

Planet earth

0.3I

Global surface

0.I4-0.I6

Global cloud

0.23

Cumulonimbus

0.9

Stratocumulus

0.6

Cirrus

0.4-0.5

Fresh snow

0.8-0.9

Melting snow

0.4-0.6

Sand

0.30-0.35

Grass, cereal crops

0.I8-0.25

Deciduous forest

0. I 5-0. I 8

Coniferous forest

0.09-0. I 5

Tropical rainforest

0.07-0. I 5

Water bodies*

0.06-0.I0

Note: 'Increases sharply at low solar anj

'les.

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Responses

  • Ovidio
    Why is solar receipt at the surface lower?
    8 years ago
  • Melissa
    What is a solar radiation receipt?
    8 years ago
  • Matta Boffin
    Why are values for surface solar receipt shown in figures 32 and 33 lower?
    4 years ago
  • ELELETA
    Why are values for surface solar reciept shown in figure 32 and 33?
    4 years ago
  • Katharina
    What are the effects of cloud cover on the solar radiation and heat exchange in the atmosphere?
    3 years ago
  • bibiana
    Why are the values for surface solar receipt shown in figure 32 and 33 lower?
    1 year ago
  • Maximilian
    Why is solar receipt at the surface of the earth lower?
    8 days ago

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