Heat Budget Of The Earth

We can now summarize the net effect of the transfers of energy in the earth-atmosphere system averaged over the globe and over an annual period.

The incident solar radiation averaged over the globe is

Solar constant X %r2 / 4nr2

where r = radius of the earth and 4nr2 is the surface area of a sphere. This figure is approximately 342 W m-2, or 11 X 109J m-2yr-1(109J = 1GJ); for convenience we will regard it as 100 units. Referring to Figure 3.21, incoming radiation is absorbed in the stratosphere (3 units), by ozone mainly, and 20 units are absorbed in the troposphere by carbon dioxide (1), water vapour (13), dust (3) and water droplets in clouds (3). Twenty units are reflected back to space from clouds, which

Heat Budget The Earth

Figure 3.21 The balance of the atmospheric energy budget. The transfers are explained in the text. Solid lines indicate energy gains by the atmosphere and surface in the left-hand diagram and the troposphere in the right-hand diagram. The exchanges are referred to 100 units of incoming solar radiation at the top of the atmosphere (equal to 342 W m-2).

Figure 3.21 The balance of the atmospheric energy budget. The transfers are explained in the text. Solid lines indicate energy gains by the atmosphere and surface in the left-hand diagram and the troposphere in the right-hand diagram. The exchanges are referred to 100 units of incoming solar radiation at the top of the atmosphere (equal to 342 W m-2).

Source: After Kiehl and Trenberth (1997) From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

Planetary Wave
Figure 3.22 Planetary short- and long-wave radiation (Wm-2). (A) Mean annual absorbed short-wave radiation for the period April 1979 to March 1987. (B) Mean annual net planetary long-wave radiation (Ln) on a horizontal surface at the top of the atmosphere.

Sources: (A) Ardanuy et al. (1992) and Kyle et al. (1993) From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society. (B) Stephens et al. (1981).

cover about 62 per cent of the earth's surface on average. units reach the earth either directly (Q = 28) or as diffuse

A further nine units are similarly reflected from the radiation (q = 21) transmitted via clouds or by down-

surface and three units are returned by atmospheric ward scattering.

scattering. The total reflected radiation is the planetary The pattern of outgoing terrestrial radiation is quite albedo (31 per cent or 0.31). The remaining forty-nine different (see Figure 3.22). The black-body radiation, assuming a mean surface temperature of 288 K, is equivalent to 114 units of infra-red (long-wave) radiation. This is possible since most of the outgoing radiation is reabsorbed by the atmosphere; the net loss of infra-red radiation at the surface is only nineteen units. These exchanges represent a time-averaged state for the whole globe. Recall that solar radiation affects only the sunlit hemisphere, where the incoming radiation exceeds 342 W m-2. Conversely, no solar radiation is received by the night-time hemisphere. Infra-red exchanges continue, however, due to the accumulated heat in the ground. Only about twelve units escape through the atmospheric window directly from the surface. The atmosphere itself radiates fifty-seven units to space (forty-eight from the emission by atmospheric water vapour and CO2 and nine from cloud emission), giving a total of sixty-nine units (Lu); the atmosphere in turn radiates ninety-five units back to the surface (Ld); thus L + L, = L is negative.

These radiation transfers can be expressed symbolically:

Rn = (Q + q) (1 - a) + Ln where Rn = net radiation, (Q + q) = global solar radiation, a = albedo and Ln = net long-wave radiation. At the surface, Rn = 30 units. This surplus is conveyed to the atmosphere by the turbulent transfer of sensible heat, or enthalpy (seven units), and latent heat (twenty-three units):

n where H = sensible heat transfer and LE = latent heat transfer. There is also a flux ofheat into the ground (B.5, this chapter), but for annual averages this is approximately zero.

Figure 3.22 summarizes the total balances at the surface (± 144 units) and for the atmosphere (± 152 units). The total absorbed solar radiation and emitted radiation for the entire earth-atmosphere system is estimated to be ±7 GJ m-2 yr-1 (± 69 units). Various uncertainties are still to be resolved in these estimates. The surface short-wave and long-wave radiation budgets have an uncertainty of about 20 W m-2, and the turbulent heat fluxes of about 10 W m-2.

Satellite measurements now provide global views of the energy balance at the top of the atmosphere. The incident solar radiation is almost symmetrical about the equator in the annual mean (cf. Table 3.1). The mean annual totals on a horizontal surface at the top of the atmosphere are approximately 420 W m-2 at the equator and 180 W m-2 at the poles. The distribution of the planetary albedo (see Figure 3.13B) shows the lowest values over the low-latitude oceans compared with the more persistent areas of cloud cover over the continents. The highest values are over the polar ice-caps. The resulting planetary short-wave radiation ranges from 340 Wm-2 at the equator to 80 Wm-2 at the poles. The net (outgoing) long-wave radiation (Figure 3.22B) shows the smallest losses where the temperatures are lowest and highest losses over the largely clear skies of the Saharan desert surface and over low-latitude oceans. The difference between Figure 3.22A and 3.22B represents the net radiation of the earth-atmosphere system which achieves balance about latitude 30°. The consequences of a low-latitude energy surplus and a high-latitude deficit are examined below.

The diurnal and annual variations of temperature are related directly to the local energy budget. Under clear skies, in middle and lower latitudes, the diurnal regime of radiative exchanges generally shows a midday maximum of absorbed solar radiation (see Figure 3.23A). A maximum of infra-red (long-wave) radiation (see Figure 3.1) is also emitted by the heated ground surface at midday, when it is warmest. The atmosphere re-radiates infra-red radiation downward, but there is a net loss at the surface (Ln). The difference between the absorbed solar radiation and L is the net radiation, R ; this is n ' n generally positive between about an hour after sunrise and an hour or so before sunset, with a midday maximum. The delay in the occurrence of the maximum air temperature until about 14:00 hours local time (Figure 3.23B) is caused by the gradual heating of the air by convective transfer from the ground. Minimum Rn occurs in the early evening, when the ground is still warm; there is a slight increase thereafter. The temperature decrease after midday is slowed by heat supplied from the ground. Minimum air temperature occurs shortly after sunrise due to the lag in the transfer of heat from the surface to the air. The annual pattern of the net radiation budget and temperature regime is closely analogous to the diurnal one, with a seasonal lag in the temperature curve relative to the radiation cycle, as noted above (p. 47).

There are marked latitudinal variations in the diurnal and annual ranges of temperature. Broadly, the annual range is a maximum in higher latitudes, with extreme

Annual Radiation Budget LatitudeAnnual Temperature Variation Curve

Figure 3.23 Curves showing diurnal variations of radiant energy and temperature. (A) Diurnal variations in absorbed solar radiation and infra-red radiation in the middle and low latitudes. (B) Diurnal variations in net radiation and air temperature in the middle and low latitudes. (C) Annual (A) and diurnal (D) temperature ranges as a function of latitude and of continental (C) or maritime (M) location.

Continental Maritime Climate

Figure 3.24 The mean annual temperature range (°C) at the earth's surface.

Source: Monin, Crowley and North (1991). Courtesy of the World Meteorological Organization.

Figure 3.24 The mean annual temperature range (°C) at the earth's surface.

Source: Monin, Crowley and North (1991). Courtesy of the World Meteorological Organization.

values about 65°N related to the effects of continentality and distance to the ocean in interior Asia and North America (Figure 3.24). In contrast, in low latitudes the annual range differs little between land and sea because of the thermal similarity between tropical rainforests and tropical oceans. The diurnal range is a maximum over tropical land areas, but it is in the equatorial zone that the diurnal variation of heating and cooling exceeds the annual one (Figure 3.23C), due to the small seasonal change in solar elevation angle at the equator.

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Renewable Energy 101

Renewable Energy 101

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  • Jayden
    What is annual and dirual heat budget?
    2 years ago
  • luigia trevisano
    How does global warming affect heat budget of the earth?
    2 years ago
  • Marina
    How heat budget affect weather and climate?
    2 years ago
  • Ruairi
    What is heat budget in weather temperatures?
    20 days ago

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