Atmospheric Energy And Horizontal Heat Transport

So far, we have given an account of the earth's heat budget and its components. We have already referred to two forms of energy: internal (or heat) energy, due to the motion of individual air molecules, and latent energy, which is released by condensation of water vapour. Two other forms of energy are important: geopotential energy due to gravity and height above the surface, and kinetic energy associated with air motion.

Geopotential and internal energy are interrelated, since the addition of heat to an air column not only increases its internal energy but also adds to its geopotential as a result of the vertical expansion of the air column. In a column extending to the top of the atmosphere, the geopotential is approximately 40 per cent of the internal energy. These two energy forms are therefore usually considered together and termed the total potential energy (PE). For the whole atmosphere potential energy ~ 1024 J

kinetic energy ~ 1010 J

In a later section (Chapter 6C), we shall see how energy is transferred from one form to another, but here we consider only heat energy. It is apparent that the receipt of heat energy is very unequal geographically and that this must lead to great lateral transfers of energy across the surface of the earth. In turn, these transfers give rise, at least indirectly, to the observed patterns of global weather and climate.

The amounts of energy received at different latitudes vary substantially, the equator on the average receiving 2.5 times as much annual solar energy as the poles. Clearly, if this process were not modified in some way the variations in receipt would cause a massive accumulation of heat within the tropics (associated with gradual increases of temperature) and a corresponding deficiency at the poles. Yet this does not happen, and the earth as a whole is approximately in a state of thermal equilibrium. One explanation of this equilibrium could be that for each region of the world there is equalization between the amount of incoming and outgoing radiation. However, observation shows that this is not so (Figure 3.25), because, whereas incoming radiation varies appreciably with changes in latitude, being highest at the equator and declining to a minimum at the poles, outgoing radiation has a more even latitudinal percent of hemisphere surface percent of hemisphere surface

Radiation Sun Newell Gabites

Figure 3.25 A meridional illustration of the balance between incoming solar radiation and outgoing radiation from the earth and atmosphere* in which the zones of permanent surplus and deficit are maintained in equilibrium by a poleward energy transfer.f

Sources: *Data from Houghton; after Newell (1964) and Scientific American. fAfter Gabites.

Figure 3.25 A meridional illustration of the balance between incoming solar radiation and outgoing radiation from the earth and atmosphere* in which the zones of permanent surplus and deficit are maintained in equilibrium by a poleward energy transfer.f

Sources: *Data from Houghton; after Newell (1964) and Scientific American. fAfter Gabites.

distribution owing to the rather small variations in atmospheric temperature. Some other explanation therefore becomes necessary.

1 The horizontal transport of heat

If the net radiation for the whole earth-atmosphere system is calculated, it is found that there is a positive budget between 35°S and 40°N, as shown in Figure 3.26C. The latitudinal belts in each hemisphere separating the zones of positive and negative net radiation budgets oscillate dramatically with season (Figure 3.26A and B). As the tropics do not get progressively hotter or the high latitudes colder, a redistribution of world heat energy must occur constantly, taking the form of a continuous movement of energy from the tropics to the poles. In this way the tropics shed their excess heat and the poles, being global heat sinks, are not allowed to reach extremes of cold. If there were no meridional interchange of heat, a radiation balance at each latitude would be achieved only if the equator were 14°C warmer and the North Pole 25°C colder than today. This poleward heat transport takes place within the atmosphere and oceans, and it is estimated that the former accounts for approximately two-thirds of the required total. The horizontal transport (advection of heat) occurs in the form of both latent heat (that is, water vapour, which subsequently condenses) and sensible january january

Atmosphere Poleward Heat Transport
165°W 120°W 60°W 0° 60°E 120°E 180°E

annual

Figure 3.26 Mean net planetary radiation budget (Rn) (W m-2) for a horizontal surface at the top of the atmosphere (i.e. for the earth-atmosphere system). (A) January. (B) July. (C) Annual.

Sources: Ardanuy et al. (I992) and Kyle et al. (I993). Stephens et al. (I98I). (A), (B) By permission of the American Geophysical Union. (C) From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

annual

Figure 3.26 Mean net planetary radiation budget (Rn) (W m-2) for a horizontal surface at the top of the atmosphere (i.e. for the earth-atmosphere system). (A) January. (B) July. (C) Annual.

Sources: Ardanuy et al. (I992) and Kyle et al. (I993). Stephens et al. (I98I). (A), (B) By permission of the American Geophysical Union. (C) From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

heat (that is, warm airmasses). It varies in intensity according to the latitude and the season. Figure 3.27B shows the mean annual pattern of energy transfer by the three mechanisms. The latitudinal zone of maximum total transfer rate is found between latitudes 35° and 45° in both hemispheres, although the patterns for the individual components are quite different from one another. The latent heat transport, which occurs almost wholly in the lowest 2 or 3 km, reflects the global wind belts on either side of the subtropical high-pressure zones (see Chapter 7B). The more important meridional transfer of sensible heat has a double maximum not only latitudinally but also in the vertical plane, where there are maxima near the surface and at about 200 mb. The high-level transport is particularly significant over the

Poleward Heat Transfer

Laltlude

Figure 3.27 (A) Net radiation balance for the earth's surface of 101 W m-2 (incoming solar radiation of 156 W m-2, minus outgoing long-wave energy to the atmosphere of 55 W m-2); for the atmosphere of -I0I W m-2 (incoming solar radiation of 84 W m-2, minus outgoing long-wave energy to space of I85 W m-2); and for the whole earth-atmosphere system of zero. (B) The average annual latitudinal distribution of the components of the poleward energy transfer (in I0I5 W) in the earth-atmosphere system.

Laltlude

Figure 3.27 (A) Net radiation balance for the earth's surface of 101 W m-2 (incoming solar radiation of 156 W m-2, minus outgoing long-wave energy to the atmosphere of 55 W m-2); for the atmosphere of -I0I W m-2 (incoming solar radiation of 84 W m-2, minus outgoing long-wave energy to space of I85 W m-2); and for the whole earth-atmosphere system of zero. (B) The average annual latitudinal distribution of the components of the poleward energy transfer (in I0I5 W) in the earth-atmosphere system.

Source: From Sellers (I965).

subtropics, whereas the primary latitudinal maximum of about 50° to 60°N is related to the travelling low-pressure systems of the westerlies.

The intensity of the poleward energy flow is closely related to the meridional (that is, north-south) temperature gradient. In winter this temperature gradient is at a maximum, and in consequence the hemispheric air circulation is most intense. The nature of the complex transport mechanisms will be discussed in Chapter 7C.

As shown in Figure 3.27B, ocean currents account for a significant proportion of the poleward heat transfer in low latitudes. Indeed, recent satellite estimates of the required total poleward energy transport indicate that the previous figures are too low. The ocean transport may be 47 per cent of the total at 30 to 35°N and as much as 74 per cent at 20°N; the Gulf Stream and Kuro Shio currents are particularly important. In the southern hemisphere, poleward transport is mainly in the Pacific and Indian Oceans (see Figure 8.30). The energy budget equation for an ocean area must be expressed as

n where tA = horizontal advection of heat by currents and G = the heat transferred into or out of storage in the water. The storage is more or less zero for annual averages.

2 Spatial pattern of the heat budget components

The mean latitudinal values of the heat budget components discussed above conceal wide spatial variations. Figure 3.28 shows the global distribution of the annual net radiation at the surface. Broadly, its magnitude decreases poleward from about 25° latitude. However, as a result of the high absorption of solar radiation by the sea, net radiation is greater over the oceans - exceeding 160 W m-2 in latitudes 15 to 20° - than over land areas, where it is about 80 to 105 W m-2 in the same latitudes. Net radiation is also lower in arid continental areas than in humid ones, because in spite of the increased insolation receipts under clear skies there is at the same time greater net loss of terrestrial radiation.

Figures 3.29 and 3.30 show the annual vertical transfers of latent and sensible heat to the atmosphere. Both fluxes are distributed very differently over land and seas. Heat expenditure for evaporation is at a maximum in tropical and subtropical ocean areas, where it

Insolation Map World
Figure 3.28 Global distribution of the annual net radiation at the surface, in W m 2. Source: After Budyko et al. (1962).

160' 140" 1IO" 100* >0' 60" 40" 10* 0" 20* 40' 40' W 100' Via' 140" 1»0* 1*0"

Figure 3.29 Global distribution of the vertical transfer of latent heat, in W m 2. Source: After Budyko et al. (1962).

160' 140" 1IO" 100* >0' 60" 40" 10* 0" 20* 40' 40' W 100' Via' 140" 1»0* 1*0"

Figure 3.29 Global distribution of the vertical transfer of latent heat, in W m 2. Source: After Budyko et al. (1962).

160" HO" 110* 100" Í0" «0* <0* 10* 0 JO* iO" Ml' «0* 100" 110* 140* iao* HO'

Figure 3.30 Global distribution of the vertical transfer of sensible heat, in W m 2. Source: After Budyko et al. (I962).

160" HO" 110* 100" Í0" «0* <0* 10* 0 JO* iO" Ml' «0* 100" 110* 140* iao* HO'

Figure 3.30 Global distribution of the vertical transfer of sensible heat, in W m 2. Source: After Budyko et al. (I962).

exceeds 160 W m-2. It is less near the equator, where wind speeds are somewhat lower and the air has a vapour pressure close to the saturation value (see Chapter 3A). It is clear from Figure 3.29 that the major warm currents greatly increase the evaporation rate. On land, the latent heat transfer is largest in hot, humid regions. It is least in arid areas with low precipitation and in high latitudes, where there is little available energy.

The largest exchange of sensible heat occurs over tropical deserts, where more than 80 W m-2 is transferred to the atmosphere (see Figure 3.30). In contrast to latent heat, the sensible heat flux is generally small over the oceans, reaching only 25-40 W m-2 in areas of warm currents. Indeed, negative values occur (transfer to the ocean) where warm continental airmasses move offshore over cold currents.

SUMMARY

Almost all energy affecting the earth is derived from solar radiation, which is of short wavelength (<4 Jm) due to the high temperature of the sun (6000 K) (i.e. Wien's Law). The solar constant has a value of approximately I368 W m-2. The sun and the earth radiate almost as black bodies (Stefan's Law, F = oT4), whereas the atmospheric gases do not. Terrestrial radiation, from an equivalent black body, amounts to only about 270 W m-2 due to its low radiating temperature (263 K); this is infra-red (long-wave) radiation between 4 and I00 Jm. Water vapour and carbon dioxide are the major absorbing gases for infra-red radiation, whereas the atmosphere is largely transparent to solar radiation (the greenhouse effect). Trace gas increases are now augmenting the 'natural' greenhouse effect (33 K). Solar radiation is lost by reflection, mainly from clouds, and by absorption (largely by water vapour). The planetary albedo is 3I per cent; 49 per cent of the extraterrestrial radiation reaches the surface. The atmosphere is heated primarily from the surface by the absorption of terrestrial infra-red radiation and by turbulent heat transfer. Temperature usually decreases with height at an average rate of about 6.5°C/km in the troposphere. In the stratosphere and thermosphere, it increases with height due to the presence of radiation absorbing gases.

The excess of net radiation in lower latitudes leads to a poleward energy transport from tropical latitudes by ocean currents and by the atmosphere. This is in the form of sensible heat (warm airmasses/ocean water) and latent heat (atmospheric water vapour). Air temperature at any point is affected by the incoming solar radiation and other vertical energy exchanges, surface properties (slope, albedo, heat capacity), land and sea distribution and elevation, and also by horizontal advection due to airmass movements and ocean currents.

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  • niina
    What is the name the horizontal transport of heat?
    8 years ago
  • adalfredo
    How is energy transported horizontally in the atmosphere?
    3 years ago
  • lavinia
    How energy is transport in atmospher?
    2 years ago
  • raimo karlsson
    What is hoizontol transprt of atmosphric heat?
    2 years ago
  • Brodie
    How does heat energy transport in the atmosphere?
    1 year ago
  • semhar
    How heat is transported in the atmosphere?
    10 months ago
  • FRANZISKA
    Why does latent heat transport occur close to the surface?
    8 months ago
  • markus
    How is energy cycled through the climate?
    5 months ago
  • Holly
    What is horizontal energy transport?
    2 months ago
  • Tobold Brandybuck
    How is energy distributed in the atmosphere?
    2 months ago
  • irene
    Where does the maximum heat transport generally occur in the atmosphere?
    1 month ago

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