The Arctic and the global heat budget 311 The radiation balance

Considered as a whole and for long-term annual means, and assuming a stationary climate, the Earth is in a state of radiative equilibrium. The ultimate energy source to the Earth is solar radiation. About 99.9% of the radiation emitted by the Sun is in wavelengths of 0.15 to 4 ^m with a peak intensity near 0.5 ^m. About 50% of the total emitted energy is within the visible spectrum (approximately 0.4-0.7 ^m). The net incoming (i.e., the available) solar radiation to the Earth system, defined as that which is measured at the top of the atmosphere (TOA) is

where S is the solar constant (1367 W m-2), A is the planetary albedo (approximately 0.3), and R is the radius of the Earth (6371 km). The area of a disk, rather than the surface area of a sphere is used as the area of intercepted solar radiation. This is equivalent to the area of the shadow cast behind the Earth. The planetary albedo (A) differs from the surface albedo (a) (see Chapter 5) in that it includes the effects of scattering and absorption by clouds, aerosols and atmospheric gases, as well as the surface albedo. However, to a first order, absorption of solar radiation by the atmosphere itself is relatively minor. Put differently, most of the solar radiation that is not scattered back to space by the atmosphere reaches the surface.

In radiative equilibrium, the net TOA solar radiation flux is balanced by the radiation emitted to space by the Earth's atmosphere and surface. This outgoing TOA radiation flux is in different coin, namely, longwave radiation:

Outgoing longwave = a T*4n R2

where a is the Stefan-Boltzmann constant (5.7 x 10-8 W m-2 K-4), and Te is the effective radiation emission temperature of the Earth. This longwave radiation is in wavelengths of about 4-300 ^m, peaking at about 10 ^m. In contrast to solar radiation, the atmosphere is semi-opaque with respect to longwave radiation. This effect is included in Te. Te depends on both the physical temperature and the longwave emissiv-ity of the atmosphere and surface considered as a whole. The emissivity is a measure of how efficiently the atmosphere and surface both absorbs and emits longwave radiation. Further details are provided in Chapter 5. The surface is heated primarily through the absorption of solar radiation, and longwave radiation emitted towards the surface from the lower atmosphere. The atmosphere is heated primarily from the surface upward through vertical turbulent heat fluxes (sensible and latent heating) and complicated longwave radiation exchanges whereby radiation emitted by the surface is absorbed by the lower atmosphere, and re-emitted both downward and upward. The longwave radiation emitted to space is primarily from the atmosphere. Direct emission to space from the surface occurs through various atmospheric "windows" (e.g., 3-5 ^m and 8-14 ^m, the former overlapping with the solar spectrum), within which atmospheric absorption is small.

However, when we look at the Earth not as a whole, but instead examine annual zonal averages (averages for different latitude circles), we find a strong latitudinal dependency of the net solar flux. The equatorial regions receive more while the polar regions receive less. This uneven distribution of solar heating arises because the Earth is a sphere. Solar radiation strikes the top of the atmosphere at a shallower (more grazing) angle in higher latitudes compared to lower latitudes. This basic pattern is in turn modified somewhat by latitudinal variations in the planetary albedo. If radiative equilibrium held at all latitudes, the latitudinal distribution of TO A outgoing longwave radiation would match that of the net TOA solar radiation. But this is not what is observed. The best available data for the TOA radiation budget are from the Earth Radiation Budget Experiment (ERBE) (February 1985 through April 1989). These data show that for annual averages, between about 38° N and 38° S, the Earth receives more radiation than it emits to space. Poleward of these latitudes, the earth emits more radiation than it receives (Figures 3.1 and 3.2).

Global Balance Heat

Figure 3.1 Zonal averages of the annual mean net radiation budget at the top of the atmosphere based on ERBE data (from Trenberth and Caron, 2001, by permission of AMS).

90 60 40 30 20 10 EQ 10 20 30 40 60 9C "S Latitude

90 60 40 30 20 10 EQ 10 20 30 40 60 9C "S Latitude

Figure 3.1 Zonal averages of the annual mean net radiation budget at the top of the atmosphere based on ERBE data (from Trenberth and Caron, 2001, by permission of AMS).

0 30£ 60E. SOE 120E 1S0E 1 SO 150W 12DW SOW 6DW 30W 0

Figure 3.2 The global pattern of the annual mean net radiation budget (W m-2) at the top of the atmosphere based on ERBE data (from Trenberth et al., 2001, by permission of Springer-Verlag).

0 30£ 60E. SOE 120E 1S0E 1 SO 150W 12DW SOW 6DW 30W 0

Figure 3.2 The global pattern of the annual mean net radiation budget (W m-2) at the top of the atmosphere based on ERBE data (from Trenberth et al., 2001, by permission of Springer-Verlag).

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