The radiation balance

The simplest form of the global radiation balance is portrayed in Figure 2.3. Details of the spatial characteristics of the radiation balance are provided by observational and remote sensing data discussed in Chapters 3, 4, and 5. The radiation balance at the top of the Earth's atmosphere and the radiation balance at the Earth's surface are important to hydroclimatology because they emphasize the unequal distribution of energy that is the primary forcing for the climate system. Furthermore, the global condition of the radiation balance serves as a benchmark for assessing the variations in the radiation balance for regions and for individual sites. The nature of the global radiation balance is an important initial step in understanding the related regional and local components discussed later in this chapter.

The Earth's radiation balance is driven by the annual receipt of solar radiation received at the upper edge of the Earth's atmosphere minus the reflected shortwave energy that constitutes the Earth's albedo. The combined downward and upward shortwave fluxes form the net shortwave radiative flux at the top of the atmosphere (Fig. 7.1). The solar radiation input component is driven by differences in the intensity and duration of solar radiation produced by the annual cycle of Earth-Sun geometry, and the Earth's albedo modulates the solar input. Averaged globally, the incoming solar radiation is 342 Wm~2, the reflected solar radiation is 107Wm"2, and the net shortwave balance is 235 Wm~2.

Global albedo increases monotonically from equatorial to polar regions. Minimum values of 20% and less occur over the tropical oceans where clouds are sparse, while the albedo of subtropical continents is around 30%. Albedo is a maximum at polar latitudes exceeding 60% due to snowcover, clouds, and large solar zenith angles (Hartmann, 1994). Albedo variability due to land and water

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Fig. 7.1. Global annual mean net solar radiation at the top of the atmosphere. Units in Wm~2. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

contrasts and the presence and absence of clouds accounts for much of the spatial complexity between 40° N and 40° S evident in the net shortwave flux in Figure 7.1. Expressed as a net value, the atmosphere above the equatorial region receives significantly greater solar radiation than the atmosphere above the polar regions. The systematic solar radiation decrease with increasing latitude indicates the Earth-Sun geometry dominance on the solar radiation input before the solar radiation begins its transit through the Earth's atmosphere.

Upwelling longwave terrestrial radiation at the top of the Earth's atmosphere (Fig. 7.2) is the thermal radiation emitted by the Earth-atmosphere system. The influence of the solar radiation input on the distribution of outgoing longwave terrestrial radiation is evident in the inverse relationship between outgoing terrestrial radiation and latitude. However, the latitudinal gradient is much weaker for outgoing terrestrial radiation than for solar radiation. The highest values of 280 Wm~2 and greater occur over the warm subtropical deserts and over tropical oceans where clouds occur infrequently. This pattern is a response to outgoing thermal radiation being related to the temperature of the emitting surface. The poles produce the lowest values of 180 Wm~2 and less. Cold poles and cold cloud tops in tropical latitudes produce lower values while warm surfaces and relatively clear sky regions produce higher values.

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Fig. 7.2. Global annual mean outgoing longwave radiation at the top of the atmosphere. Units in Wm~2. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

The atmospheric influence on radiation exchanges and the different thermal responses of land and water account for much of the greater complexity of the outgoing terrestrial radiation pattern. Albedo influences contribute to quantitative differences at specific latitudes and energy redistribution within the climate system supports larger outgoing terrestrial radiation fluxes at high latitudes compared to solar radiation values at similar latitudes. The global average thermal energy flux at the upper edge of the Earth's atmosphere is 235Wm~2, which equals the net solar radiation flux at this level. The zonal mean top of the atmosphere radiation balance (see Fig. 2.6) highlights the radiation balance excess in the tropics and the deficit at the high latitudes.

Absorption and reflection of solar radiation by the atmosphere depletes the quantity of solar radiation reaching the Earth's surface. The net shortwave flux at the Earth's surface is represented by the first two variables on the right side of Equation 2.11. The spatial distribution of this quantity is an inverse relationship with latitude evident in conditions at the top of the atmosphere (see Fig. 7.1), but with complexity added due to solar radiation being absorbed by the atmosphere and reflected by the atmosphere and the Earth's surface. Since water has a lower albedo than land at high sun angles, the equatorial oceans have a lower albedo than adjacent continents and a larger available solar radiation flux.

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Fig. 7.3. Global annual mean net radiation at the Earth's surface. Units in Wm 2. (NCEP Reanalysis data courtesy of NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at http://www.cdc.noaa.gov/.)

The concept of net radiation expressed in Equation 2.11 is a useful paradigm at the Earth's surface for defining available energy to drive non-radiative processes. Figure 7.3 shows that over most of the globe the net surface radiation is downward, but polar regions experience a net radiation loss. Variations in the global pattern of net radiation are related to the spatial variation in solar radiation, cloud cover, and the albedo and thermal differences of land and water surfaces. Two readily identified characteristics of the spatial variation in net radiation result from the interplay of radiation exchanges. A systematic decrease in net radiation with increasing latitude demonstrates the dominant role incoming solar radiation plays in quantifying annual net radiation. The radiation flux decreases from values of 160 W m-2 or greater near the equator to values of -40Wm-2 poleward of 80° latitude. The intensity and duration of solar radiation at the surface is related to latitude through fundamental Earth-Sun geometry responsible for delivering the solar beam and the Earth's rotation which distributes the energy longitudinally. The relationship between net radiation and latitude is most evident over the world's oceans.

A second feature apparent in Figure 7.3 is that net radiation is greater over oceans than over land at the same latitude. Net shortwave radiation varies due to spatial differences in atmospheric attenuation ofshortwave radiation and surface albedo. Since solar radiation is related to latitude, it is reasonable to expect that incoming shortwave radiation at the top of the atmosphere is similar for all locations at the same latitude. The net radiation difference between land and water surfaces must be due to other factors that include atmospheric attenuation of solar radiation, atmospheric absorption of solar and terrestrial radiation, surface albedo differences, and the large heat storage capacity of water. The slow thermal response of water moderates the emission of longwave thermal radiation from the oceans and permits the water to conserve more of the incident solar radiation. Another potentially significant contributor is the low albedo of water that permits a higher proportion of the solar radiation to be absorbed by the water.

Another notable feature in Figure 7.3 is the low net radiation values over the subtropical deserts. High surface albedo and low cloudiness and humidity combine to produce large solar radiation inputs, but these shortwave fluxes are offset by high thermal surface emissions that exceed atmospheric thermal inputs. The low net radiation values for these land surfaces contrast markedly with oceans at the same latitude.

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