Absorption of Solar Radiation by Clouds

Although clouds form on existing particles, the solutions formed are quite dilute and hence are not expected to absorb solar radiation significantly. As a result, it has been commonly accepted that clouds will predominantly reflect solar radiation and that absorption will not be significant. However, as early as 1951, it was suggested that clouds appeared to be absorbing 

FIGURE 14-49 Absorption of light from an overhead sun by water associated with a 1-km stratus cloud with its top at an altitude of 2 km. The solid line is the absorption due to liquid water, the dashed line water vapor inside the cloud, and the dotted line water vapor in a column in the atmosphere (adapted from Davies et al., 1984). 

Several different approaches have been taken to investigate whether there is more absorption of visible light by clouds than expected based on current models of radiative transfer in the atmosphere. Some of these approaches and results are discussed in Box 14.3. 

While there thus appears to be evidence for apparent excess absorption of solar radiation by clouds, there is substantial controversy over whether this is indeed true absorption or whether there is some other explanation for the discrepancies (e.g., see Stephens and 

Similarly, Chou et al. (1998) used measurements of surface radiative fluxes and satellite radiance data in the Pacific warm pool region to conclude that the effect of clouds was similar to that expected, i.e., that the excess absorption, if it exists, is small. 

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Y. Zhou, Absorption of Solar Radiation by Clouds Observations versus Models, Science, 267, 496-503 (1995). 

Cess, R. D., M. H. Zhang, Y. Zhou, X. Jing, and V. Dvortsov, Absorption of Solar Radiation by Clouds Interpretations of Satellite, Surface, and Aircraft Measurements, J. Geophys. Res., 101, 23299-23309 (1996b). 

Chylek, P., and J. Hallett, Enhanced Absorption of Solar Radiation by Cloud Droplets Containing Soot Particles in Their Surface, Q. J. R. Meteorol. Soc., 118, 167-172 (1992). 

King, M. D., L. F. Radke, and P. V. Hobbs, Determination of the Spectral Absorption of Solar Radiation by Marine Stratocumulus Clouds from Airborne Measurements within Clouds, J. Atmos. Sci, 47, 894-907 (1990). 

A further effect of aerosols is the evaporation of clouds as a consequence of atmospheric heating caused by absorption of solar radiation by aerosols. This phenomenon reduces cloudiness, increasing absorption of solar radiation by the Earth-atmosphere system, a so-called semi-direct effect (Hansen et al., 1997 Satheesh and Ramanathan, 2000 Lohmann and Feichter, 2001). 

If this excess absorption by clouds is ultimately shown to be a real phenomenon, then an increased cloud formation and extent due to anthropogenic emissions may alter the radiative balance of the atmosphere not only through increased reflectance but also through increased absorption of solar radiation. Such an effect could impact atmospheric temperatures, their vertical distribution, and circulation, as well as surface wind speeds and the surface latent heat flux (Kiehl et al., 1995). Hence establishing if this is truly excess absorption, and if so, its origins, is a critical issue that remains to be resolved. 

Interstellar dusts could have a layered structure, as proposed by Greenberg (1998), of an icy mantle covering an organic-silicate core. On the way from an interstellar molecular cloud to protosolar nebula, interstellar dust could be modified by any thermal events, e.g., nebula gas heated by shock waves on an accreting disk or the absorption of solar radiation. 

Aerosols influence climate directly by the scattering and absorption of solar radiation and indirectly through their role as cloud condensation nuclei. The magnitude of the direct forcing of aerosols (measured in W m ) at a particular time and location depends on the amount of radiation scattered back to space, which itself depends on the size and optical properties of the particles, their abundance, and the solar zenith angle. The so-called indi-... 

Fig. 2.13 Radiation and heat balance of the system earth-atmosphere. Percentages are given in relation to the incoming solar radiation (100% = 343 W m ). I interception of solar radiation by molecules/particles (p) clouds (c), and earth surface (e), D diffuse radiation by molecules/particles (p) clouds (c), R reflexion (albedo) by molecules/particles (p) clouds (c), and earth surface (e), IR infrared dissipation to space by molecules/particles (p) clouds (c), T terrestrial radiation back to space (without absorption), A absorption of terrestrial radiation by molecules, AB atmospheric back-radiation, SH and LH sensible and latent heat, resp.

Diamond et al. [127] have estimated UVR doses in wetlands using this approach. Typical UVR doses were estimated by first generating maximal solar radiation doses for each day using a radiative transfer model, SBDART [113]. The model produced values for the full spectrum of solar radiation, from 280 to 700 nm, for cloudless conditions. These maximal values were then modified based on cloud cover effect estimates from 30 yr of historical solar radiation data (National Renewable Energy Laboratory, Department of Energy http //rredc.nrel.gov/solar/). The values derived in this procedure were estimated daily terrestrial, spectral (2 nm increments from 280 to 700 nm) solar radiation doses. Water column doses were then derived from absorption coefficients and spectral attenuation data described by Peterson et al. [128]. Although the focus of this effort was to characterize risk of UV-B radiation effects in amphibians, the procedure is directly applicable to phototoxicity, and the resulting UV-A radiation and spectral doses could be directly incorporated into calculation of possible effects. 

Aerosols may also play an important role in cHmate change. Natural aerosol emissions, similar to those caused by volcano eruptions and forest fires, can affect the radiation balance around the planet and, therefore, affect global temperatures quite distinctly from the heat directly released in such phenomena. Atmospheric aerosol emissions resulting from human industrial and deforestation activities can have the same effect, distinct from the associated greenhouse gas emissions. In both cases, these aerosols influence climate through the scattering of solar radiation, the absorption of terrestrial radiation, and through their effects on the properties of clouds [128, 129]. 

In or near the high energy zones, ions will trap electrons in reducing zones to form neutral atoms. When light of frequencies corresponding to electronic transitions of the atoms is passed through the atomic cloud, such frequency light will be absorbed by the atoms and excite them. The extent of such absorption is a measure of what is present and in what quantity. This phenomenon was first observed on solar radiation by Fraunhofer, who noticed precisely defined black lines in the solar spectrum. This led to the identification of the then- as-yet undiscovered new element, helium. Since then, atomic absorption spectrometry (AAS) has become one of the trace elemental, particularly metal, analytical chemists favorite techniques. 

In addition to the artificial radiation hght sources above described, sun-hght can be used to illuminate the photocatalysts. The earth receives about 1.7 X lO kW of solar radiation (1.5 x 10 kWh per year). Extraterrestrial radiation has an intensity of 1367 W m in a wavelength range between 200 and 5000 nm, which is reduced to 280-4000 nm when it reaches the ground, due to absorption phenomena by atmospheric compounds such as ozone, oxygen, carbon dioxide, aerosols, water vapor, clouds, etc. 

The troposphere ranges form -300 km (temperature about 320 K) to 50 km (temperature about 53 K). In the pressure range of 50 to 100 bar water clouds are assumed. In the range of 20 to 40 bar clouds of ammonium hydrosulfide, between 3 and 10 bar hydrogen sulfide and at 1 to 2 bar methane clouds can be found. At the tropopause, the temperature is 53 K, then it increases in the stratosphere due to absorption of solar UV radiation by methane and other hydrocarbons. At the outermost layer of its atmosphere, the temperature is about 800 K. 

A non-uegligible fraction of the solar radiation incident on the earth is lost by reflection from the top of the atmosphere and tops of clouds back into outer space. For the radiation penetrating the earth s atmosphere, some of the incident energy is lost due to scattering or absorption by air molecules, clouds, dust and aerosols. The radiation that reaches the earth s surface... 

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