BOA Radiation

From this brief discussion of TOA solar radiation balance we jump to the situation at the surface below the atmosphere where the solar radiation balance is confounded by atmospheric transmissivity and surface albedo. The former depends mostly on cloudiness and cloud properties, but also on dust and other aerosols. The latter, which has a small influence on downward radiation, depends on surface properties, which are influenced by land use and climate.

As solar radiation traverses the atmosphere it is absorbed and reflected by gases and non-gaseous particles [17]. Ozone is responsible for absorption of most of the UV radiation, that is, the solar radiation at wavelengths below 0.29 mm; at larger wavelengths oxygen and ozone absorption is negligible. Water vapour is a significant absorber in the infra-red portion of the solar spectrum above 0.7 mm. Carbon dioxide absorption of solar radiation is negligible. Aerosols can scatter and reflect some of the radiation back to space. Clouds can reflect most of the radiation back to space. Radiation reflected from the earth's surface can be re-reflected back, and so surface albedo can influence the downward flux. Thus, BOA solar radiation is much less than that at TOA, and is commonly divided into two fluxes: direct radiation coming from a 2.5 5° angle centred in the direction of the sun, and diffuse radiation arriving from the rest of the sky hemisphere above the observer. The total of these two, that is, global radiation (Eg#), is the total solar energy available at the surface.

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