The downward solar radiation flux 521 The clearsky flux

The total downward flux of solar radiation Fsw represents a combination of direct beam and diffuse beam components, which together are often termed global radiation. Diffuse radiation is largely isotropic (i.e., the flux is roughly the same no matter what direction it is coming from), although the intensity is higher underneath the portion of the celestial dome nearest the Sun. The fundamental control on the global radiation flux reaching the surface is the TOA (or extraterrestrial) flux. The TOA flux is zonally symmetric. At the North Pole, the TOA flux is zero from the autumnal to spring equinoxes. By contrast, although the solar zenith angle (the angle between zenith and the Sun) at the Pole at local noon on the summer solstice is still a large 66.5°, the attendant 24-hour daylight yields a daily mean TOA flux of 522 W m-2, compared to a value of only 383 W m-2 at the equator. Because of the combined effects of day length and solar zenith angle, the monthly mean TOA flux for northern high latitudes actually increases with latitude during May through August, decreasing with latitude in other months.

Because there is an intervening atmosphere, the solar flux at the surface is smaller than the TOA flux. In the absence of cloud cover, the solar radiation received at the surface (the clear sky flux, or Gclr) depends on latitudinal variations in the solar zenith angle and elevation (which determine atmospheric path length), and associated path-length dependencies of non-cloud atmospheric scattering and absorption. Solar radiation is attenuated primarily through absorption by water vapor in several bands between 0.9 and 2.1 ^m, absorption by ozone in three bands (0.20-0.31, 0.31-0.35 and 0.45-0.85 ^m) and absorption and scattering by aerosols. Ozone absorption is nearly complete at wavelengths shorter than 0.285 ^m (in the ultraviolet region). As outlined in Chapter 4, most ozone absorption occurs in the stratosphere. The surface albedo also has a minor effect on Gclr by promoting multiple scattering between the surface and atmosphere. Gcir can be estimated with the aid of radiative transfer models (Schweiger and Key, 1994).

As discussed in Chapter 2, the term Arctic haze has come into widespread use to describe the frequent occurrence of aerosol layers that are especially noticeable in spring. The haze tends to be concentrated near the top of the Arctic inversion layer

(Bridgman et al., 1989). The effect of aerosols on incoming solar radiation involves both scattering and absorption, depending on their properties. In the Arctic, observations document the presence of several types of aerosol mineral dust, carbonaceous and sulfur particles.

The clear-sky transmittance over the Arctic, taken as the ratio between Gclr and the TOA fluxes, typically ranges from 0.70 to 0.90. The clear-sky transmittance decreases as latitude increases because the atmospheric path length increases (solar radiation strikes the Earth at a more grazing angle - the longer resulting atmospheric path length means more scatterers and absorbers). As path length decreases with elevation (the higher the elevation, the fewer scatterers and absorbers), clear-sky fluxes will be higher over surfaces such as the Greenland Ice Sheet. Measurements over the Greenland Ice Sheet by Konzelmann and Ohmura (1995) point to a clear-sky transmittance of about 0.80. While to a first order, the clear-sky atmosphere is hence relatively transparent with respect to solar radiation, non-cloud scattering and absorption is by no means insignificant in the Arctic.

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