The solar radiation field

The solar flux crossing a horizontal plane depends on the Sun s elevation angle but is independent of azimuth. The infrared radiation held is always azimuthally symmetric. Therefore, because only fluxes contribute to heating rates, we are interested solely in azimuth-independent radiation fields in our analysis. By analogy with Eq. (2.5.10), the appropriate equation for describing the transfer of radiation in the visible channel is 

Because only fluxes are physically significant in our problem, the two-stream approximation is deemed adequate. Hence the radiation fields are restricted to the directions fio = l i = —At-i = 1 J i- It can be shown (Samuelson, 1983) that a solution of Eq. (9.1.4) leads to a flux divergence 

Finally, in order to assign the distribution of solar heating rates correctly, it is necessary to quantify the relative fractions of visible and conservative radiation incident at the top of the atmosphere. If q is the fraction associated with the visible channel, we have 

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