Introduction

The absorption and scattering of the incoming solar radiation depends on the molecular composition of the atmosphere, the presence of clouds and aerosols in the atmosphere, and on the radiative properties of the Earth's surface with water, snow and ice playing an important role. It is only weakly dependent on the vertical thermal structure. Incoming solar radiation follows a downward path through the atmosphere to the surface and through reflection an upward path through the atmosphere out to space. Incoming direct solar radiation, as we have seen in Chapter 5, is characterized by a direction given by the solar zenith angle z, or ¡1 = cos z, which depends on latitude and time of year. Radiation that has undergone scattering or isotropic reflection, i.e. diffuse radiation, can be characterized by a fixed value ¡i = 3/5, the standard diffusivity approximation for converting diffuse transmission to an equivalent direct transmission, as was discussed in §3.5.5. In reality, the radiation undergoes multiple scattering between the various atmospheric layers, Fig. 6.1, and the Earth's surface, especially over highly reflecting surfaces such as ice or snow. The effect of the multiple reflections between the ground and clouds or aerosols is to increase atmospheric and ground absorption. The incoming solar flux can be divided into two broad spectral regions comprising 60% mainly UV-visible radiation (0.2 < A < 0.85 ¡m) and 40% near-infra-red radiation (0.85 < A < 5.0 ¡m), and their transfer through each atmospheric layer needs to be treated separately. The UV-visible component in

FlG. 6.1. Multiple reflections between scattering layer and the Earth's surface.

SOLAR RADIATION TRANSFER IN THE ATMOSPHERE uv -visible transfer near-infrared transfer

Ozone absorption_

Rayleigh scattering C02 absorption H20

Cloud scattering Cloud scattering/absorption

Cloud scattering Cloud scattering/absorption

Earth's surface Earth's surface

FlG. 6.2. Scattering and absorption of UV-visible and near-infra-red solar radiation by components of the atmosphere.

Earth's surface Earth's surface

FlG. 6.2. Scattering and absorption of UV-visible and near-infra-red solar radiation by components of the atmosphere.

its transfer through the atmosphere (Fig. 6.2) undergoes cloud scattering, absorption mainly by ozone, Rayleigh scattering by air molecules, absorption and scattering by aerosols, and some very weak absorption by water vapour. The near-infra-red component undergoes cloud and aerosol scattering and absorption, absorption by water vapour, carbon dioxide, methane and ammonia. Near and below 0.2 ¡m, O2 plays a very significant role in ultraviolet absorption, but this mainly affects the Earth's photochemistry, as will be discussed in Chapter 7. Both components of the solar radiation undergo reflection and absorption by the Earth's surface.

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