How near-forward scattering impacts measurements of direct solar radiation?

What is it about?

Atmospheric ice crystals, cloud droplets and large aerosol particles can scatter substantial amounts of solar radiation in near-forward directions. In measurements, the near-forward scattered radiation cannot be easily distinguished from the direct (i.e., non-scattered) solar radiation. In fact, all measurements of "direct" solar radiation at the surface are contaminated by some scattered radiation. This work discusses how the amount of near-forward scattered radiation depends on the properties of atmospheric particles and the instrument field of view. It also provides simple means for accounting for the near-forward scattering in radiation calculations.

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Why is it important?

Near-forward scattered radiation is an important issue for interpreting the measurements of direct solar radiation made with pyrheliometers and sun photometers. It is also relevant for concentrating solar energy applications. The parametrizations provided in this paper allow for a simple computation of the "apparent" direct solar radiation, for example, in numerical weather prediction models.

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