Parameterization of topographic effect on surface solar radiation

Topographic impact on the surface solar (shortwave (SW)) radiation was investigated using a 40 m high‐resolution digital topographic data model and a radiative transfer model. With applications to the complex terrain of Taiwan, we found that the spatial variation of the surface SW radiation has a strong diurnal cycle. The standard deviation of the SW radiation, σ f , over Taiwan is nearly constant for a given solar zenith angle (SZA). The maximum σ f occurs at SZA ∼55°, corresponding to midmorning (early morning) and midafternoon (late afternoon) in winter (summer). For a spatial resolution of 40 m, the σ f over Taiwan attains a maximum value of 300 W m −2 , which is ∼50% of the domain mean SW radiation. The topographic impact on the spatial variation of SW radiation increases with decreasing grid box size, Δ x . In mesoscale land and atmospheric models, Δ x is small, and the topographic impact is strong. Over Taiwan, the standard deviation of the difference between the daily mean SW radiation computed with a spatial resolution of 40 m and that computed with degraded spatial resolution reaches 25 W m −2 for Δ x = 1 km and 8 W m −2 for Δ x = 10 km, indicating the importance of topographic impact on the surface SW radiation in mesoscale land and atmospheric models. We developed a parameterization of the topographic impact on the surface SW radiation that scales the SW radiation computed for a flat surface without shading by surrounding terrains. The scaling is separately applied to the direct and diffuse radiation. It is applicable to all clear, aerosol‐laden, and cloudy conditions and to all spatial resolutions with Δ x > 40 m.