Modelling topographic effects on surface radiation – A fast algorithm to compute Horizon and Sky View Factor from large high-resolution Digital Elevation Models

Abstract:

Surface radiation is strongly modulated by adjacent terrain in regions with complex topography. Incoming shortwave radiation can be reduced by shading (direct-beam part) and altered by partial sky-obstruction and terrain reflection (diffuse part). Similarly, outgoing longwave radiation can be reduced by interception from neighbouring sloping terrain. These effects are often neglected in global and regional climate models, which apply the two-stream approximation for radiative transfer. The effects can be considered in a parametrised form by computing topographic parameters like the Horizon and the Sky View Factor from a Digital Elevation Model (DEM). However, deriving these quantities is computationally expensive as non-local terrain information needs to be processed. We present a fast algorithm, which is able to compute these quantities from DEMs like ASTER or NASADEM, which cover large parts of the globe and provide surface elevation at a high spatial resolution of one arc second (~30 m). The algorithm is able to process equally spaced DEM data both on latitude/longitude grids and on cartesian map projections. Slope and aspect angle, which are required to account for self-shading, are additionally computed. The algorithm is compared with existing algorithms to evaluate its accuracy and computational efficiency. The so-computed topographic parameters can be used in radiation schemes of weather and climate models, to account for resolved and/or subgrid-scale topographic effects. Furthermore, the algorithm also provides a useful tool for solar energy modelling.