The influence of the spatial resolution of topographic input data on the accuracy of 3-D UV actinic flux and irradiance calculations
1Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna, Austria
2Division for Biomedical Physics, Innsbruck Medical University, Innsbruck, Austria
3Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
4Department of Applied Physics and Applied Mathematics, Columbia University, New York, USA
5School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester
6Institute of Atmospheric Sciences and Climate, ISAC-CNR, Rome, Italy
Abstract. The aim of this study was to investigate the influence of the spatial resolution of a digital elevation map (DEM) on the three-dimensional (3-D) radiative transfer performance for both spectral ultraviolet (UV) irradiance and actinic flux at 305 nm. Model simulations were performed for clear sky conditions for three case studies: the first and second one using three sites in the Innsbruck area and the third one using three sites at the Sonnblick Observatory and surrounding area. It was found that DEM resolution may change the altitude at some locations by up to 500 m, resulting in changes in the sky obscured by the horizon of up to 15%. The geographical distribution of UV irradiance and actinic flux shows that with larger pixel size, uncertainties in UV irradiance and actinic flux determination of up to 100% are possible. These large changes in incident irradiance and actinic flux with changing pixel size are strongly connected to shading effects. The effect of DEM pixel size on irradiance and actinic flux was studied at the six locations, and it was found that significant increases in irradiance and actinic flux with increasing DEM pixel size occurred at one valley location at high solar zenith angles in the Innsbruck area as well as for one steep valley location in the Sonnblick area. This increase in irradiance and actinic flux with increasing DEM resolution is most likely to be connected to shading effects affecting the reflections from the surroundings.