The Distribution Of Clear-sky Radiation Over Varying Terrain

This research explores the topographic distribution of clear-sky incoming solar radiation over varying terrain. Using a two-stream atmospheric radiation model and digital elevation models (DEM) of several different physiographic regions, we simulate clear-sky radiation at IS times throughout the day for three dates, December IS, March 15 and June 15. Geostatistical analysis is used to characterize the spatio­ temporal variability in modeled radiation over each terrain grid. This analysis reveals that, for any particular elevation model and set of atmospheric conditions, the variance and spatial autocorrelation of the data depend on sun angle and on probability density functions of slope. By considering the behavior of the radiation mean and variance as a function of sun angle and slope, we derive predictive equations for these statistics. Mean irradiance is a function of slope distribution, but can be approximated as function of mean slope only. The variance in irradiance is approximately proportional to sin2S, where S is the mean slope across the region. In addition, there is one particular solar zenith angle at which variance is maximized, and this is the same for all landscapes. This angle is a function of optical depth only and independent of any particular terrain considerations.