Investigating the topographic influence on short-wave irradiance measurements: A case study for Kanzelhohe Observatory, Austria

We investigate terrain effects on short-wave radiation measurements at Kanzelhohe Observatory (KSO), a mountainous radiation monitoring site in the Austrian Southern Alps, using three-dimensional (3-D) radiative transfer (RT) modeling and observations. The magnitude of terrain effects on global radiation is experimentally assessed through the comparison of 10-min measurement segments ex - /including terrain reflected radiation. The reflected component is ex - /included in global solar irradiance (GLO) measurements through a horizon shaped metal frame raised/lowered in 10-min intervals. Additionally we assess terrain influence in 3-D RT model simulations performed with an updated version of GRIMALDI, allowing for digital elevation model inclusion. The DEM used for terrain representation is the one arc second Shuttle Radar Topography Mission (SRTM) digital elevation model. For GRIMALDI simulations we consider an area averaged albedo of 0.30 for summer and 0.36 for winter (0.30 for forests and 0.90 for fresh snow), respectively. The influence of the horizon and surface albedo on a grid cell of interest (KSO location) is derived through comparison of the solar irradiance incident on a domain grid cell surface (SI-GCS) with SI-GCS reduced for the reflected component. Applying a 2a threshold for the significance of terrain effects in GLO measurements, we find no significant influence of the surrounding during summertime. Also during wintertime, we find for the vast majority of observational increments no significant influence of the surrounding. Above a 2a threshold, exceedances are small, on average 0.2 W m(-2) Differences between 3-D RT model simulations including and excluding terrain effects are small during winter conditions, i.e. always below 1% and never exceeding 1 W m(-2). Slightly larger differences are found for 3-D RT simulations including/excluding the reflected component during summertime, reaching at solar noon up to 6.1 W m(-2). Both observations and model simulations indicate that KSO is well suited for radiation monitoring as terrain effects are small (close to negligible considering instrument specifications) throughout the year.