The HelioMont method for assessing solar irradiance over complex terrain: Validation and improvements

- We validate HelioMont, an algorithm retrieving surface radiation from satellite data.
- We focus on three sites in the Alps, encompassing different topographic conditions.
- The estimation error for radiation components is strong under cloud free conditions.
- Using monthly climatological values of aerosol characteristics has a big impact.
- Better results are obtained with daily local-scale aerosol input.

This study evaluates the suitability of the method HelioMont, developed by MeteoSwiss, for estimating solar radiation from geostationary satellite data over the Alpine region. The algorithm accounts for the influence of topography, clouds, snow cover and the atmosphere on incoming solar radiation. The main error sources are investigated for both direct and diffuse solar radiation components by comparison with ground-based measurement taken at three sites, namely Bolzano (IT), Davos (CH) and Payerne (CH), encompassing different topographic conditions. The comparison shows that the method provides high accuracy of the yearly cycle: the Mean Absolute Bias (MAB) is below 5 W m− 2 at the lowland station Payerne and below 12 W m− 2 at the other two mountainous stations for the monthly averages of global and diffuse radiation. For diffuse radiation the MAB is in the range 11-15 W m− 2 for daily means and 34-40 W m− 2 for hourly means. It is found that the largest errors in diffuse and direct radiation components on shorter time scales occur during summer and for cloud-free days. In both Bolzano and Davos the errors for daily-mean diffuse radiation can exceed 50 W m− 2 under such conditions. As HelioMont uses monthly climatological values of atmospheric aerosol characteristics, the effects of this approximation are investigated by simulating clear-sky solar radiation with the radiative transfer model (RTM) libRadtran using instantaneous aerosol measurements. Both ground-based and satellite-based data on aerosol optical properties and water vapor column amount are evaluated. When using daily atmospheric input the estimation of the hourly averages improves significantly and the mean error is reduced to 10-20 W m− 2. These results suggest the need for a more detailed characterization of the local-scale clear-sky atmospheric conditions for modeling solar radiation on daily and hourly time scales.