Forward scattering of snow as a factor in local surface energy balance and winter PV production

Because of its high albedo, snow changes the surface energy balance very signiKcantly and this introduces a strong positive feed-back loop with the local climate. In complex, mountainous terrain, the high albedo causes shortwave solar radiation to be reQected multiple times between slopes, which is known to increase the total incoming surface radiation as a result. Snow is also known to have a highly anisotropic reQectance function with a peak for forward scattering. This should inQuence the radiative transfer over snow in complex terrain. However, models have never been developed to include the combined effect of multiple terrain scattering and forward scattering of snow. We present the new numerical model module GROUNDEYE as part of the Alpine3D modelling environment, which has speciKcally been designed to consider the forward scattering of snow surfaces together with radiation-terrain interactions. Decomposing the hemisphere above each grid point into solid angles, a computationally eYcient radiative transfer model has been developed, which allows to consider also the incoming radiation on solar panels (PV) with an arbitrary geometry. The results of the model are veriKed against rigorous theoretical results and validated against data from a test site. We show that forward scattering of snow locally enhances radiative Quxes in particular for sunny slopes in valleys at the expense of shady slopes. In combination with multiple terrain scattering the effect is maximized. The local energy balance of the snow surface itself is not heavily changed through the forward scattering effect because of the generally high albedo of snow. However, if an object with a low albedo is placed where a lot of radiation is received, the effect of anisotropy and multiple scattering can be large. This has implications for the placement of solar panels in snowy environments. By placing them at optimal locations and with a rather steep inclination, winter electricity production can be signiKcantly increased, which is attractive in many mid-latitude environments, where renewable energy production may be low in winter.