Impact of the near-field effects on radiative transfer simulations of the bidirectional reflectance factor and albedo of a densely packed snow layer

Abstract Phenomenological radiative transfer models commonly assume that particles in media reside in each other’s far zone and, thus, are defined by their single-scattering properties. This so called far-field assumption becomes questionable for dense particulate snow layers since the location of the snow grains inside is close and in each other’s near zone. Electromagnetic interactions among the particles can occur, which potentially change their single-scattering properties. In this paper, the near-field effects in radiative transfer simulations of the snow surface bidirectional reflectance factor (BRF) and albedo of a snow layer in the wavelength range of 300 - 2500 nm is investigated using the phenomenological radiative transfer model SCIATRAN. The snow layer is composed of densely packed snow grains of droxtal shape with a maximum dimension of 60 µm. The snow grain shape assumption is justified by the good agreement of simulated top of atmosphere BRF with satellite measurements over a pure snow surface in Greenland. To evaluate the error of the far-field assumption, the single-scattering properties of the dense particulate snow layer has been modified according to the dense-medium light-scattering theory. By applying the far-field approximation, the BRF is overestimated by less than 0.039 % at forward-scattering angles and underestimated by less than 0.006 % at backscattering angles, respectively. The albedo of the dense particulate snow layer is overestimated by less than 0.012 %. In the considered case, the near-field effects on snow surface BRF and albedo of radiative transfer simulations appear negligibly small. Consequently, the simulation of snow surface BRF and albedo in the wavelength range of 300 - 2500 nm by phenomenological radiative transfer models under the far-field assumption is well justified.