Investigation of the effect of surface heterogeneity and topography on the radiation environment of Palmer Station, Antarctica, with a hybrid 3-D radiative transfer model

Abstract

We have developed and used a Monte Carlo radiative transfer code to investigate how surface topography and heterogeneous snow/ice distributions affect the downwelling irradiance at Palmer Station, Antarctica (64.76°S, 64.07°W). The Monte Carlo calculations treat a three-dimensional (3-D) atmospheric volume which extends from the surface to 100 km altitude and has a 20 km X 20 km footprint on the southwest coast of Anvers Island. The radiative transfer calculations include the effects of molecular absorption, Rayleigh scattering, and clouds. The surface interaction is modeled explicitly. The trajectories of reflected photons are computed from stochastic bidirectional-directional reflectance functions, and their paths are traced through multiple interactionswith complex surface features. Computed results for a range of cloud optical depth, solar zenith angle, and surface albedo are presented. Comparisons of the 3-D model calculations to plane-parallel model predictions show that the effective albedo which characterizes a given ice distribution is affected by regions surprisingly far from the point of interest. Under low clouds (Zcloud = 1 km), surface irradiance measurements over a snow surface are significantly affected by the dark ocean surface more than 7 km away. For the opposite case of irradiance observations over ocean, the effect of a distant snow surface is not significant at ranges greater than 2 or 3 km. Since the radius of influence depends on atmospheric transmission adn surface albedo, the effective albedo varies spectrally. Neglect of this nonlocal albedo effect may significantly degrade the accuracy of radiation diagnostics that depend on spectral intensity ratios.