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

We have developed and used a Monte Carlo radiative transfer code to in vestigate how surface topography and heterogeneous snow/ice distributi ons affect the downwelling irradiance at Palmer Station, Antarctica (6 4.76 degrees S, 64.07 degrees W). The Monte Carlo calculations treat a three-dimensional (3-D) atmospheric volume which extends from the sur face to 100 km altitude and has a 20 km x 20 km footprint on the south west coast of Anvers Island. The radiative transfer calculations inclu de the effects of molecular absorption, Rayleigh scattering, and cloud s. The surface interaction is modeled explicitly. The trajectories of reflected photons are computed from stochastic bidirectional-direction al reflectance functions, and their paths are traced through multiple interactions with complex surface features. Computed results for a ran ge 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 (Z(cloud) = 1 km), surface irr adiance 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 surfa ce is not significant at ranges greater than 2 or 3 km. Since the radi us of influence depends on atmospheric transmission and surface albedo , the effective albedo varies spectrally. Neglect of this nonlocal alb edo effect may significantly degrade the accuracy of radiation diagnos tics that depend on spectral intensity ratios.