THEORY AND MEASUREMENTS OF THE COMPLETE BEAM SPREAD FUNCTION OF SEA-ICE

The beam spread function (BSF) of sea ice is of interest for several r easons. The BSF characterizes beam propagation through sea ice. Its eq uivalent, the point spread function, is essentially the optical impuls e response of the medium, which has many useful connections to radiati ve transfer theory. In-ice measurements of the BSF over the full angul ar range 0-180 degrees, using a novel method, were made of first-and m ultiyear ice off the shore of Barrow, Alaska. All of the measured sea ice BSFs were drastically different than the BSF of ocean water, and t hey strongly indicated that sea ice is a highly scattering medium, wit h a single scattering albedo generally >0.97 over the visible spectrum . At pathlengths >30 cm, the BSF was found to be nearly identical to t he computed asymptotic radiance distribution. The rapid approach to th e asymptotic state and the high single scattering albedo of sea ice su ggest that photon diffusion theory should accurately describe radiativ e transfer in sea ice away from boundaries. Equating the results bf di ffusion theory with asymptotic radiative transfer theory yields a simp le expression that relates the asymptotic attenuation coefficient K-in finity to the inherent optical property coefficients and the asymmetry parameter g of the scattering phase function. It is shown that the ne cessary optical parameters for computing g can be obtained from the me asured BSE Thus, all the information necessary for modeling optical pr opagation in sea ice can be obtained from the BSF measurements using t he method described here.