Theory and measurements of the complete beam spread function of sea ice1

Abstract

The beam spread function (BSF) of sea ice is of interest for several reasons. The BSF characterizes beam propagation through sea ice. Its equivalent, the point spread function, is essentially the optical impulse response of the medium, which has many useful connections to radiative transfer theory. In‐ice measurements of the BSF over the full angular range 0–180°, using a novel method, were made of first‐ and multiyear ice off the shore of Barrow, Alaska. All of the measured sea ice BSFs were drastically different than the BSF of ocean water, and they strongly indicated that sea ice is a highly scattering medium, with a single scattering albedo generally >0.97 over the visible spectrum. At pathlengths >30 cm, the BSF was found to be nearly identical to the computed asymptotic radiance distribution. The rapid approach to the asymptotic state and the high single scattering albedo of sea ice suggest that photon diffusion theory should accurately describe radiative transfer in sea ice away from boundaries. Equating the results of diffusion theory with asymptotic radiative transfer theory yields a simple expression that relates the asymptotic attenuation coefficient K∞, to the inherent optical property coefficients and the asymmetry parameter g of the scattering phase function. It is shown that the necessary optical parameters for computing g can be obtained from the measured BSE Thus, all the information necessary for modeling optical propagation in sea ice can be obtained from the BSF measurements using the method described here.