Derivation and comparison of bi-directional reflectance functions of first-year and multi-year sea ice types in the Southern Ocean

Abstract

Surface spectral bi-directional reflectance function (BRF) is a fundamental surface property in radiation interaction with the atmosphere-Earth surface system. Knowledge of surface BRFs is also crucial to estimating surface albedo from remote sensing data sets. During two austral summer cruises in 1999 and 2000, surface spectral directional reflectance patterns of first-year (FY) and multi-year (MY) sea ice were measured under clear skies for 512 spectral channels between 350-1050 nm. After smoothing raw data, calibrating spectral albedo of the reference panel, and removing the impact of sky diffuse radiation, representative patterns of BRF of FY and MY sea ice types in the Southern Ocean have been produced. The solar incidence angles (50 to 65 degrees) associated with the derived patterns are moderate. Some common features are seen in the BRF patterns of both FY and MY sea ice types. A moderately strong peak occurs in the principal plane when the sensor is looking into the azimuth from which the solar illumination comes. When the sensor is viewing towards the azimuth perpendicular to the principal plane, the BRF values are close to the surface albedo. However, the FY ice exhibits stronger surface anisotropy than the MY ice because the former has a smoother surface than the latter. The BRF pattern of the FY ice has a minimum in the backward direction. From the backward minimum towards the forward peak, BRF values of the FY sea ice show a monotonous increase along almost all great circle scans in the hemisphere. On the contrary, the MY ice looks similar to an isotropic (or Lambertian) surface when it is viewed from almost all directions except for the forward viewing azimuth with large viewing zenith angles (60-80 degrees). Besides, there is a discernible secondary peak in the backward direction of the MY ice BRF, presumably caused by reflectance from the fore-slopes of surface undulation. The resulting patterns will be especially useful in deriving surface albedo from NASA moderate resolution imaging spectrometer (MODIS).