Comparison among simulated sea ice surface bidirectional reflectance factor (BRF), hemispherical-directional reflectance factor (HDRF) and field measurements

Broadband albedo is an important geophysical parameter in the Earth surface-atmosphere interaction in global climate change, hydrological cycle and snowmelt runoff studies. To derive the broadband albedo accurately from satellite optical sensor observation at limited bands and at a single observation angle, the bidirectional reflectance factor (BRF) has to be quantitatively specified. When we tried to validate snow covered sea ice surface BRF model results using field-measured directional reflectance (FMDR). we recognized that the field measurements are actually the hemispherical-directional reflectance factor (HDRF) instead of BRF because of the existence of diffuse irradiance even under clear skies. Therefore, we made a comparison among FMDR and simulated BRF and HDRF. Our logic is as follow: if the HDRF patterns agree with FMDR while the simulated BRF patterns do not, we still consider that the simulated BRF patterns are validated. The comparison indicates that the simulated HDRF generally agrees with FMDR for the main part of the viewing hemisphere. This agreement suggests that the BRF model provides a good approximation of snow surface BRF for the central (viewing zenith angle <50°) and side looking parts of the viewing hemisphere. The study also suggests that the surface roughness and surface heterogeneity may also affect the actual surface BRF pattern.