Influence of spatial heterogeneity of surface albedo on its retrieval from airborne irradiance measurements
Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. In contrast, ground-based measurements deliver the local surface albedo. This work studies the effect of the heterogeneity of the local surface albedo on the area-averaged surface albedo. This helps to understand, how well the surface albedo can be spatially-resolved by airborne measurements. To quantify this effect as a function of heterogeneity, aerosol optical depth (AOD) and flight altitude, spatially heterogeneous surface albedo maps were input into a 3-dimensional (3D) Monte Carlo radiative transfer model to simulated 3D irradiance fields at artificial flight altitudes. The calculated up-and downward irradiances are used to derive the area-averaged surface albedo using an iterative retrieval that removes the atmospheric masking of the surface. For the case of adjacent land and sea surfaces a parameterization is presented which quantifies the horizontal distance to the coastline that is required to measure an undisturbed surface albedo (either land or sea surface albedo). In addition, the deviation between area-averaged and local surface albedo is determined and parameterized for more complex surface albedo maps.