Influence of spatial heterogeneity of local surface albedo on the area-averaged surface albedo retrieved from airborne irradiance measurements

[摘要] Spectral airborne upward and downward irradiance measurements areused to derive the area-averaged surface albedo. Real surfaces arenot homogeneous in their reflectivity. Therefore, this work studiesthe effects of the heterogeneity of surface reflectivity on thearea-averaged surface albedo to quantify how well aircraftmeasurements can resolve the small-scale variability of the localsurface albedo. For that purpose spatially heterogeneous surfacealbedo maps were input into a 3-dimensional (3-D) Monte Carloradiative transfer model to simulate 3-D irradiance fields. Thecalculated up- and downward irradiances in altitudes between 0.1 and 5 km are used to derive the area-averaged surface albedo usingan iterative retrieval method that removes the effects due toatmospheric scattering and absorption within the layer beneath theconsidered level. For thecase of adjacent land and sea surfaces,parametrizations are presented which quantify the horizontaldistance from the coastline that is required to reduce surfaceheterogeneity effects on the area-averaged surface albedo to a givenlimit. The parametrization which is a function of altitude, aerosoloptical depth, single scattering albedo, and the ratio of local landand sea albedo was applied for airborne spectral measurements. Inaddition, the deviation between area-averaged and local surfacealbedo is determined for more complex surface albedo maps. Formoderate aerosol conditions (optical depth less than 0.4) and awavelength range between 400 and 1000 nm, the altitude and theheterogeneity of the surface albedo are the dominant factorsdetermining the mean deviation between local and area-averagedsurface albedo. A parametrization of the mean deviation is appliedto an albedo map that was derived from a Landsat image of an area inEast Anglia (UK). Parametrization and direct comparison of local andarea-averaged surface albedo show similar mean deviations (20% vs. 25%)over land.

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