Spatial scaling of reflectance and surface albedo over a mixed-use, temperate forest landscape during snow-covered periods

Albedo products from the MODerate resolution Imaging Spectroradiometer (MODIS) have been validated extensively over spatially homogeneous sites with snow-cover. This study evaluated the spatial scaling of albedo and related reflectance-based quantities over areas of high spatial heterogeneity in temperate mixed forest, deciduous forest, urban and built-up, and cropland/natural mosaic lands under snow covered conditions. Reflectance-based quantities evaluated included spectral radiance, surface directional reflectance, and spectral and broadband albedo derived from ground- and tower-based measurements and high-resolution (5 m) hyperspectral imagery (HSI) to coarse resolution (~ 500 m) MODIS satellite data.Our approach first compared ground- and tower-based spectral and broadband reflectance quantities to HSI data, then to evaluate HSI with MODIS reflectance-based products. Over snow-covered pasture, HSI directional reflectance was biased lower than ground-based measurements collected using a spectroradiometer, and greatly underestimated at wavelengths less than 450 nm. Tower-based shortwave broadband albedo (280–2800 nm) and HSI albedo agreed within ± 0.04. HSI directional spectral reflectance agreed well with tower-based measurements of spectral albedo collected using a spectroradiometer and remote cosine receptor above a mixed forest canopy with underlying snow cover. Spectral albedo collected over a dormant deciduous broadleaf canopy increased from 0.10 for snow-free conditions to 0.14–0.18 when snow-cover was present under the canopy. Canopy shortwave broadband albedo was not very sensitive to underlying snowpack depth, indicative of strong vegetation masking.
Next, HSI data were spatially aggregated and averaged to 500 m MODIS grids and compared to two MODIS albedo products: (1) MODIS/Terra and Aqua Combined Bidirectional Reflectance Distribution Function (BRDF) Albedo V005 (MCD43A, magnitude inversion), and (2) MODIS/Terra Snow Cover Daily L3 Global 500 m Grid (MOD10A1). An assessment of surface heterogeneity demonstrated that MODIS products generally perform well for snow-covered landscapes with high spatial heterogeneity, with biases between ± 0.04 and RMSEs less than 0.085 for mixed and deciduous broadleaf forested and urban & built-up land cover classes, and less than 0.11 for cropland/natural mosaic. Biases were generally lower for MOD10A1 compared to MCD43A3.
Mean shortwave broadband albedo from MODIS and HSI over deciduous broadleaf and mixed forest with underlying snow cover ranged from 0.14 to 0.28. Albedo over MODIS grids characterized as a mosaic of cropland, forests, grasslands, and shrublands (e.g., cropland/natural mosaic) was substantially higher than forested grids under snow-covered conditions, ranging from 0.39 to 0.43.