The MODIS (Collection V005) BRDF/albedo product: Assessment of spatial representativeness over forested landscapes

Abstract

A new methodology for establishing the spatial representativeness of tower albedo measurements that are routinely used in validation of satellite retrievals from global land surface albedo and reflectance anisotropy products is presented. This method brings together knowledge of the intrinsic biophysical properties of a measurement site, and the surrounding landscape to produce a number of geostatistical attributes that describe the overall variability, spatial extent, strength of the spatial correlation, and spatial structure of surface albedo patterns at separate seasonal periods throughout the year. Variogram functions extracted from Enhanced Thematic Mapper Plus (ETM+) retrievals of surface albedo using multiple spatial and temporal thresholds were used to assess the degree to which a given point (tower) measurement is able to capture the intrinsic variability of the immediate landscape extending to a satellite pixel. A validation scheme was implemented over a wide range of forested landscapes, looking at both deciduous and coniferous sites, from tropical to boreal ecosystems. The experiment focused on comparisons between tower measurements of surface albedo acquired at local solar noon and matching retrievals from the MODerate Resolution Imaging Spectroradiometer (MODIS) (Collection V005) Bidirectional Reflectance Distribution Function (BRDF)/albedo algorithm. Assessments over a select group of field stations with comparable landscape features and daily retrieval scenarios further demonstrate the ability of this technique to identify measurement sites that contain the intrinsic spatial and seasonal features of surface albedo over sufficiently large enough footprints for use in modeling and remote sensing studies. This approach, therefore, improves our understanding of product uncertainty both in terms of the representativeness of the field data and its relationship to the larger satellite pixel.

A Multisensor Approach to Global Retrievals of Land Surface Albedo

“…Similar to BSRN processing, we discard any observations where the albedo is unphysical or the incoming irradiance data fails the BSRN quality test for extremely large cases. The evaluation of the satellite albedo estimates with in situ observations is unavoidably a point-to-pixel comparison, where the representativeness of the in situ measured albedos for the albedo of the larger surrounding area can have a large impact on the results [31,52]. The Surface Albedo Validation Sites (SAVS) database [53] provides metrics of areal representativeness for albedo evaluation sites.…”

Mapping Surface Broadband Albedo from Satellite Observations: A Review of Literatures on Algorithms and Products

“…From 2002 to now, researchers paid more attention to the improvement and validation of the current surface albedo products [49][50][51][52][53]. In this period, the highlighted papers include the algorithm for the snow albedo product [54] and the direct-estimation algorithm [55] (labeled with green in Figure 3).…”

“…However, the most important issue is that the footprint of ground measurements and the satellite pixel are sometimes not comparable, thus requiring an up-scaling of ground-measured albedo with the aid of high-resolution albedo products [49]. In recent years, more and more studies [51,117] have evaluated the representativeness of ground stations with geostatistical attributes, and the data with high representativeness are screened for validating the generated surface albedo products.…”

Reducing the Uncertainties in Direct Aerosol Radiative Forcing

“…Several of the aerosol retrieval approaches self-consistently derive surface reflectance parameters (e.g., Martonchik et al 2009;Lyapustin and Wang 2009;Pinty et al 2000;Govaerts et al 2010), and the standard MODIS BRDF product (Schaaf et al 2002) is widely used. Limited validation data are available for these products (e.g., Roman et al 2009), but it is beyond the scope of the current paper to review this subject in greater depth.…”