Bidirectional reflectance of selected BOREAS sites from multiangle airborne data

Abstract

Abstract. Hyperspectral, multiangle reflected radiances were obtained using the airborne advanced solid-state array spectroradiometer (ASAS) over Boreal Ecosystem-Atmosphere Study (BOREAS) sites in Canada during four field campaigns in 1994. Atmospherically corrected bidirectional reflectance factors and estimates of spectral hemispherical reflectance for three canopies in the BOREAS southern study area (old aspen, old black spruce, and old jack pine) are presented. The multiangle spectral reflectance factors derived from data acquired July 21, 1994 (for a limited solar zenith angle range of 340-37 ø) for the forested sites showed distributions of high backscatter and consistently low forward scatter due to shadowing. Position and shape of the retrosolar maximums for the three sites varied. Spectral distinction among the sites was evident in the red, where reflectance factors for the jack pine exceeded those for spruce and aspen, and in the near infrared (NIR), where the aspen reflectance factors were twice those for the conifers. Data presented here suggest that spectral reflectances acquired at 26 ø backscatter in the principal plane would provide better discrimination among the major cover types than those obtained from a nadir view. Red reflectance was strongly dependent on view geometry for the spruce and jack pine sites due to the varying amounts of deep shadow and red-reflecting materials observed as a function of view azimuth and zenith. At the aspen site the red reflectance displayed much less variation with changing view zenith and azimuth. Accordingly, angular effects on the normalized difference vegetation index were large for the spruce and jack pine canopies but small for the aspen site.

Remote sensing in BOREAS: Lessons learned

“…This method yielded a strong agreement (r 2 = 0.8) with fluxes determined by eddy covariance from five different boreal ecosystems (Rahman et al, 2001). the confirmation that BRDF signatures vary predictably with the dominant land-cover types, offering a novel means of mapping vegetation for the boreal forest region (see Russell et al, 1997;Sandmeier & Deering, 1999). Furthermore, the specular component of the BRDF helps isolate inundated areas such as fens (e.g., Vanderbilt et al, 2002), which are important regional sources of methane.…”

“…Moreover, BOREAS has been successful in clarifying the contribution of multi-scale and directional radiative effects necessary to improve radiation schemes in climate models Li et al, 1997;Russell et al, 1997;Sandmeier & Deering, 1999). Scaling strategies consisting of comparing remotely sensed radiances from tower scale to patch scale and finally to regional scale (including water bodies) are leading to a new generation of radiative transfer models which account for grouping effects across scales.…”

“…Activities were greatly facilitated by a science staff that assisted in the overall project coordination, field logistics, and data archiving, all of which were essential for the integrative aspects of the project (Newcomer et al, 2001). Large amounts of project data were archived at the Oak Ridge National Lab's Distributive Active Archive Center (ORNL DAAC, accessible at http://www- Lefsky et al, 1999Lefsky et al, , 2002 Polarization and Directionality of Earth's Radiation (POLDER) helicopter and C-130 spectral radiance and BRDF Bicheron et al, 1997 Advanced Solid-State Array Spectrometer (ASAS) C-130 spectral radiance and BRDF Russell et al, 1997;Sandmeier & Deering, 1999 (Newcomer et al, 2001). BOREAS remains a ''work in progress.''…”

“…A number of multidirectional sensors, including (1) the Advanced Solid-state Array Spectroradiometer instrument (ASAS; Russell, Irons, & Dabney, 1997;Sandmeier & Deering, 1999), (2) were used in BOREAS primarily to characterize the bidirectional reflectance distribution function (BRDF). While the primary applications were to characterize the structural and radiative properties of boreal stands, a fortuitous by-product of these studies was Fig.…”

Variability in surface BRDF at different spatial scales (30m–500m) over a mixed agricultural landscape as retrieved from airborne and satellite spectral measurements

“…In line with this assumption, earlier studies have treated satel-61 lite BRDF/albedo retrievals as being observed over a homogeneous landscape; thus allowing di-62 rect -point-to-pixel‖ comparisons ( sub-grid scale spatial heterogeneity, and other underlying sources of variation that introduce 69 measurement uncertainties within the ground resolution element (GRE) of satellite retrievals; the 70 BRDF (and thus albedo) of certain ecosystems can only be correctly sampled with airborne mul-71 tiangle measurements (Walthall et al 2000). Among key biomes affected by these sources of er-72 ror are mixed-forest canopies (Johnson 1994;Russell et al 1997b), tropical savannas (Hill et al 73 2008; Georgiev et al 2009), shrublands (Chopping et al 2004), as well as snow-covered envi-74 ronments (Lyapustin et al 2009). Furthermore, recent studies contend that subpixel vegetation 75 structure is only detectable when obtaining measurements near the principal plane; i.e., where 76 BRDF effects are most pronounced (Chen et al 2005).…”