Albedo time series of the European Alps based on NOAA AVHRR data (1985 – 2012)

Consortium: PI: Stefan Wunderle (GIUB), CoPI: Reto Stöckli (MeteoSwiss); Support: Crystal Schaaf (Boston University)
Funding source: SNSF
 

Albedo is a dominant factor of the radiation balance and thus of great importance for the Earth's climate. Therefore, the albedo project focuses on processing and analyzing a long-time series of land surface albedo covering the European Alps, which will bring new insights on albedo changes during the last 25 years. The main goals are to show the spatial and temporal distribution of albedo and to generate a consistent albedo dataset with a high spatial and temporal resolution.

When deriving surface albedo it has to be considered, that unlike simple theoretical models, Earth's surface does not reflect isotropically. To take the anisotropic surface reflectance into account, knowledge of the Bidirectional Reflectance Distribution Function (BRDF) is required. The BRDF describes the scattering of a parallel beam of incident light from one direction in the hemisphere into another direction in the hemisphere and represents the intrinsic reflectance properties of a surface.

Illustration of forward and backward scatters
Figure: Illustration of forward (observer A) and backward (observer B) scattering. Leaf 1 is just about perpendicular to the sunbeam and thus brightly illuminated while leaf 2 is nearly parallel to the sunbeam and only dimly lit. For observer A, leaf 2, which is dimly lit, is clearly visible but he will not see much of leaf 1. Observer B will see the brightly lit leaf 1 well and will not see much of the dimly lit leaf 2.

As a ratio of infinitesimals, the BRDF is not directly measurable and a BRDF model is needed. The RSG of the University of Berne (RSGB) makes use of a semiempirical kernel-driven BRDF model. The theoretical basis of these models is that the land surface reflectance is modeled as weighted sum of three kernels representing the basic scattering types, namely, isotropic, volumetric and geometric scattering. For the application of these models a good sampling of illumination and viewing angles is required. To obtain the directional sampling of surface reflectances the RSGB will accumulate sequential AVHRR images over a specified time period and compute multi-day clear sky composites covering these angles.

 

Publications:

Sütterlin, Melanie; Stöckli, R.; Schaaf, C. B.; Wunderle, Stefan (2016). Albedo climatology for European land surfaces retrieved from AVHRR data (1990-2014) and its spatial and temporal analysis from green-up to vegetation senescence. Journal of Geophysical Research: Atmospheres, 121(14), pp. 8156-8171. American Geophysical Union10.1002/2016JD024933
Sütterlin, Melanie; Schaaf, C.B.; Stöckli, R.; Sun, Q.; Hüsler, Fabia; Neuhaus, Christoph; Wunderle, Stefan (2015). Albedo and reflectance anisotropy retrieval from AVHRR operated onboard NOAA and MetOp satellites: Algorithm performance and accuracy assessment for Europe. Remote sensing of environment, 168, pp. 163-176. Elsevier 10.1016/j.rse.2015.06.023