Coupling Diffuse Sky Radiation and Surface Albedo

Abstract

New satellite instruments have been delivering a wealth of information regarding land surface albedo. This basic quantity describes what fraction of solar radiation is reflected from the earth's surface. However, its concept and measurements have some ambiguity resulting from its dependence on the incidence angles of both the direct and diffuse solar radiation. At any time of day, a surface receives direct radiation in the direction of the sun, and diffuse radiation from the various other directions in which it may have been scattered by air molecules, aerosols, and cloud droplets. This contribution proposes a complete description of the distribution of incident radiation with angles, and the implications in terms of surface albedo are given in a mathematical form, which is suitable for climate models that require evaluating surface albedo many times. The different definitions of observed albedos are explained in terms of the coupling between surface and atmospheric scattering properties. The analytical development in this paper relates the various quantities that are retrieved from orbiting platforms to what is needed by an atmospheric model. It provides a physically simple and practical approach to evaluation of land surface albedo values at any condition of sun illumination irrespective of the current range of surface anisotropic conditions and atmospheric aerosol load. The numerical differences between the various definitions of albedo for a set of typical atmospheric and surface scattering conditions are illustrated through numerical computation.

Quality assessment and improvement of the EUMETSAT Meteosat Surface Albedo Climate Data Record

“…The BHR (blue sky albedo) is the quantity measured by the in situ instrument. Blue sky albedo values are expected to lie between the corresponding BHR iso and DHR values (Pinty et al, 2005). This expected behaviour is confirmed in most of the cases for the 0DEG mission (Fig.…”

“…Throughout this paper we use the variable directional hemispherical reflectance (DHR) to refer to black sky albedo and the variable bi-hemispherical albedo under isotropic illumination (BHR iso ) to refer to white sky albedo. These two parameters can be used to estimate the actual albedo, the blue sky or BHR under ambient conditions (Pinty et al, 2005). EUMETSAT has undertaken an activity to generate a surface albedo data set (Meteosat Surface Albedo, MSA) climate data record (CDR), spanning over more than 2 decades from measurements acquired by the Meteosat Visible and Infrared Imager (MVIRI) instrument onboard the Meteosat First Generation (MFG) satellites (EUMETSAT, 2011).…”

Atmospheric effect on the ground-based measurements of broadband surface albedo

“…Although a lot of thorough scientific work concerning the diffuse and direct fraction of solar radiation has been carried out (e.g. Marsden et al, 2005;Pinty et al, 2005;Kaskaoutis and Kambezidis, 2009), the effect of the atmospheric optical depth on the pyranometer-measured albedo has thus far not been studied. This effect is important only when pyranometers are used for characterisation of the black-sky surface albedo, because in that case the albedo values are affected by the atmosphere due to the modification of the surface radiant flux density spectra by atmospheric absorption while also scattering, and the relative contributions of direct and diffuse radiant flux density vary with atmospheric optical depth.…”

“…The black-sky and white-sky albedo quantities represent the extreme cases under completely direct and completely diffuse illumination (Pinty et al, 2005;Román et al, 2010).…”

Towards Multidecadal Consistent Meteosat Surface Albedo Time Series

“…The data product contains information about the isotropic bi-hemispherical spectral albedo for a specific satellite (BHR iso,λ ) and the directional-hemispherical albedo at a sun zenith angle of 30 degree (DHR 30,λ ) [14]. These two quantities allow to calculate the actual surface albedo for any combination of diffuse and direct sky radiation [15]. The subscript λ indicates spectral albedo quantities, while the subscript bb will indicate broadband quantities throughout the paper.…”