Blue Sky Albedo

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Blue Sky Albedo
The broad-band surface albedo is defined as the ratio of up-welling to down-welling radiation fluxes in a given wavelength range. It is not a true surface property, but rather a characteristic of the coupled surface-atmosphere system, therefore defined as Blue-Sky Albedo (BSA). The BSA is a critical physical variable, which influences the Earth’s climate by affecting the energy budget and distribution in the Earth-atmosphere system. Its role is highly significant in both global and local scales; hence, BSA measurements provide a quantitative means for better constraining global and regional scale climate modelling efforts. The Moderate Resolution Imaging Spectroradiometer (MODIS) sensor, on board NASA’s Terra and Aqua platforms, provides the parameters needed for the computation of BSA on an 8-day temporal scale 500 x 500 m spatial resolution. Here, the evaluation of BSA is based on albedo model parameters available from the MODIS Bidirectional Reflectance Distribution Function (BRDF) product (product code: MCD43B1), on an 8-day basis at 500 x 500 m on a global basis. These parameters are derived from multi-angular reflectance observations, through the inversion of a BRDF model, after atmospheric correction and cloud screening. The algorithm makes use of a kernel-driven, linear BRDF model, whereby kernel weights are derived from a best fitting procedure to observational data. For each 500 x 500 m pixel, these weights are included in the MCD43A1 product at seven wavelengths in the visible, near infrared and shortwave infrared. Using these weights, and after applying the corresponding quality flags, the broadband (0.25-4.0 μm) directional-hemispherical surface reflectance (Black-Sky Albedo: BSA) for all possible solar zenith angles (in hourly basis) and the bi-hemispherical surface reflectance (White-Sky Albedo: WSA) are computed. Then, the true BSA is estimated using a linear relationship of BSA and WSA, depending on the fraction of diffuse radiation, which is a function of the Aerosol Optical Thickness (AOT). For the BSA computation, AOT data came from the MODIS Level 3 product; among other parameters, this product includes AOT at 550 nm, on an 8-day average basis that temporally coincides with the BSA, and at 1° Χ 1° spatial resolution. All computations are performed in the Google Earth Engine (GEE). More information on the BSA product provided here is available in the following articles:
Mitraka, Z., Benas, N., Gorelick, N. and Chrysoulakis, N., 2016. Global land surface albedo maps from MODIS using the Google Earth Engine. Geophysical Research Abstracts, 18, EGU2016-11679, EGU General Assembly 2016.
Benas, N. and Chrysoulakis, N., 2015. Estimation of the land surface albedo changes in the broader Mediterranean area, based on 12 years satellite observations. Remote Sensing, 7, 16150 - 16163.
Benas, N., Mitraka, Z., Chrysoulakis, N., Marconcini, M., Esch, T. and Lantzanakis, G., 2015. Global estimates of Urban Surface Albedo time series with the use of cloud computing. In: Book of Abstracts of the Mapping Urban Areas from Space (MUAS 2015) Conference, European Space Agency, pp. 32 - 33.
Benas, N. and Chrysoulakis, N., 2014. Estimation of land surface albedo time series and trends based on MODIS data. In: Proceedings of SPIE Remote Sensing 2014. SPIE 9239, Remote Sensing for Agriculture, Ecosystems, and Hydrology XVI, 92390Q; doi:10.1117/12.2066473.
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