Abstract
Observing the Earth radiation budget (ERB) from satellites is crucial for monitoring and understanding Earth’s climate. One of the major challenges for ERB observations, particularly for reflected shortwave radiation, is the conversion of the measured radiance to the more energetically relevant quantity of radiative flux, or irradiance. This conversion depends on the solar-viewing geometry and the scene composition associated with each instantaneous observation. We first outline the theoretical basis for algorithms to convert shortwave radiance to irradiance, most commonly known as empirical angular distribution models (ADMs). We then review the progression from early ERB satellite observations that applied relatively simple ADMs, to current ERB satellite observations that apply highly sophisticated ADMs. A notable development is the dramatic increase in the number of scene types, made possible by both the extended observational record and the enhanced scene information now available from collocated imager information. Compared with their predecessors, current shortwave ADMs result in a more consistent average albedo as a function of viewing zenith angle and lead to more accurate instantaneous and mean regional irradiance estimates. One implication of the increased complexity is that the algorithms may not be directly applicable to observations with insufficient accompanying imager information, or for existing or new satellite instruments where detailed scene information is not available. Recent advances that complement and build on the base of current approaches, including machine learning applications and semi-physical calculations, are highlighted.
Highlights
Introduction distributed under the terms andMonitoring the amount of solar radiation reflected back to space by Earth is vital for understanding and modelling our climate system and its evolution [1,2]
To lessen the impact of the resolution differences between GERB and the observations used to generate the CERES-TRMM angular distribution models (ADMs), the ADMs are not assigned at the GERB pixel scale (~50 km at nadir), but rather on sub-regions within each GERB footprint corresponding to 3× 3
ADMs: radiances collected over an extended time time petraditionally been taken to generate ADMs: radiances collected over an extended riod at at a large are period a largerange rangeofofsolar-viewing solar-viewinggeometries geometries aresorted sortedinto intoangular angularbins binsand andaveraveraged, a process that is performed separately for scene types with distinct anisotropy
Summary
While some of the finer details regarding the conversion of radiance to irradiance have evolved over time, the general approach remains the same and is outlined . Discrete ADMs, angularan bins are defined, and has a large number of observed radiances generate empirical approach traditionally been adopted. (a) The synthetic “truth” represents the continuous anisotropic function for this scene It is calculated for angle anglebin bin. . Avariations normalization is applied to each radiance observation I 0 ijk to (i, j, k)for account for variations in Earth-sun distance and solar zenith angle as follows:. Nk are the total number angular bins inbin viewing and relative j and widths,Nor estimated numerically withof fixed angular widthszenith and angle an additional azimuth angle, respectively, and w and w are weights associated with the hemispheric j k weighting by cosine of the viewing zenith angle (e.g., via Gaussian quadrature).
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have