Abstract
Abstract. The Cloud and the Earth's Radiant Energy System (CERES) instruments on NASA's Terra, Aqua and Soumi NPP satellites are used to provide a long-term measurement of Earth's energy budget. To accomplish this, the radiances measured by the instruments must be inverted to fluxes by the use of a scene-type-dependent angular distribution model (ADM). For permanent snow scenes over Antarctica, shortwave (SW) ADMs are created by compositing radiance measurements over the full viewing zenith and azimuth range. However, the presence of small-scale wind blown roughness features called sastrugi cause the BRDF (bidirectional reflectance distribution function) of the snow to vary significantly based upon the solar azimuth angle and location. This can result in monthly regional biases between −12 and 7.5 Wm−2 in the inverted TOA (top-of-atmosphere) SW flux. The bias is assessed by comparing the CERES shortwave fluxes derived from nadir observations with those from all viewing zenith angles, as the sastrugi affect fluxes inverted from the oblique viewing angles more than for the nadir viewing angles. In this paper we further describe the clear-sky Antarctic ADMs from Su et al. (2015). These ADMs account for the sastrugi effect by using measurements from the Multi-Angle Imaging Spectro-Radiometer (MISR) instrument to derive statistical relationships between radiance from different viewing angles. We show here that these ADMs reduce the bias and artifacts in the CERES SW flux caused by sastrugi, both locally and Antarctic-wide. The regional monthly biases from sastrugi are reduced to between −5 and 7 Wm−2, and the monthly-mean biases over Antarctica are reduced by up to 0.64 Wm−2, a decrease of 74 %. These improved ADMs are used as part of the Edition 4 CERES SSF (Single Scanner Footprint) data.
Highlights
The Clouds and the Earth’s Radiant Energy System (CERES) instruments are used to estimate the amount of reflected shortwave (SW) flux and emitted longwave (LW) flux at the top of the atmosphere (TOA) (Wielicki et al, 1996)
The CS15 albedos show a slight variation with solar azimuth; the dependence is greatly reduced. This indicates that the CS15 albedo retrievals behave more realistically with the solar azimuth than the KL05 angular distribution model (ADM), and that the anisotropic factors are more accurately compensating for the change in www.atmos-meas-tech.net/8/3163/2015/
In our attempt to find a solution to this issue we have incorporated the multi-angle information from Multi-Angle Imaging Spectro-Radiometer (MISR) into the creation of the CERES clear-sky Antarctic snow ADMs
Summary
The Clouds and the Earth’s Radiant Energy System (CERES) instruments are used to estimate the amount of reflected shortwave (SW) flux and emitted longwave (LW) flux at the top of the atmosphere (TOA) (Wielicki et al, 1996). These fluxes are widely used in studies of Earth’s energy budget. In order to derive the flux from the radiance we use scene-dependent angular distribution models (ADMs) (Loeb et al, 2005). ADMs relate the reflected radiance at a given satellite viewing geometry to the total reflected flux. We focus on details pertaining to the development and testing of the ADMs over clear Antarctic scenes that were briefly described in Su et al (2015)
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