Abstract
One of the most crucial tasks of measuring top-of-atmosphere (TOA) radiative flux is to understand the relationships between radiances and fluxes, particularly for the reflected shortwave (SW) fluxes. The radiance-to-flux conversion is accomplished by constructing angular distribution models (ADMs). This conversion depends on solar-viewing geometries as well as the scene types within the field of view. To date, the most comprehensive observation-based ADMs are developed using the Clouds and the Earth’s Radiant Energy System (CERES) observations. These ADMs are used to derive TOA SW fluxes from CERES and other Earth radiation budget instruments which observe the Earth mostly from side-scattering angles. The Earth Polychromatic Imaging Camera (EPIC) onboard Deep Space Climate Observatory observes the Earth at the Lagrange-1 point in the near-backscattering directions and offers a testbed for the CERES ADMs. As the EPIC relative azimuth angles change from 168◦ to 178◦, the global daytime mean SW radiances can increase by as much as 10% though no notable cloud changes are observed. The global daytime mean SW fluxes derived after considering the radiance anisotropies at relative azimuth angles of 168◦ and 178◦ show much smaller differences (<1%), indicating increases in EPIC SW radiances are due mostly to changes in viewing geometries. Furthermore, annual global daytime mean SW fluxes from EPIC agree with the CERES equivalents to within 0.5 Wm−2 with root-mean-square errors less than 3.0 Wm−2. Consistency between SW fluxes from EPIC and CERES inverted from very different viewing geometries indicates that the CERES ADMs accurately quantify the radiance anisotropy and can be used for flux inversion from different viewing perspectives.
Highlights
The Deep Space Climate Observatory (DSCOVR) was launched on Feb. 11, 2015 and is the first Earthobserving satellite at the Lagrange-1 (L1) point, about 1.6 million kilometers from Earth
DSCOVR is in an elliptical Lissajous orbit around the L1 point where the Earth Polychromatic Imaging Camera (EPIC) and NISTAR view the Earth from a small range of relative azimuth angle from 168◦ to 178◦
Applying the Clouds and the Earth’s Radiant Energy System (CERES) angular distribution models (ADMs) to EPIC observations offers an opportunity to test the performance of radiance-to-flux conversion in the near back-scattering angles
Summary
The Deep Space Climate Observatory (DSCOVR) was launched on Feb. 11, 2015 and is the first Earthobserving satellite at the Lagrange-1 (L1) point, about 1.6 million kilometers from Earth. This paper will examine how Isw and Rsw vary with relative azimuth angles and compare Fsw from EPIC derived at different backscattering directions with CERES counterparts to investigate if CERES ADMs capture the radiance anisotropy changes at different backscattering angles. The narrowband-to-broadband regression coefficients are derived using collocated Moderate Resolution Imaging Spectrometer (MODIS) narrowband reflectances (469, 550, and 645 nm) and CERES broadband reflectances within the CERES Single Scanner Footprint TOA/ Surface Fluxes and Clouds (SSF) Edition 4 A product separately for ocean and non-ocean surfaces for all-sky conditions These narrowband-to-broadband regressions are applied to the EPIC measurements to derive the “EPIC broadband” SW reflectance for each EPIC pixel. Changes in clouds and viewing geometries are the dominant factors affecting the magnitude of Isw. For May, Isw of 2017 are greater than those of 2020 during the first half of the month when the relative azimuth angles differ the most between 2017 and 2020. The radiances from many Earth scenes can be very different when viewed at these different azimuth angles (Gatebe and King, 2016)
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