Shortwave Radiance to Irradiance Conversion for Earth Radiation Budget Satellite Observations: A Review  
[PERSON]  
1 Coperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA 12N0 AA Chemical Sciences Laboratory, Boulder, CO 80305, USA 20 NAA Chemical Sciences Laboratory, Boulder, CO 80305, USA 20 Joint of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA; [EMAIL_ADDRESS] 20 NASA Goddard Institute for Space Studies, New York, NY 10025, USA; [EMAIL_ADDRESS] 20 NASA Goddard Institute for Space Studies, New York, NY 10025, USA 20 NASA Goddard Institute for Space Studies  
radiation budget (ERB) satellite instruments flown in low-Earth orbit [3; 4; 5; 6; 7; 8]. Among these are relatively simple wide field-of-view (WFOV) non-scanning instruments that sample the entire Earth disk as a single pixel, and narrow field-of-view (NFOV) instruments that sample only a small area but provide coverage by scanning. The WFOV instruments present unique challenges for developing an ERB data record [9], and NFOV scanners have therefore emerged as the instrument of choice. From the perspective of the satellite, it is only possible to instantaneously observe the reflected shortwave radiation in the direction that a NFOV scanner observes, known as the radiance. However, what is ultimately of most interest for quantifying the ERB [10; 11; 12; 13; 14], understanding climate forcing and feedbacks [15; 16], and for constraining and improving climate models [17; 18; 19; 20] is the cosine-weighted reflected shortwave radiance leaving Earth integrated over all directions above the scene that the satellite observes, known as the radiative flux (strictly the radiative flux density) or irradiance. Since the angular distribution of reflected shortwave radiation can vary dramatically from one scene to another, the conversion of the directional satellite-observed radiance to the energetically relevant hemispherically integrated quantity of irradiance is non-trivial. We must rely on a model for this conversion that depends on the direction that the scene is being viewed by the satellite instrument relative to the sun (i.e., the solar-viewing geometry) and the scene properties, such as cloud cover or surface type. Such models are referred to as angular distribution models (ADMs), and have been generated from instruments on a variety of satellites over a series of decades [21; 22; 23; 24]. While the underlying theory that defines the relationship between radiance and irradiance remains unchanged, satellites have observed at progressively finer spatial resolution, for longer duration, and with more advanced supporting imager information leading to a drastic variation in the approach to generating shortwave ADMs over time. The purpose of this article is to provide a synthesis of shortwave ADMs and their application from the earliest ERB satellite missions up until the present day, and highlight recent advances that support the conversion of shortwave radiance to irradiance addressing some of the outstanding issues that have emerged along the way. While reviews of ERB satellite missions already exist in the literature [7; 8], none have been dedicated to the decades-long research and important processing step of shortwave radiance to irradiance conversion. Many of the concepts discussed here also apply to the emitted longwave radiation by Earth, which also requires conversion from radiance to irradiance, but we focus exclusively on the shortwave for two reasons. Firstly, the shortwave typically exhibits a greater dynamic range in anisotropy and therefore presents the most challenging case for the conversion from radiance to irradiance. Secondly, at present there is renewed focus on shortwave ADMs in light of the upcoming _Libera_ mission that was recently selected by the National Aeronautics and Space Administration (NASA) to continue the ERB data record into the next generation.