The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission will provide a calibration laboratory in orbit for the purpose of accurately measuring and attributing climate change. CLARREO measurements establish new climate change benchmarks with high absolute radiometric accuracy and high statistical confidence across a wide range of essential climate variables. CLARREO's inherently high absolute accuracy will be verified and traceable on orbit to Système Internationale (SI) units. The benchmarks established by CLARREO will be critical for assessing changes in the Earth system and climate model predictive capabilities for decades into the future as society works to meet the challenge of optimizing strategies for mitigating and adapting to climate change. The CLARREO benchmarks are derived from measurements of the Earth's thermal infrared spectrum (5–50 μm), the spectrum of solar radiation reflected by the Earth and its atmosphere (320–2300 nm), and radio occultation refractivity from which accurate temperature profiles are derived. The mission has the ability to provide new spectral fingerprints of climate change, as well as to provide the first orbiting radiometer with accuracy sufficient to serve as the reference transfer standard for other space sensors, in essence serving as a “NIST [National Institute of Standards and Technology] in orbit.” CLARREO will greatly improve the accuracy and relevance of a wide range of space-borne instruments for decadal climate change. Finally, CLARREO has developed new metrics and methods for determining the accuracy requirements of climate observations for a wide range of climate variables and uncertainty sources. These methods should be useful for improving our understanding of observing requirements for most climate change observations

[1] Subvisual cirrus clouds that are defined as those whose optical thickness is less than ∼0.3 are found in ∼50% of global observations. Passive remote-sensing instruments, such as the Moderate Resolution Imaging Spectroradiometer (MODIS), generally fail to detect these optically thin clouds. The launch of NASA's Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite provides an unprecedented ability to detect thin cloud layers globally. Also, the Clouds and the Earth's Radiant Energy System (CERES) provides accurate measurements of top-of-atmosphere radiation. By using CERES, MODIS, and CALIPSO measurements in a synergistic manner, a quantitative assessment of the influence of subvisual clouds on the Earth's shortwave (SW) radiation is accomplished. The difference between clear-sky radiation flux and the flux obtained with the presence of subvisual clouds clearly shows the cooling effect of subvisual clouds in the SW. The subvisual clouds increase the diurnal mean reflected SW flux by ∼2.5 W m −2 . The subvisual clouds' effect on outgoing longwave radiation is also studied using a radiative-transfer model. The model results show that a layer of subvisual clouds having optical thickness of 0.1 can have a warming effect of ∼15 W m −2 . These clouds can also affect the polarization of the reflected SW radiation and the accuracy of aerosol retrieval with satellite measurements. This work demonstrates that the study of subvisual clouds is necessary for an accurate and detailed understanding of Earth-atmosphere radiation.

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On-orbit calibration requirements for a space-based climate observing system include long-term sensor response stability and reliable inter-calibration of multiple sensors, both contemporaneous and in succession. The difficulties with achieving these for reflected solar wavelength instruments are well known. The Moon can be considered a diffuse reflector of sunlight, and its exceptional photometric stability has enabled development of a lunar radiometric reference, manifest as a model that is queried for the specific conditions of Moon observations. The lunar irradiance model developed by the Robotic Lunar Observatory (ROLO) project has adequate precision for sensor response temporal trending, but a climate-quality lunar reference will require at least an order of magnitude improvement in absolute accuracy. To redevelop the lunar calibration reference with sub-percent uncertainty and SI traceability requires collecting new, high-accuracy Moon characterization measurements. This paper describes specifications for such measurements, along with a conceptual framework for reconstructing the lunar reference using them. Three currently active NASA-sponsored projects have objectives to acquire measurements that can support a climate-quality lunar reference: air-LUSI, dedicated lunar spectral irradiance measurements from the NASA ER-2 high altitude aircraft; ARCSTONE, dedicated lunar spectral reflectance measurements from a small satellite; and Moon viewing opportunities by CLARREO Pathfinder from the International Space Station.

Abstract. Reflected solar radiance from the Earthatmosphere system is polarized. Radiance measurements can be affected by light's state of polarization if the radiometric sensor has polarization dependence. To enable the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission for inter-calibration of the imagers with polarization dependence, such as the MODIS, the polarization state of the light must be known with sufficient accuracy. For this purpose, the polarized solar radiation from the ocean-atmosphere system is studied with an adding-doubling radiative transfer model (ADRTM). The Cox-Munk ocean wave slope distribution model is used in calculation of the reflection matrix of a wind-ruffled ocean surface. An empirical foam spectral reflectance model and an empirical spectral reflectance model for water volume below the surface are integrated in the ocean-surface model. Solar reflectance from the ADRTM is compared with that from the discrete-ordinate radiative transfer (DISORT) model. Sensitivity studies are conducted for various ocean-surface and atmospheric conditions for the stratification of polarization distribution models (PDMs), which are to be used in the inter-calibration of the polarization-sensitive imager measurements with the CLARREO data. This report presents the first accurate approach for making the spectral PDMs over broad solar spectra, which cannot be achieved by empirical PDMs based on the data from polarimetric sensors.