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SI-traceable absolute radiometry for measuring top-of-atmosphere radiation balance: new detectors, measurements, and opportunities
by Dr. John Lehman, Prof. Peter Pilewskie, Mr. Malcolm White, Dr. Dave Harber, Dr. Michelle Stephens, Dr. Cameron Straatsma, Dr. Nathan Tomlin, Mr. Karl Heuerman, Dr. Chris Yung


Requirements for the measurements of Earth’s top-of-atmosphere energy budget can be derived on the basis of resolving an imbalance between incoming solar radiation and outgoing Earth radiation, currently estimated to be 0.6 W m-2. Over the past two decades, significant reduction in the uncertainty of solar irradiance has been achieved that is within this climate-quality constraint. The uncertainty in outgoing radiation from Earth, on the other hand, while also reduced over the last two decades, is not yet at a level sufficient to resolve the imbalance. It is anticipated that future measurements of outgoing emitted longwave and reflected shortwave radiance will have uncertainties that are a factor of 5 to 10 lower than current measurements, but still insufficient for resolving an imbalance on the order of 1 W m-2 or less. The focus of this paper is on state-of-the-art SI-traceable absolute radiometry for measuring top-of-atmosphere incoming and outgoing radiation. Advances in detector technology (microbolometers using vertically aligned carbon nanotubes), control (closed-loop electrical substitution) and implementation (from single detectors for solar total and spectral irradiance and rapid scanning of Earth limb-to-limb broad-band radiance to linear arrays for spatially resolved Earth radiance) that are already being implemented in current and for future missions. Features of this radiometry have several goals: lower uncertainty because of substantial reduction to inequivalence and lower noise (demonstrated by modeling and intra-comparisons); room temperature operation; faster and cheaper fabrication; and lower implementation costs because of reduced size, weight, and power. This will provide a path for data continuity and multiple redundant missions dispersed over time. Enhanced long-wavelength sensitivity and spectral uniformity to enhance measurement and imaging of cooler earth features such as clouds and oceans. We have recently undertaken a decadal revalidation of the LASP Total-solar-irradiance Radiometer Facility (TRF), and established a replacement room temperature standard known as Not Another Cryogenic Radiometer (NACR). The results of this extensive measurement campaign bridge the past TRF radiometer comparison against the NIST primary standard (POWR – Primary Optical Watt Radiometer) of 2008 to the present radiometric assessment of NACR and serves as a basis for validating upcoming missions including the Compact Total Irradiance Monitor mission (CTIM), the Black Array of Broadband Absolute Radiometers (BABAR), Libera, and others. NACR is a very portable primary standard, designed for irradiance measurements at solar power levels of 1361 W m-2. It is capable of being directly compared to other solar viewing instrumentation deployed in the field.

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Topic : Theme 1: Earth Energy Balance.
Reference : T1-C5

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