The Decadal Survey of the U.S. National Oceanic and Atmospheric Administration (NOAA) and the U.S. National Aeronautics and Space Administration (NASA) by the U.S. National Research Council (NRC) recommended that NASA deploy a Climate Absolute Radiance and Refractivity Observatory (CLARREO) as one of its four highest priorities. This recommendation came in response to a request from NASA and NOAA to suggest what satellite missions should be flown to form a national climate research program that is responsive to societal demands (National Research Council, Committee on Earth Science and Applications from Space 2007). Society demands data sets deemed trustworthy for trend detection and sufficiently

Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

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accurate to test climate models according to their predictive capability, and for this reason the U.S. NRC recommended the CLARREO mission to NASA as a top priority. In short, society needs tools that usefully predict future climates depending on its own decisions regarding future economic development and energy production. Those tools are climate models and, pursuant to the scientific paradigm, must be tested empirically.

CLARREO calls for three instruments: a GNSS (global navigation satellite system) occultation instrument, an instrument to measure emitted infrared radiation with high spectral resolution, and an instrument to measure reflected shortwave radiation with high spectral resolution. In order to ensure their data are absolutely accurate, it is required that they are assured traceable to the international standards that define the units of their observables on-board with overall uncertainties sufficient to test climate models (Ohring 2007). It is also required that their sampling patterns be sufficiently dense and uniform so that mission accuracy requirements are met. The optimal configuration for monitoring the emitted infrared spectrum with three satellites has them in perfectly polar orbits (90° inclination) spaced 60° in longitude of ascending node (Kirk-Davidoff et al. 2005). Such a configuration affords robustness in that the bias in annual averages induced by the diurnal cycle would be minimal should one or two satellites fail. Should all three satellites be operational, bias in seasonal averages induced by the diurnal cycle would be minimized.

The data types called for by CLARREO were selected because their traceability to international standards is possible. The NRC Decadal Survey's recommendation should answer societal demands, which in this case pertain to testing climate models. In what ways the data types of CLARREO can be used to test climate models according to their predictive capability remains an open question. In testing climate models, the scientific assessments of the Intergovernmental Panel on Climate Change focus much of their efforts in comparing the overall sensitivities of climate models, which is the surface air temperature increase predicted by a climate model when subjected to a prescribed forcing by increased carbon dioxide. Certainly, the sensitivity of the climate system must be modeled correctly, but a trustworthy model must attain the correct sensitivity for the right physical reasons. There are many ways to explain the sensitivity of a climate model (and the actual climate system), and, for the sake of simplicity, we analyze the sensitivity in the language of radiative feedbacks.

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