The Climate Absolute Radiance and Refractivity Observatory (CLARREO) is one of four Tier 1 missions recommended by the recent NRC Decadal Survey report on Earth Science and Applications from Space (NRC, 2007). The CLARREO mission addresses the need to rigorously observe climate change on decade time scales and to use those observations as the most critical method for determining the accuracy of climate change projections such as those used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). The foundation of CLARREO is the ability to produce highly accurate climate records to test climate projections in order to improve models and enable sound policy decisions. The CLARREO mission accomplishes this critical objective through rigorous SI traceable decadal change observations that are sensitive to many of the key uncertainties in climate radiative forcings, responses, and feedbacks that in turn drive uncertainty in current climate model projections. These same uncertainties also lead to uncertainty in attribution of climate change to anthropogenic forcing.

The CLARREO breakthrough in decadal climate change observations is to achieve the required levels of accuracy and traceability to SI standards for a set of observations sensitive to a wide range of key decadal change variables. The required accuracy levels are determined so that climate trend signals can be detected against a background of naturally occurring variability. Climate system natural variability therefore determines what level of accuracy is overkill, and what level is critical to obtain.  In this sense the CLARREO mission requirements are considered optimal from a science value perspective.  The accuracy for decadal change traceability to SI standards includes uncertainties associated with instrument calibration, satellite orbit sampling, and analysis methods. Unlike most space missions, the CLARREO requirements are judged not by the instantaneous accuracy of the measurements, but by accuracy in the large time/space scale averages that are key to understanding decadal changes.

The NRC Decadal Survey concluded that the single most critical issue for decadal change observations was their lack of accuracy and low confidence in observing the small but critical climate change signals. CLARREO is the recommended response to this challenge, and builds on the last decade of climate observation advances in the Earth Observing System, advances in on-orbit testing and verification standards, as well as metrological advances at National Institute of Standards and Technology (NIST) and other standards laboratories.

The presentation will summarize the planned CLARREO observations, science objectives and requirements.  The approach includes a dual strategy to provide climate change benchmarks directly from time/space averaged CLARREO observations including the use of optimal spectral fingerprinting, as well as to serve as a set of reference spectrometers in orbit capable of improving the calibration of other weather and climate sensors. 

The CLARREO observations include a nadir viewing infrared interferometer covering the spectral region of 200 to 2000 cm^-1, with 1 cm^-1 spectral resolution, and radiance accuracy with an expanded uncertainty on orbit of 0.07K (95% confidence bound, or k=2).  Note that "expanded uncertainty" is the NIST recommended term used to quantitatively define measurement uncertainty (see NIST technical note 1297 for details).  A unique feature of the infrared interferometer is the inclusion of on-orbit absolute standards to directly verify performance, rather than depend on assumed stability.  The mission also includes a reflected solar spectrometer, pointed by a two-axis gimbal, covering the spectral region from 320 to 2300 nm, with 4 nm spectral sampling, and nadir reflectance accuracy with a relative expanded uncertainty of 0.3% (95% confidence bound).  The solar spectrometer will be capable of pointing to the moon and sun for calibration, as well as tracking time/angle/space-matched observations when used for Reference Intercalibration of other radiometers such as CERES or VIIRS.   Finally, the observations include radio occultation receivers and antennas to allow use of both the GPS and Galileo Global Navigation Satellite Systems.   

The CLARREO mission is planned for two 90 degree polar orbits to enable global sampling of the nadir infrared and reflected solar spectra, sampling of the complete diurnal cycle every 3 months, and Reference Intercalibration sampling in all climate regimes and orbit conditions from equator to pole.  The orbit sampling has been shown to meet the CLARREO mission accuracy requirements for global and zonal space scales, as well as seasonal and annual time scales.