CLARREO Pathfinder Mission: Calibrating Climate Observing Systems of the Future

Calibration of satellite instruments required to observe climate change remains a major challenge (NRC, 2007; NRC, 2015, NRC, 2018). Figure 1 provides an example of the impact of calibration on the length of record required to detect trends (Wielicki et al. 2013). The figure uses the example of trends in reflected shortwave cloud radiative forcing (CRF) or cloud radiative effect (CRE). Trends in shortwave CRF are directly related to the magnitude of low cloud feedback (Soden et al. 2008). The range of these trends found in the IPCC climate model feedbacks is shown at the lower left of the figure. Low cloud feedback is the largest uncertainty in climate sensitivity. Figure 1 shows that current instruments measuring global average cloud radiative forcing (CERES) or their associated cloud properties (MODIS) have calibration absolute accuracy limitations that can lead to 20 year or longer delays in narrowing uncertainty in cloud feedback and therefore in climate sensitivity. Recent economic analysis has shown that advancing in time knowledge of cloud feedbacks and climate sensitivity would be worth $10 to $20 Trillion U.S. dollars to the world economy (Cooke et al. 2014, 2015, 2016; Hope 2015).

The 2007 and 2018 U.S. Academy of Sciences Decadal Surveys for Earth Science recommended a major improvement in calibration for both reflected solar and infrared space based instruments (NRC, 2007). The 2007 report recommended a Tier 1 mission called CLARREO (Climate Absolute Radiance and Refractivity Observatory) to serve as reference calibration spectrometers in orbit. These instruments would anchor activities such as the Global Space Based InterCalibration System (GSICS) which has requested CLARREO or equivalent reference spectrometers (Goldberg et al. 2011). The Global Climate Observing System (GCOS, 2017) has made similar recommendations. The recent 2018 Decadal Survey listed similar reference radiance intercalibration as one of its Most Important Targeted Observables. The spectrometers are designed to achieve SI standard traceable accuracy in orbit of 0.3% at 95% confidence for the reflected solar spectrum, and 0.07K at 95% confidence for the infrared spectrum (NRC, 2007; Wielicki et al. 2013). While the full CLARREO mission has been delayed due to limits in NASA funding, instrument and science development has continued.

The presentation will summarize progress and current status of a CLARREO reference calibration mission. Ground demonstration instruments have been built for both the infrared and reflected solar spectrometers, with both instruments having successfully reached TRL-6. The reflected solar instrument has demonstrated greatly improved accuracy in orbit using 30km high altitude balloon flights (Kopp et al. 2017). The infrared instrument has been demonstrated in two laboratory test implementations. Methodologies for high accuracy inter-calibration have also been demonstrated and published, considering requirements for instrument offset, gain, non-linearity, and polarization dependence.

A CLARREO Pathfinder mission for the reflected solar spectrometer entered Phase A development in January 2016. The low cost Pathfinder mission (< $100 million U.S. dollars) is designed to demonstrate the new higher accuracy SI traceability on orbit as well as the intercalibration of multiple sensors. The spectrometer is based on the University of Colorado LASP HySICS balloon demonstration instrument, and is planned for launch as an attached payload on the International Space Station (ISS) in early 2022. Accuracy goal for the Pathfinder is 0.3% (k=1) or a factor of 3 to 10 better than current reflected solar instruments. The Pathfinder instrument on the ISS would be capable of obtaining time/space/angle/spectral matched data across the full scan swath of low earth orbit and geostationary orbit reflected solar instruments such as CERES, MODIS, VIIRS, Landsat, Sentinel 2a/b, GERB, SEVERI, ABI, as well as land imager constellations and ocean color sensors. The spectrometer has contiguous spectral response with 3 nm sampling from 350 nm to 2300 nm. Instantaneous field of view is 500 m, with a swath width of 70 km. A 2-axis gimbal allows matched angle of view for the full swath of scanning instruments.

While the world still lacks an observing system specifically designed for climate observations, the CLARREO and CLARREO Pathfinder missions represent a foundation for the much higher accuracy observations critical to narrowing uncertainty in future climate change.

References

Cooke, Roger M. et al., 2015. Integrated Assessment Modeling of Value of Information in Earth Observing Systems, Climate Policy

Cooke, Roger M. et al., 2016. Real Option Value for New Measurements of Cloud Radiative Forcing, RFF DP 16-19.

GCOS 2016 Implementation Plan, 2016. WMO GCOS-200, 315pp.

Hope, C. 2015. The $10 trillion value of better information about the transient climate response. Philosophical Transactions of the Royal Society A 373

Kopp, G., et al. 2017. Radiometric flight results from the HyperSpectral Imager for Climate Science. Geosci. Instrum. Method. Data Syst., 6, 169–191.

NRC, 2007. Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond. National Academy Press, 428 pp.

NRC, 2015. Continuity of NASA Earth Observations from Space: A Value Framework. National Academies Press.

NRC, 2018. Thriving on our Changing Planet: A Decadal Strategy for Earth Observation from Space. National Academy Press, 700 pp.

Soden, B.J., et al. 2008. Quantifying climate feedbacks using radiative kernels. Journal of Climate 21: 3504–3520.

Wielicki, B. A. et al. (2013): Achieving Climate Change Absolute Accuracy in Orbit. Bulletin of the American Meteorological Society, 94, 1519–1539.