Progress Toward Achieving an IR Radiance Record for Decadal Climate Trending: Fundamental Questions, Challenges, and Assessments

Infrared radiance spectra can be measured with very high accuracy (0.1 K 3-sigma brightness temperature at scene brightness temperature) and contain a high degree of information about the state of the Earth's climate, including temperature, water vapor, and trace gas profiles, lapse rates, plus cloud and surface properties. How best to make use of existing EOS-AIRS and Suomi NPP/JPSS-CrIS/IASI observations and how to move toward more extensive future observations is a fundamental question that the climate community and world's space agencies need to address. A major step toward achieving climate quality observations was realized through the successes of the operational sounders IASI-A & B on MetOp and the CrIS on Suomi NPP. Both instrument types are expected to continue as part of the Joint Polar Satellite system through at least 2040 (with upgrades to IASI and hopefully to CrIS as well). IASI covers the 0930 time slot and CrIS the 1330 slot for operational weather forecasting applications. This represents a major step toward the unbiased sampling needed for accurate climate trending products. However, there is still a hole at the 0530 dawn/dusk time slot that needs to be filled. The new Chinese hyperspectral sounder, HIRAS, is expected to fulfill this need when FY3E is launched in 2018. In addition to improved sampling, the system of operational Fourier transform sounding instruments offer other measurement accuracy advantages over earlier instruments. This talk will review some of these, including review of CrIS accuracy estimates, discussion of several specific error sources, and the fundamental ability of these instruments to produce well-defined, stable, and largely instrument independent Instrument Line Shape (ILS) functions. In this context, we will address EOS to Suomi NPP spectral radiance continuity issues. Finally, optimizing this system still requires an on-orbit 0.1 K 3-sigma calibration reference that is not susceptible to long-term drifts and that can be used to correct inconsistencies among the different sensors. The new technologies needed to provide this capability have been developed under NASA ESTO and the CLARREO program, and should be flown as soon as possible. For the infrared it is widely accepted that a Fourier transform instrument provides the best possible reference sensor.