747 Achieving Climate Quality Inter-Calibration with Accurate Spectral Solar Irradiance (SSI) Measurements from NASA's SORCE and TSIS Missions

Tuesday, 24 January 2017
4E (Washington State Convention Center )
Dong L. Wu, NASA/GSFC, Greenbelt, MD; and A. Marshak, J. N. Lee, Y. Yang, M. T. DeLand, S. marchenko, N. A. Krotkov, P. Pilewskie, T. N. Woods, J. Harder, and E. Richard

Spectral solar irradiance (SSI) plays a critical role in calibrating the radiances of UV-VIS-SWIR bands on weather satellites. Although reflectance factor measurements (upwelling radiance normalized by SSI) are less affected by the SSI accuracy, radiance measurements (e.g., Geostationary observations) and inter-sensor calibration are sensitive to errors in the irradiance spectrum used. Modern reflective solar sensors are able to detect 1-2% errors in radiometric calibration and demand a better (<1%) accuracy of SSI measurements. In this study we analyzed 9 widely used SSI spectra for radiometric calibration evaluation.  We found that differences in the so-called “calibrated radiances” could reach as large as +/-2% in VIS-SWIR bands and +/-7% in UV bands, depending on what SSI spectrum is used. Convolution of a higher resolution SSI reference spectrum to a broader remote sensing measurement band can also introduces some uncertainty. Such a large uncertainty has been a major challenge not only for achieving climate-quality inter-calibration but also for making accurate SSI measurements directly. NASA’s SORCE (Solar Radiation and Climate Experiment, 2003-present) and future TSIS (Total and Spectral Solar Irradiance Sensor) missions are aimed to provide climate-quality SSI measurements. While the SORCE SSI accuracy is ~2% at present, TSIS-1 (to be launched to International Space Station in late 2017 or early 2018) is tasked to improve the SSI accuracy to 1% or better.
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