Abstract
Deep Space Climate Observatory (DSCOVR) measurements of Earth’s reflected solar and emitted thermal radiation permit a unique model/data comparison perspective that is not readily available from other satellite data. The key factor is the unique Lissajous orbital viewing geometry from the Lagrangian L1 point, which enables a continuous view of Earth’s sunlit hemisphere. The National Institute of Standards and Technology Advanced Radiometer (NISTAR) is the DSCOVR Mission energy budget instrument, which views the reflected and emitted radiation of the Earth’s sunlit hemisphere by means of single pixel active cavity full-spectrum (Band-A, 0.2–100 μm) and filtered solar wavelength (Band-B, 0.2–4.0 μm; and Band-C, 0.7–4.0 μm) radiometer measurements. An additional solar wavelength photodiode channel (0.3–1.1 μm) provides a calibration reference. The objective of this study is the assessment of climate GCM performance via direct model/data comparisons. Such comparisons are difficult due to quasi-chaotic natural variability present in real-world observational data and in climate GCM simulations. This is where the unique DSCOVR viewing geometry makes possible the longitudinal data slicing methodology for more direct model/data comparison. The key point of the longitudinal slicing approach is that data integration over the entire sunlit hemisphere eliminates the quasi-chaotic meteorological weather-scale noise, while preserving intra-seasonal and planetary-scale variability. The rotation of the Earth that retrieves this climate-style, large-scale longitudinal and seasonal variability. The hemispheric averaging is accomplished automatically in NISTAR measurements with its single-pixel view of the Earth. For climate GCMs, this requires implementing the Sunlit Hemisphere Sampling (SHS) scheme to operate on the GCM run-time output data, utilizing the DSCOVR Satellite Ephemeris data to assure precise viewing geometry between NISTAR measurements and GCM output data, while averaging out the meteorological weather noise. However, GCM generated data are radiative fluxes, while NISTAR (and EPIC) measurements are near-backscattered radiances. Conversing NISTSR measurements into radiative fluxes cannot be accomplished using NISTAR data alone, even with detailed support from conventional satellite data. But the identical viewing geometry of Earth’s sunlit hemisphere, and synergistic analyses of EPIC data make it feasible for this conversion of NISTAR near-backscatter radiances into radiative fluxes.
Highlights
Because the climate system is highly variable both in space and in time with differences in viewing geometry, spatial resolution, and diurnal cycle sampling time between observations and climate model output, this can produce biases and distortion. Both climate GCMs and the real world operate in quasi-chaotic fashion, which produces a “natural” uncorrelated variability that is an unavoidable source of uncertainty in model/ data comparisons
In our experience with NISTAR and with the sunlit hemisphere averaged EPIC data, where the information content resides in the longitudinal ordering of the data timeseries, we found that the optimum in display to be the Hovmöller format contour map
This paper describes a unique model/data comparison approach
Summary
The DSCOVR Satellite is not precisely located at the Lagrangian L1 point. The solar illumination of the earth is determined by the Solar Ephemeris whereby (in the GCM simulations) the grid-box local solar zenith angle changes are updated on an hourly basis, as is the seasonal change in solar irradiance due to the Earth’s orbital motion around the Sun. But, for viewing Earth from the Lissajous orbit, the DSCOVR Satellite viewing geometry must conform to the specification given by the DSCOVR Satellite Ephemeris
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