the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 May 2026
Towards ground-to-space radiative closure in the mid- to far-infrared
Abstract. Despite containing up to half of the Earth's thermal emission to space, the far-infrared spectral region (FIR, defined here as 100–667 cm-1 or 15–100 µm) has seldom been observed from satellites. This has contributed to substantial uncertainties in the spectroscopy of water vapour, radiative properties of clouds, and surface spectral emissivity; these in turn limit confidence in modelled FIR energy flows. With the advent of the Polar Radiant Energy in the Far-InfraRed Experiment (PREFIRE), a step is taken towards new, systematic, spectral observations of the Earth in the FIR. Launched in 2024, the two PREFIRE CubeSats offer a new perspective of Earth's outgoing longwave energy with moderate spectral resolution. However, the observational uncertainty budget requires further consolidation.
In this study we assess the accuracy of PREFIRE spectral measurements through a ‘ground-to-space’ closure experiment. Using zenith-viewing observations from the ground-based Far INfrarEd Spectrometer for Surface Emissivity (FINESSE), we gauge the representativity of atmospheric data from in situ sensors and reanalysis. With these data we simulate PREFIRE-observed radiances for an overflight of our field site in eastern Canada. Simulations of the FINESSE radiances are in very good agreement with observations, while those from PREFIRE indicate some bias beyond the calculated uncertainties. We find that atmospheric water vapour specification, uncertain surface properties, and instrument noise dominate the uncertainties. Based on these findings, we highlight proposed techniques for closure experiments using terrestrial and satellite instruments alike. Such experiments will provide a ground-truth for validation of future FIR satellite missions.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2443', Anonymous Referee #1, 13 Jul 2026
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RC2: 'Comment on egusphere-2026-2443', Anonymous Referee #2, 20 Jul 2026
This paper provides, what the authors believe, is the first simultaneous downwelling and upwelling radiometric spectral closure experiment in the far-infrared. The ground-based radiometric observations were from the zenith-viewing viewing Far INfrarEd Spectrometer for Surface Emissivity (FINESSE), whereas the space-borne observations where from the PRE-FIRE satellite (the MIR-2, FIR-1, and FIR-2 spectral regions). Data were evaluated over the Gault field site at McGill University.
There are many challenging aspects to this analysis. In no particular order, the ground-based instruments were on the side of hill about 130 m MSL whereas the surrounding terrain was closer to 30 m MSL, only a single case was found to be clear sky during the 1-month field campaign, the radiosonde launched during this period was found to be about 9 to 15% dry biased (relative to either the statistical analysis with ERA5 or the instantaneous comparison with the same), the skin temperature was uncertain, and more. That being said, the authors did a very nice job trying to account for all of these issues in their analysis.
Thus, I only really have two concerns. The first is that the entire analysis is only a single case, and thus there is a question about how representative these results are overall. I would like to have the authors add more discussion in the final section of the paper about this.
Second: there were other ground-based spectrally resolved instruments at the site also. Why were these not included in the analysis? For example, the AERI covers a good fraction of the same spectral bands as the FINESSE, and showing that either the AERI and FINESSE are in good agreement, or similarly that the AERI agrees well with the ERA5-DW profile, would provide significantly more support that the FINESSE results are solid.
Third, and this is a recommendation not a requirement: The authors show that there is a near-surface temperature error in the ERA5-DW and InSitu-DW results, as well as some water vapor challenges in the latter. The Mlawer et al. 2019 and Mlawer et al. 2024 (JGR) papers use thermodynamic profiles retrieved from the AERI assuming spectroscopy from RHUBC-I is well-validated. Would your results and interpretation change if you used a retrieved thermodynamic profile in your closure analysis?
The authors have done a very good job explaining what they have done, and evaluating/representing the various sources of uncertainty well. The writing is clear, and the figures are clean and very useful. If the authors can address my points above, I believe that this is worth of publication.
Citation: https://doi.org/10.5194/egusphere-2026-2443-RC2
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- 1
“Towards ground-to-space radiative closure in the mid- to far-infrared” by Pery et al. conducts a validation study of a mid and far infrared spectrum measured by the new PREFIRE cube-sat mission using line-by-line radiative transfer simulations to coordinate observations from a PREIFRE overpass with ground based FTIR and radiosonde data collected coincidentally in January 2025 near McGill University. As the authors acknowledge, the study is quite limited, being that only a single overpass is compared, but I agree with the authors that the analysis is nevertheless useful. The results provide an initial benchmark for PREFIRE uncertainty and a rare glimpse into far-IR measurement uncertainty. The paper is suitable for AMT and I think could be published once the authors address my comments below. These comments are mostly minor in nature, though the first will likely require some re-arranging and re-printing of figures.
Main Comment:
The Methods section describes a great deal of effort to constrain the input data for LBLRTM, but then in section 4.1 and again in section 5, the study second-guesses this work and proposes an alternate preparation of data. In section 4.1 we learn that the surface skin temperature could be better constrained and in section 5 evidence is provided suggesting that the radiosonde humidities were biased dry, which is not a surprising error. There is little independent evidence of the skin temperature bias, but the biases in the CO2 band (~650-750 cm-1) in Figs. 6b and 6c suggest a positive bias also in the near-surface air temperature supplied to LBLRTM of up to +1 K, since FINESSE is most likely better calibrated than this. I think that the authors should constrain the input data as best as they can up-front in Methods (including better near-surface air temperature, their best estimate of skin temperature, and the bias-corrected humidities) and provide a single best-estimate simulation for comparison in Results. Then, in Discussion provide the transparency by reminding the reader of some of this data preparation work and then commenting on how it affected the results.
Minor Comments:
Title: The title as it stands is a little overstated for comparison of a single measurement. Maybe “Validation of PREFIRE mid- and far-infrared radiances during the WHAFFERS campaign” or similar.
L73-74: Could you provide some specifics on the meteorology? The FIR would be semi-transparent for precipitable water vapor < ~ 1 cm. What was the PWV?
L90: There are a lot of sensors given only as their acronyms in the paper; AERI, CrIS, IASI, as well as models (PCRTM), some of WHAFFFERS. I know it’s a lot of words, but maybe a table would help: full name, spectral resolution, absolute accuracy, reference.
L117: What are the criteria for valid spectra? You mean clear skies?
L118: Why is nighttime a problem? Sounds optimal to me.
L123-124: “good stability”. Can you quantify this, perhaps for several hours around the overpass? It seems like a useful data point for assessing uncertainty.
L149-150: Can you elaborate on this bias?
L151-154: FIR surface emissivity seems like a really large source of uncertainty. It would neither be well-known nor easily matched to the surface cover in the actual FOV, which is at least partly mixed and sloped (FIR surface emissivity of a partially snow-covered forested hill, yikes!). I would like a better sense of what sort of uncertainty poorly constrained surface emissivity causes.
L155: I don’t see ozone listed here, but it was clearly included based on Figure 6.
L205: Did ERA5 assimilate this radiosonde?
Figure 5: For uncertainties that are not NESR, what are we looking at, mean bias, RMSE?
Figure 6: One thing that really stands out in this comparison is the difference in the CO2 rotational band from ~650-750 inverse cm. Looks like the surface temperature input to LBLRTM is biased high by an amount I’m guessing is close to 1 K and it seems like the error in that value should be smaller. … Ah, I see the text explanation now (L305): I don’t think you want to hand-wave this away. The near-surface can likely be constrained better and it might be important to the UW data for PREFIRE.
L350-351: I don’t understand “instrument effects”. Maybe be more specific here. And what evidence is there to “believe” this?