the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jun 2023
IMK/IAA MIPAS retrievals version 8: CH4 and N2O
Abstract. Using the IMK/IAA data processor, methane and nitrous oxide distributions were retrieved from version 8 limb emission spectra recorded with the Michelson Inferferometer for Passive Atmospheric Sounding (MIPAS). The dataset includes measurements from the Nominal, UTLS-1, Middle Atmosphere, Upper Atmosphere and Noctilucent Cloud observation modes. The processing differs from the previous version 5 data with respect to the atmospheric state variables jointly retrieved with the target gases CH4 and N2O, the treatment of the radiance offset, the selection of microwindows, the regularization, the spectroscopic data used and the treatment of horizontal variability of the atmospheric state. Besides the regular data product, a coarse-grid representation of the profiles with unity averaging kernels is available, as well as a specific research product for Middle Atmosphere measurements resulting from a slightly different retrieval approach. The CH4 errors are dominated by the large spectroscopic uncertainty for line intensities, which probably is too pessimistic, and estimated to be 21–34 % in the altitude range 6–68 km for northern midlatitude summer day conditions. The N2O errors are 7–17 % below 45 km. At higher altitudes they increase strongly due to nearly vanishing N2O amounts. Analysis of the horizontal averaging kernels reveals that for both gases the horizontal resolution is sampling-limited, i.e., information is not smeared over consecutive limb scans. Zonal mean seasonal composites of both CH4 and N2O exhibit the typical distribution of source gases with strong upwelling in the tropics and subsidence above the winter poles. Comparison with the previous data version shows several improvements: First, the vertical resolution of the retrieved CH4 (N2O) profiles has generally been significantly enhanced and varies between 2.5 (2.5) and 4 (5) km at altitudes between 10 and 60 km, with the best resolution around 30 km for both species. Secondly, the number of not converged retrievals has been clearly reduced, and thirdly, formerly strongly oscillating profiles are now considerably smoother.
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RC1: 'Comment on egusphere-2023-919', Chris Boone, 05 Jul 2023
Well written and well organized. Just a few relatively minor comments, the most significant being the question of why CF4 is not included in the analysis.
Are the molecules indicated in Tables 3 and 4 all the interferers considered in the analysis? On page 13, line 8, reference is made to H2O, O3, and 14 other species in relation to interferers, but only 14 species appear in the tables (H2O, O3, plus 12 other interferers). Conspicuously missing is CF4. The microwindow 1282.5-1283.55 is presumably included because it contains the strong Q-branch of the molecule, and the CF4 signal should extend quite high in altitude (i.e., above the 30 km lower altitude limit of the microwindow). Excluding it from the analysis would be problematic.
Also absent is HDO. I gather differences in isotopologues are generally ignored (both N2O and CH4 have lines from subsidiary isotopologues in the given wavenumber region, which will have slightly different VMR profiles than the main isotopologues), but there are HDO lines in some of the low altitude microwindows (i.e., microwindows that extend down to 6 km), and atmospheric fractionation for HDO relative to the main isotopologue is around a factor of 2 different from the H/D reference factor assumed when scaling HITRAN intensities to isotopic abundance. If using the same VMR profile as main isotopologue H2O, the HDO signal will be significantly overestimated. Can that really be ignored in the analysis?
There are some other molecules that could contribute weakly to the signal in this region (e.g., C2F6), but perhaps those contributions are adequately accounted for by the ‘continuum’ fitting parameters.
In this wavenumber region, the spectroscopy for HNO3 has been improved in recent years, but there remain missing hot bands in the HITRAN database, which could impact the analysis results.
Page 19: line 16: “We suspect that this bias might be due to the spectroscopic data used”
Likely a major (maybe the biggest) factor, but not necessarily the only one. It is perhaps worth noting strong correlations between the CH4 and N2O differences in Figure 8. Between latitudes 40 S to 40 N and altitudes 10 to 30 km, there is a common pattern: an increase relative to V5 between 10 and 15-20 km, a decrease relative to V5 near 20 km, and an increase relative to V5 between 25 and 30 km. I gather the two increases worsen the level of agreement with correlative data. The fact that N2O and CH4 share this pattern suggests a common source for the differences, something mechanical in the retrieval (changes in tangent heights, changes in pressure and temperature profiles, etc.). This in turn suggests there could be contributions to the biases other than just spectroscopy.
Minor issues:
Page 2, line 32: a formatting issue (truncated sentence that is continued on the next page).
Page 16, line 21: “even some more”
even more
Citation: https://doi.org/10.5194/egusphere-2023-919-RC1 -
AC1: 'Reply on RC1', Norbert Glatthor, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-919/egusphere-2023-919-AC1-supplement.pdf
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AC1: 'Reply on RC1', Norbert Glatthor, 08 Nov 2023
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RC2: 'Comment on egusphere-2023-919', Anonymous Referee #2, 04 Oct 2023
The paper is well written and well suited for AMT. I'd suggest publication after a few minor corrections, listed below.
Page 1, lines 4-5: “…with respect to the atmospheric state variables jointly retrieved with the target gases CH4 and N2O…” is awkwardly phrased. Does this mean that CH4 and N2O are now retrieved simultaneously and weren’t previously?
Page 2, line 18: “Submillimeter Wave Radiometer” should be “Sub-Millimetre Radiometer” (note the spelling of sub-millimetre)
Page 3, line 30: Please specify what the high bias is with respect to
Page 5, lines 14-16: I assume you mean H2O and HNO3 are interfering species that are no longer joint-fitted. If so, please clarify in the text. Also, please explain what is meant by “hardly any influence”
Section 3.1.2: Please briefly explain the physical justification for how the radiance offset could be altitude-dependent?
Page 8, line 11: If CO2 is an interfering species, why isn’t its VMR listed as an uncertainty in Table 3?
Page 17, line 4: for consistency, “methane” should probably be “CH4”
Page17, line 8: it seems a bit odd to be using vague phrases like “had been as high as” and “could be reduced to” and then give very specific values. Why not just give the rates for both V5 and V8 (as done in the next sentence), or say the version that had a non-convergence rate of 8.4%?
Figure 6: in my opinion, there’s more data here than necessary, which makes it difficult to see the region of interest. Maybe consider only showing the northern latitudes to highlight the oscillations better.
Citation: https://doi.org/10.5194/egusphere-2023-919-RC2 -
AC2: 'Reply on RC2', Norbert Glatthor, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-919/egusphere-2023-919-AC2-supplement.pdf
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AC2: 'Reply on RC2', Norbert Glatthor, 08 Nov 2023
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RC3: 'Comment on egusphere-2023-919', Anonymous Referee #3, 09 Oct 2023
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AC3: 'Reply on RC3', Norbert Glatthor, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-919/egusphere-2023-919-AC3-supplement.pdf
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AC3: 'Reply on RC3', Norbert Glatthor, 08 Nov 2023
Status: closed
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RC1: 'Comment on egusphere-2023-919', Chris Boone, 05 Jul 2023
Well written and well organized. Just a few relatively minor comments, the most significant being the question of why CF4 is not included in the analysis.
Are the molecules indicated in Tables 3 and 4 all the interferers considered in the analysis? On page 13, line 8, reference is made to H2O, O3, and 14 other species in relation to interferers, but only 14 species appear in the tables (H2O, O3, plus 12 other interferers). Conspicuously missing is CF4. The microwindow 1282.5-1283.55 is presumably included because it contains the strong Q-branch of the molecule, and the CF4 signal should extend quite high in altitude (i.e., above the 30 km lower altitude limit of the microwindow). Excluding it from the analysis would be problematic.
Also absent is HDO. I gather differences in isotopologues are generally ignored (both N2O and CH4 have lines from subsidiary isotopologues in the given wavenumber region, which will have slightly different VMR profiles than the main isotopologues), but there are HDO lines in some of the low altitude microwindows (i.e., microwindows that extend down to 6 km), and atmospheric fractionation for HDO relative to the main isotopologue is around a factor of 2 different from the H/D reference factor assumed when scaling HITRAN intensities to isotopic abundance. If using the same VMR profile as main isotopologue H2O, the HDO signal will be significantly overestimated. Can that really be ignored in the analysis?
There are some other molecules that could contribute weakly to the signal in this region (e.g., C2F6), but perhaps those contributions are adequately accounted for by the ‘continuum’ fitting parameters.
In this wavenumber region, the spectroscopy for HNO3 has been improved in recent years, but there remain missing hot bands in the HITRAN database, which could impact the analysis results.
Page 19: line 16: “We suspect that this bias might be due to the spectroscopic data used”
Likely a major (maybe the biggest) factor, but not necessarily the only one. It is perhaps worth noting strong correlations between the CH4 and N2O differences in Figure 8. Between latitudes 40 S to 40 N and altitudes 10 to 30 km, there is a common pattern: an increase relative to V5 between 10 and 15-20 km, a decrease relative to V5 near 20 km, and an increase relative to V5 between 25 and 30 km. I gather the two increases worsen the level of agreement with correlative data. The fact that N2O and CH4 share this pattern suggests a common source for the differences, something mechanical in the retrieval (changes in tangent heights, changes in pressure and temperature profiles, etc.). This in turn suggests there could be contributions to the biases other than just spectroscopy.
Minor issues:
Page 2, line 32: a formatting issue (truncated sentence that is continued on the next page).
Page 16, line 21: “even some more”
even more
Citation: https://doi.org/10.5194/egusphere-2023-919-RC1 -
AC1: 'Reply on RC1', Norbert Glatthor, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-919/egusphere-2023-919-AC1-supplement.pdf
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AC1: 'Reply on RC1', Norbert Glatthor, 08 Nov 2023
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RC2: 'Comment on egusphere-2023-919', Anonymous Referee #2, 04 Oct 2023
The paper is well written and well suited for AMT. I'd suggest publication after a few minor corrections, listed below.
Page 1, lines 4-5: “…with respect to the atmospheric state variables jointly retrieved with the target gases CH4 and N2O…” is awkwardly phrased. Does this mean that CH4 and N2O are now retrieved simultaneously and weren’t previously?
Page 2, line 18: “Submillimeter Wave Radiometer” should be “Sub-Millimetre Radiometer” (note the spelling of sub-millimetre)
Page 3, line 30: Please specify what the high bias is with respect to
Page 5, lines 14-16: I assume you mean H2O and HNO3 are interfering species that are no longer joint-fitted. If so, please clarify in the text. Also, please explain what is meant by “hardly any influence”
Section 3.1.2: Please briefly explain the physical justification for how the radiance offset could be altitude-dependent?
Page 8, line 11: If CO2 is an interfering species, why isn’t its VMR listed as an uncertainty in Table 3?
Page 17, line 4: for consistency, “methane” should probably be “CH4”
Page17, line 8: it seems a bit odd to be using vague phrases like “had been as high as” and “could be reduced to” and then give very specific values. Why not just give the rates for both V5 and V8 (as done in the next sentence), or say the version that had a non-convergence rate of 8.4%?
Figure 6: in my opinion, there’s more data here than necessary, which makes it difficult to see the region of interest. Maybe consider only showing the northern latitudes to highlight the oscillations better.
Citation: https://doi.org/10.5194/egusphere-2023-919-RC2 -
AC2: 'Reply on RC2', Norbert Glatthor, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-919/egusphere-2023-919-AC2-supplement.pdf
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AC2: 'Reply on RC2', Norbert Glatthor, 08 Nov 2023
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RC3: 'Comment on egusphere-2023-919', Anonymous Referee #3, 09 Oct 2023
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AC3: 'Reply on RC3', Norbert Glatthor, 08 Nov 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-919/egusphere-2023-919-AC3-supplement.pdf
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AC3: 'Reply on RC3', Norbert Glatthor, 08 Nov 2023
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