the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Stratospheric trace gas profile retrievals from balloon-borne limb imaging of mid-infrared emission spectra
Jeff Langille
Daniel Zawada
Adam Bourassa
Doug Degenstein
Abstract. The Limb Imaging Fourier-Transform Spectrometer Experiment (LIFE) instrument is a balloon-borne prototype of a satellite instrument designed to take vertical images of atmospheric limb emission spectra in the 700 to 1400 cm−1 wavenumber range from the upper troposphere/lower stratosphere (UTLS) altitude region of the atmosphere. The prototype builds on the success of past and existing instruments while reducing the complexity of the imaging design. This paper details the results of a demonstration flight on a stabilized stratospheric balloon gondola from Timmins, Canada in August 2019. Retrievals of vertical trace gas profiles for the important greenhouse gases H2O, O3, CH4 and N2O, as well as HNO3, are performed using an Optimal Estimation Approach and the SASKTRAN radiative transfer model. The retrieved profiles are compared to approximately coincident observations made by the ACE-FTS solar occultation satellite instrument. An evaluation of the LIFE measurements is performed and areas of improvement are identified. This work increases the overall technical readiness of the approach for future balloon, aircraft and space applications.
Ethan Runge et al.
Status: final response (author comments only)
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RC1: 'Comment on amt-2023-9', Anonymous Referee #1, 01 Mar 2023
The manuscript has already been in a good shape when I provided my access review. The authors have carefully considered my few comments made then, thus I do not have much to add and suggest publication of the paper as is. With respect to the high bias of methane, the authors may speculate that inconsistence in the spectroscopic data in the spectral region used by LIFE and in the spectral region used by ACE-FTS might be part of the explanation.
Citation: https://doi.org/10.5194/amt-2023-9-RC1 -
RC2: 'Comment on amt-2023-9', Anonymous Referee #2, 20 Mar 2023
Review of manuscript amt-2023-9: “Stratospheric trace gas profile retrievals from balloon-borne limb imaging of mid-infrared emission spectra” by Ethan Runge et al.
General comments:
The authors present results of a demonstration flight of the imaging Fourier transform spectrometer LIFE on a stratospheric balloon. After a comprehensive description of the data processing, the authors present vertical profiles of the trace gases H2O, O3, HNO3, CH4, and N2O. These profiles are compared to observations of the ACE-FTS satellite instrument. The manuscript is clearly written and well-structured and will be of interest to scientists involved in projects with imaging spectrometers. I therefore recommend publishing this manuscript in AMT after addressing the comments below.
Major comment:
My main concern affects the comparison of the trace gas profiles measured with LIFE and obtained with ACE (since the authors consider this comparison as a kind of validation of the LIFE measurements). The differences in measured vertical trace gas profiles (mainly N2O and CH4, but also H2O and O3) between both instruments are quite large in some altitude regions, exceeding the combined errors of both instruments. Some differences may be explained because the temperature was not fitted prior to the LIFE trace gas retrievals. Another reason may be the poor coincidence between LIFE and ACE observations since the ACE measurements were recorded several days later than the LIFE measurements. I therefore strongly recommend that the authors also compare Microwave Limb Sounder (MLS) profiles that should be available for the gases H2O, O3, HNO3, and N2O close to the time and the region of the LIFE measurements and plot them as an additional mean profile (including standard deviation) in Figure 11.
Specific comments:
Line 105 and Figure 2: You should add one or two sentences how you treated the temperature during the retrieval process. Obviously, the temperature profile was not retrieved and you used a meteorological temperature analysis instead. Since the assumed temperature has some influence on the trace gas retrieval results, you should explain this matter clearly.
Page 9, Figure 5, Box “Stage 2”: What means “Low only” here? Does this mean that the continuum is only taken into account at low altitudes? Please clarify.
Line 213 and Figure 6b: It is not clear to me what is meant with “high resolution” here. It is also not clear to me what exactly is plotted as high-res RTM in Figure 6b. Is it important to show this parameter? If yes, please explain this a bit more in detail. If no, please omit this parameter in the plot.
Lines 260-262 and Figures 8 and 9: You mention some “primary sources” of retrieval errors. However, from my experience inaccuracies in the temperature profile used for the trace gas retrievals is also a major error source, especially if the temperature profile is not retrieved prior to the trace gas retrievals. You should include at least one or two sentences here why you obviously neglected the temperature error. The better way would be to estimate the temperature error by test retrievals and to include this error in Figures 8 and 9.
Line 309: “While the shape of these profiles raises interesting questions for further investigations, the magnitudes recovered are plausible …”. That sounds very vague. The N2O magnitudes are only plausible between about 15 and 22 km. Above this altitude region they would only be plausible within the late winter polar vortex but not at mid-latitudes in summer. Below about 15 km, the N2O profile is completely unreasonable. Interesting questions are, for instance: Is an inaccurate temperature profile at least partly responsible for the extremely low N2O values? Are the selected microwindows strongly influenced by overlapping features of different gases due to the coarse spectral resolution (at least with regard to N2O because the retrieval seems to work better for CH4)? Please be more specific and write a few sentences about possible reasons for this strange N2O profile.
Lines 349 and 350: For ozone, the mentioned low bias is only visible above 20 km. Below this altitude, the bias is positive.
Line 352: “…there are only a few regions for which the retrievals and ACE…”. It should read like “…there are some altitude regions for which the LIFE retrievals and ACE…”.
Lines 353 and 354: “…from the turbulent nature of the atmosphere…”,. You probably mean “…from the atmospheric variability…”.
Lines 355 to 360: There is a lot of speculation here about the differences between LIFE and ACE. As mentioned at the beginning, for reasons of a better coincidence between LIFE and a comparison instrument, MLS trace gas profiles should also be shown in Figure 11, as these are available for all gases displayed (except CH4) for the day of the flight. The authors could chose mean MLS profiles around Timmins (e.g. +/-5 deg. latitude +/-20 deg. longitude) including standard deviation. The present text should then be adapted accordingly (here and in the conclusions around line 366).
Lines 364, 367, and 378: The word “valid” sounds too strong; this reminds me of a valid law, which is not the case here. You may write instead “reliable” or “confidable”.
Line 382: “…other atmospheric constituents appear promising as well”. This is a very vague statement. Which species are meant here, for example? The expected accuracy is likely to be less than that of the gases previously discussed. Please be a little more specific in your statements here.
Line 383: “…validity of a …”. This statement is again too strong. Please change to “…feasibility of a …”.
Technical corrections:
Line 13: Please define acronym ABB.
Line 20: Please define acronym CNES.
Line 26: “… designed for an aircraft …”. Please rewrite “… designed for aircraft and balloon platforms …”.
Line 29: Please include the paper by Johansson et al. (Atmos. Meas. Tech., 11, 4737–4756, doi:10.5194/amt-11-4737-2018, 2018) into this list since this paper contains the validation aspects mentioned in line 28.
Line 51: Please write “follows” (instead of “follow”).
Page 4, Fig. caption 1, line 4: Please change …” is output as the result” into “… output is the result”.
Line 123: Please define acronym FASCODE.
Line 124: Please define acronym MERRA.
Line 209: Please define acronym NOAA.
Line 211: “…is applied is shown…”. Please rewrite “…is applied and is shown…”.
Page 15, Fig. caption 8 and Page 16, Fig. caption 9: Instead of writing “…on the flight date” please give the exact date of the flight here. Please indicate the meaning of the shaded region in the left hand panel.
Page 18, Fig. caption 11: “…from around the same time and location”. Please be more specific and give the time difference in days and the maximum spatial distance.
Citation: https://doi.org/10.5194/amt-2023-9-RC2
Ethan Runge et al.
Ethan Runge et al.
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