the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Jan 2024
Scale separation for gravity wave analysis from 3D temperature observations in the MLT region
Abstract. MATS (Mesospheric Airglow/Aerosol, Tomography & Spectroscopy) is a Swedish satellite designed to investigate atmospheric dynamics in the Mesosphere and Lower Thermosphere (MLT). From observing structures in noctilucent clouds over polar regions, and oxygen A-band emissions globally, MATS will supply the research community with properties of the MLT atmospheric wave field. Individual A-band images taken by the MATS main instrument, a 6-channel limb imager, are through tomography and spectroscopy turned into three-dimensional temperature fields in which the wave structures are embedded. To identify wave properties, in particular the gravity wave momentum flux, from the temperature field, smaller-scale perturbations, associated with the targeted waves, must be separated from large-scale background variations by a method of scale separation. This paper investigates the possibilities of employing a simple method based on smoothing polynomials to separate the smaller and larger scales. By using synthetic tomography data based on the HIAMCM (High Altitude Mechanistic Circulation Model) we demonstrate that smoothing polynomials can be applied to MLT temperatures to obtain fields corresponding to a global scale separation at zonal wavenumber 18. The simplicity of the method makes it a promising candidate for studying wave dynamics in the MATS temperature fields.
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RC1: 'Comment on egusphere-2024-136', Anonymous Referee #1, 12 Feb 2024
The manuscript studies gravity wave momentum flux obtained from a snapshot of a gravity-wave-resolving global model subjected to different methods and filtering. The analysis is tailored to expected data from the MATS satellite. The objective is to evaluate methods that will be applied to MATS data with respect to a "true" global field.
The study is well designed and clearly presented (Fig. 3 is very helpful). It will be very useful for analysis of MATS data. The result is that gravity wave momentum flux derived from expected MATS observational data can be within a factor of 2 of the true values. In the real atmosphere and the real MATS data, gravity waves with less than 200~km horizontal wavelength may be dominant, and the scale separation should be even better.
The analysis focused on cuts on zonal wavenumber, Savitzky-Golay filtering and S3D transforms. Other methods that are also applied to observational data in order to analyze gravity waves like Butterworth filters, wavelet transforms or modal decompositions were not mentioned.
It may be interesting, if feasible, to discuss in more detail one specific wave event when the performance was especially bad, in order to get a feeling on what type of waves would not be well captured. For example the differences in the meridional component at 50 deg N are large, maybe due to a wave with short vertical wavelength?
Is the used HIAMCM snapshot considered representative? In a sense that different snapshots are comparable in covered gravity wave events and variability, such that it does not seem necessary to evaluate different snapshots?
- 307 missing closing bracket after (Fig.8a
Citation: https://doi.org/10.5194/egusphere-2024-136-RC1 -
AC1: 'Reply on RC1', Björn Linder, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-136/egusphere-2024-136-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2024-136', Anonymous Referee #2, 13 Feb 2024
Review of "Scale separation for gravity wave analysis from 3D temperature observations in the MLT region" by Björn Linder et al.
General comments
In this new study, Linder et al. propose and evaluate an approach for estimating gravity wave pseudomomentum flux from tomographic temperature measurements in the mesosphere and lower thermosphere, which are expected to be provided by the MATS satellite launched in November 2022. The study proposes the use of Savitzky-Golay filters for local separation of the background temperature and temperature perturbations of the MATS temperature measurements. The well-established S3D method is proposed to perform the spectral analysis to estimate the wave properties (wave amplitude and wavelength) from the temperature perturbations, which are input to the estimation of the gravity wave momentum flux.
The proposed methodology is evaluated using simulated temperature fields from the HIAMCM general circulation model, which provides realistic fields of temperature, winds, and other parameters from the surface to the thermosphere. The study compares estimates of gravity wave momentum fluxes estimated directly from the model wind fields with pseudomentum fluxes from model temperatures. It further compares ideal estimates of momentum flux with estimates derived with the SG filters and the S3D method applied to temperature fields sampled as MATS measurements, taking into account the satellite orbit, the retrieval resolution via expected averaging kernels, and the expected measurement noise.
The study has been carefully carried out and the results appear to be sound. The proposed methodology for estimating gravity wave pseudomentum flux from MATS measurements is convincing. The paper is exceptionally well written, clear and concise, in my opinion. I would recommend that the paper be considered for publication, subject to the correction of a few rather minor comments listed below.
Specific comments
Line 131: This study proposes to use the S3D spectral analysis method, but for completeness, the S-transform spectral method could be referenced as another commonly used method for analyzing the types of measurements to be provided by MATS.
Line 146: It is pointed out that the swath width of about 200 km of MATS is a bottleneck for properly estimating some of the gravity wave spectral characteristics. Why was the instrument built with such a small swath, assuming that the estimation of the gravity wave momentum flux in the MLT region is indeed a key objective of the MATS measurements?
Lines 148-150: Perhaps add the dimensions for clarity here, 600 km x 190 km x 10 km (along track x across track x vertical). It would be good to provide some rationale or reference as to why the vertical width of the S3D boxes was set to 10 km. It is pointed out that the length of the cuboids is 600 km, but the sampling would be done every 100 km. What would be the reason for this oversampling, since the results of neighboring boxes would be largely correlated?
Line 176: It is pointed out that the zonal wavenumber 18 corresponds to wavelengths of about 2200 km at the equator. If the reference method is to use zonal wavenumber 18 for scale separation, it looks like there would be some discrepancy with the measurements of the MATS instrument, which has a swath width of ~200 km, which in combination with the S3D could allow estimation of wavelengths up to ~600 km. Isn't there some kind of gap between 600 and 2200 km wavelength?
Lines 191-193: You might add that structured noise from something like stray light would also be very difficult and challenging to properly simulate and account for.
Lines 240-241: I may have missed it earlier in the paper, but for completeness you might want to mention whether the HIAMCM data is a boreal summer and austral winter case?
Lines 269-271: The proposed cuboids have an asymmetry in size along and across the track, which seems to lead to an anisotropy in spectral sensitivity of the gravity wave measurements along and across the track. Such an anisotropy can cause some difficulty in interpreting measurement results. This can be seen in the example presented here for the polar regions. Perhaps you could discuss and clarify why using such unbalanced cube sizes is still advantageous compared to e.g. using a cube size of 200 km x 200 km x 10 km?
Lines 286-287: When discussing vertical smoothing, it would be good to provide some information on the vertical resolution of the HIAMCM test data. The MATS measurements have a vertical resolution of 500 m, and averaging kernels with FWHMs of 1 km and 2 km are considered for the retrieval data. Is the HIAMCM vertical resolution comparable or better?
Technical corrections
line 201: Section -> Sect.
line 220: efficiently -> effectively (?)
Citation: https://doi.org/10.5194/egusphere-2024-136-RC2 -
AC2: 'Reply on RC2', Björn Linder, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-136/egusphere-2024-136-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Björn Linder, 22 Mar 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-136', Anonymous Referee #1, 12 Feb 2024
The manuscript studies gravity wave momentum flux obtained from a snapshot of a gravity-wave-resolving global model subjected to different methods and filtering. The analysis is tailored to expected data from the MATS satellite. The objective is to evaluate methods that will be applied to MATS data with respect to a "true" global field.
The study is well designed and clearly presented (Fig. 3 is very helpful). It will be very useful for analysis of MATS data. The result is that gravity wave momentum flux derived from expected MATS observational data can be within a factor of 2 of the true values. In the real atmosphere and the real MATS data, gravity waves with less than 200~km horizontal wavelength may be dominant, and the scale separation should be even better.
The analysis focused on cuts on zonal wavenumber, Savitzky-Golay filtering and S3D transforms. Other methods that are also applied to observational data in order to analyze gravity waves like Butterworth filters, wavelet transforms or modal decompositions were not mentioned.
It may be interesting, if feasible, to discuss in more detail one specific wave event when the performance was especially bad, in order to get a feeling on what type of waves would not be well captured. For example the differences in the meridional component at 50 deg N are large, maybe due to a wave with short vertical wavelength?
Is the used HIAMCM snapshot considered representative? In a sense that different snapshots are comparable in covered gravity wave events and variability, such that it does not seem necessary to evaluate different snapshots?
- 307 missing closing bracket after (Fig.8a
Citation: https://doi.org/10.5194/egusphere-2024-136-RC1 -
AC1: 'Reply on RC1', Björn Linder, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-136/egusphere-2024-136-AC1-supplement.pdf
-
RC2: 'Comment on egusphere-2024-136', Anonymous Referee #2, 13 Feb 2024
Review of "Scale separation for gravity wave analysis from 3D temperature observations in the MLT region" by Björn Linder et al.
General comments
In this new study, Linder et al. propose and evaluate an approach for estimating gravity wave pseudomomentum flux from tomographic temperature measurements in the mesosphere and lower thermosphere, which are expected to be provided by the MATS satellite launched in November 2022. The study proposes the use of Savitzky-Golay filters for local separation of the background temperature and temperature perturbations of the MATS temperature measurements. The well-established S3D method is proposed to perform the spectral analysis to estimate the wave properties (wave amplitude and wavelength) from the temperature perturbations, which are input to the estimation of the gravity wave momentum flux.
The proposed methodology is evaluated using simulated temperature fields from the HIAMCM general circulation model, which provides realistic fields of temperature, winds, and other parameters from the surface to the thermosphere. The study compares estimates of gravity wave momentum fluxes estimated directly from the model wind fields with pseudomentum fluxes from model temperatures. It further compares ideal estimates of momentum flux with estimates derived with the SG filters and the S3D method applied to temperature fields sampled as MATS measurements, taking into account the satellite orbit, the retrieval resolution via expected averaging kernels, and the expected measurement noise.
The study has been carefully carried out and the results appear to be sound. The proposed methodology for estimating gravity wave pseudomentum flux from MATS measurements is convincing. The paper is exceptionally well written, clear and concise, in my opinion. I would recommend that the paper be considered for publication, subject to the correction of a few rather minor comments listed below.
Specific comments
Line 131: This study proposes to use the S3D spectral analysis method, but for completeness, the S-transform spectral method could be referenced as another commonly used method for analyzing the types of measurements to be provided by MATS.
Line 146: It is pointed out that the swath width of about 200 km of MATS is a bottleneck for properly estimating some of the gravity wave spectral characteristics. Why was the instrument built with such a small swath, assuming that the estimation of the gravity wave momentum flux in the MLT region is indeed a key objective of the MATS measurements?
Lines 148-150: Perhaps add the dimensions for clarity here, 600 km x 190 km x 10 km (along track x across track x vertical). It would be good to provide some rationale or reference as to why the vertical width of the S3D boxes was set to 10 km. It is pointed out that the length of the cuboids is 600 km, but the sampling would be done every 100 km. What would be the reason for this oversampling, since the results of neighboring boxes would be largely correlated?
Line 176: It is pointed out that the zonal wavenumber 18 corresponds to wavelengths of about 2200 km at the equator. If the reference method is to use zonal wavenumber 18 for scale separation, it looks like there would be some discrepancy with the measurements of the MATS instrument, which has a swath width of ~200 km, which in combination with the S3D could allow estimation of wavelengths up to ~600 km. Isn't there some kind of gap between 600 and 2200 km wavelength?
Lines 191-193: You might add that structured noise from something like stray light would also be very difficult and challenging to properly simulate and account for.
Lines 240-241: I may have missed it earlier in the paper, but for completeness you might want to mention whether the HIAMCM data is a boreal summer and austral winter case?
Lines 269-271: The proposed cuboids have an asymmetry in size along and across the track, which seems to lead to an anisotropy in spectral sensitivity of the gravity wave measurements along and across the track. Such an anisotropy can cause some difficulty in interpreting measurement results. This can be seen in the example presented here for the polar regions. Perhaps you could discuss and clarify why using such unbalanced cube sizes is still advantageous compared to e.g. using a cube size of 200 km x 200 km x 10 km?
Lines 286-287: When discussing vertical smoothing, it would be good to provide some information on the vertical resolution of the HIAMCM test data. The MATS measurements have a vertical resolution of 500 m, and averaging kernels with FWHMs of 1 km and 2 km are considered for the retrieval data. Is the HIAMCM vertical resolution comparable or better?
Technical corrections
line 201: Section -> Sect.
line 220: efficiently -> effectively (?)
Citation: https://doi.org/10.5194/egusphere-2024-136-RC2 -
AC2: 'Reply on RC2', Björn Linder, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-136/egusphere-2024-136-AC2-supplement.pdf
-
AC2: 'Reply on RC2', Björn Linder, 22 Mar 2024
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