the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Information Content of Brightness Temperature Differences of Spaceborne Imagers with respect to Cloud Phase
Abstract. This study investigates the sensitivity of two brightness temperature differences (BTDs) in the infrared (IR) window of the SEVIRI imager to various cloud parameters in order to better understand their information content, with a focus on cloud thermodynamic phase. To this end, this study presents radiative transfer calculations, providing an overview of the relative importance of all radiatively relevant cloud parameters, including thermodynamic phase, cloud top temperature (CTT), optical thickness (τ), effective radius (Reff) and ice crystal habit. By disentangling the roles of cloud absorption and scattering, we are able to explain the relationships of the BTDs to the cloud parameters on the one hand by spectral differences in the cloud optical properties. In addition, an effect due to the nonlinear transformation from radiances to brightness temperatures contributes to the specific characteristics of the BTDs and their dependence on τ and CTT. We find that the dependence of the BTDs on phase is more complex than sometimes assumed. Although both BTDs are directly sensitive to phase, this sensitivity is comparatively small in contrast to other cloud parameters. Instead, the primary link between phase and the BTDs lies in their sensitivity to CTT, which is associated with phase. One consequence is that distinguishing high ice clouds from low liquid clouds is straightforward, but distinguishing mid-level ice clouds from mid-level liquid clouds is challenging. These findings help to better understand and improve the working principles of phase retrieval algorithms.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2024-540', Andrew Heidinger, 22 Mar 2024
This paper is a very thorough sensitivity analysis of SEVIRI IR brightness temperature differences. It will serve as a reference for future studies. Excellent text and high quality figures. I urge its publication and my comments should be considered optional.
Line 30. Many algorithms use these brightness temperature differences along with other measurements (brightness temperature of other channels, reflectances …) to retrieve phase simultaneously with other cloud properties (temperature, reff, tau). Are you saying this is necessary?
Figure 2. SEVIRI offers channels at 9.7 and 13.3um but they occur in absorption bands. These seem to offer different single scattering properties that add to the phase story. Do you see any benefit of their inclusion or is the atmospheric absorption too much in your opinion?
Line 40.0 You mention Parol (1991). One of the main tools of his analysis was to convert BTDs in beta ratios which are directly linked to the single scattering properties and remove many of the temperature, optical depth dependences. Do you have a position on the use of beta ratios in cloud phase determination?
Line 355, I am confused by the two statements “the single scattering properties
are not CTT dependent” and “cloud emissivity is not CTT dependent (as it is a function of the absorption coefficient)”. Is not the absorption coefficient a single scatter property? Emissivity is solely a function of reff and tau unless you mean the “effective emissivity” observed through brightness temperatures.Line 490. One of the big issues in satellite imager cloud phase determination is the discrepancy in cloud phase from IR and VIS/NIR approaches for mid-level clouds. Is this something you see in your analysis?
Line 550: Is this study only to show these dependencies or is targeted for improving or understanding a current or future cloud phase product?
Overall, excellent. It might add to the story to show a compelling SEVIRI scene or two.
Citation: https://doi.org/10.5194/egusphere-2024-540-CC1 -
AC3: 'Reply on CC1', Johanna Mayer, 21 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-540/egusphere-2024-540-AC3-supplement.pdf
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AC3: 'Reply on CC1', Johanna Mayer, 21 May 2024
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RC1: 'Comment on egusphere-2024-540', Anonymous Referee #1, 01 Apr 2024
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AC2: 'Reply on RC1', Johanna Mayer, 21 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-540/egusphere-2024-540-AC2-supplement.pdf
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AC2: 'Reply on RC1', Johanna Mayer, 21 May 2024
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RC2: 'Comment on egusphere-2024-540', Anonymous Referee #2, 30 Apr 2024
I think the previous two reviewers had very nice summary and evaluation of the manuscript. I agree with a lot of their comments. Overall I think this is a very rigorous study of the IR based retrieval of cloud properties and it is worthy of publication. On the other hand I think the manuscript needs some modification and revision before it can be finally accepted for publication.
First of all the manuscript reads more like a shortened Ph.D./MS. thesis than a scientific paper. It is way too long and hard to get insightful/meaningful results without having to go through many unnecessary parts. For example, the section 2 "physics background" should be significantly shortened if not removed completely as it is about something that is from textbook and well known. Similarly, I'm not sure if the Appendix A is needed either. In any case, I would suggest finding all possible ways to make the paper succinct.
The discussion of "disentangling the roles of cloud absorption and scattering" is interesting. However, it is based on the assumption that scattering is completely ignored in the IR cloud remote sensing, which is rarely the case in recent studies. As mentioned by the first reviewer, a popular method is the so-called beta ratio method, which was first proposed Parol et al., 1991 and then improved and used in many follow up studies (e.g., Pavolonis 2010, Heidinger et al. 2010; Heidinger et al., 2015;). The beta ratio method is based on the so-called similarity principle that takes into account the strong forward scattering of cloud particles in radiative transfer. I think it is important to point this out (with proper reference) and discuss it against Eq. (6).
While the manuscript covers most of the important aspects of the IR based retrieval of cloud properties, an important missing is the impacts of cloud microphysics. A good paper on this topic is Zhang et al (2010). I think it would make the study stronger if you can add some study/discussion to about this effect, perhaps along with the discussion on the effects of cloud geometrical thickness.
Pavolonis, M. J., 2010: Advances in Extracting Cloud Composition Information from Spaceborne Infrared Radiances—A Robust Alternative to Brightness Temperatures. Part I: Theory. J. Appl. Meteor. Climatol., 49, 1992–2012,
Heidinger, A. K., M. J. Pavolonis, R. E. Holz, B. A. Baum, and S. Berthier (2010), Using CALIPSO to explore the sensitivity to cirrus height in the infrared observations from NPOESS/VIIRS and GOES-R/ABI, J. Geophys. Res., 115, D00H20, doi:10.1029/2009JD012152.
Heidinger, A.K.; Li, Y.; Baum, B.A.; Holz, R.E.; Platnick, S.; Yang, P. Retrieval of Cirrus Cloud Optical Depth under Day and Night Conditions from MODIS Collection 6 Cloud Property Data. Remote Sens. 2015, 7, 7257-7271. https://doi.org/10.3390/rs70607257
Zhang, Z., S. Platnick, P. Yang, A. K. Heidinger, and J. M. Comstock (2010), Effects of ice particle size vertical inhomogeneity on the passive remote sensing of ice clouds, J. Geophys. Res., 115, D17203, doi:10.1029/2010JD013835.
Citation: https://doi.org/10.5194/egusphere-2024-540-RC2 -
AC1: 'Reply on RC2', Johanna Mayer, 21 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-540/egusphere-2024-540-AC1-supplement.pdf
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AC1: 'Reply on RC2', Johanna Mayer, 21 May 2024
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2024-540', Andrew Heidinger, 22 Mar 2024
This paper is a very thorough sensitivity analysis of SEVIRI IR brightness temperature differences. It will serve as a reference for future studies. Excellent text and high quality figures. I urge its publication and my comments should be considered optional.
Line 30. Many algorithms use these brightness temperature differences along with other measurements (brightness temperature of other channels, reflectances …) to retrieve phase simultaneously with other cloud properties (temperature, reff, tau). Are you saying this is necessary?
Figure 2. SEVIRI offers channels at 9.7 and 13.3um but they occur in absorption bands. These seem to offer different single scattering properties that add to the phase story. Do you see any benefit of their inclusion or is the atmospheric absorption too much in your opinion?
Line 40.0 You mention Parol (1991). One of the main tools of his analysis was to convert BTDs in beta ratios which are directly linked to the single scattering properties and remove many of the temperature, optical depth dependences. Do you have a position on the use of beta ratios in cloud phase determination?
Line 355, I am confused by the two statements “the single scattering properties
are not CTT dependent” and “cloud emissivity is not CTT dependent (as it is a function of the absorption coefficient)”. Is not the absorption coefficient a single scatter property? Emissivity is solely a function of reff and tau unless you mean the “effective emissivity” observed through brightness temperatures.Line 490. One of the big issues in satellite imager cloud phase determination is the discrepancy in cloud phase from IR and VIS/NIR approaches for mid-level clouds. Is this something you see in your analysis?
Line 550: Is this study only to show these dependencies or is targeted for improving or understanding a current or future cloud phase product?
Overall, excellent. It might add to the story to show a compelling SEVIRI scene or two.
Citation: https://doi.org/10.5194/egusphere-2024-540-CC1 -
AC3: 'Reply on CC1', Johanna Mayer, 21 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-540/egusphere-2024-540-AC3-supplement.pdf
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AC3: 'Reply on CC1', Johanna Mayer, 21 May 2024
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RC1: 'Comment on egusphere-2024-540', Anonymous Referee #1, 01 Apr 2024
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AC2: 'Reply on RC1', Johanna Mayer, 21 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-540/egusphere-2024-540-AC2-supplement.pdf
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AC2: 'Reply on RC1', Johanna Mayer, 21 May 2024
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RC2: 'Comment on egusphere-2024-540', Anonymous Referee #2, 30 Apr 2024
I think the previous two reviewers had very nice summary and evaluation of the manuscript. I agree with a lot of their comments. Overall I think this is a very rigorous study of the IR based retrieval of cloud properties and it is worthy of publication. On the other hand I think the manuscript needs some modification and revision before it can be finally accepted for publication.
First of all the manuscript reads more like a shortened Ph.D./MS. thesis than a scientific paper. It is way too long and hard to get insightful/meaningful results without having to go through many unnecessary parts. For example, the section 2 "physics background" should be significantly shortened if not removed completely as it is about something that is from textbook and well known. Similarly, I'm not sure if the Appendix A is needed either. In any case, I would suggest finding all possible ways to make the paper succinct.
The discussion of "disentangling the roles of cloud absorption and scattering" is interesting. However, it is based on the assumption that scattering is completely ignored in the IR cloud remote sensing, which is rarely the case in recent studies. As mentioned by the first reviewer, a popular method is the so-called beta ratio method, which was first proposed Parol et al., 1991 and then improved and used in many follow up studies (e.g., Pavolonis 2010, Heidinger et al. 2010; Heidinger et al., 2015;). The beta ratio method is based on the so-called similarity principle that takes into account the strong forward scattering of cloud particles in radiative transfer. I think it is important to point this out (with proper reference) and discuss it against Eq. (6).
While the manuscript covers most of the important aspects of the IR based retrieval of cloud properties, an important missing is the impacts of cloud microphysics. A good paper on this topic is Zhang et al (2010). I think it would make the study stronger if you can add some study/discussion to about this effect, perhaps along with the discussion on the effects of cloud geometrical thickness.
Pavolonis, M. J., 2010: Advances in Extracting Cloud Composition Information from Spaceborne Infrared Radiances—A Robust Alternative to Brightness Temperatures. Part I: Theory. J. Appl. Meteor. Climatol., 49, 1992–2012,
Heidinger, A. K., M. J. Pavolonis, R. E. Holz, B. A. Baum, and S. Berthier (2010), Using CALIPSO to explore the sensitivity to cirrus height in the infrared observations from NPOESS/VIIRS and GOES-R/ABI, J. Geophys. Res., 115, D00H20, doi:10.1029/2009JD012152.
Heidinger, A.K.; Li, Y.; Baum, B.A.; Holz, R.E.; Platnick, S.; Yang, P. Retrieval of Cirrus Cloud Optical Depth under Day and Night Conditions from MODIS Collection 6 Cloud Property Data. Remote Sens. 2015, 7, 7257-7271. https://doi.org/10.3390/rs70607257
Zhang, Z., S. Platnick, P. Yang, A. K. Heidinger, and J. M. Comstock (2010), Effects of ice particle size vertical inhomogeneity on the passive remote sensing of ice clouds, J. Geophys. Res., 115, D17203, doi:10.1029/2010JD013835.
Citation: https://doi.org/10.5194/egusphere-2024-540-RC2 -
AC1: 'Reply on RC2', Johanna Mayer, 21 May 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-540/egusphere-2024-540-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Johanna Mayer, 21 May 2024
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Bernhard Mayer
Luca Bugliaro
Ralf Meerkötter
Christiane Voigt
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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