the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Oct 2023
A new approach to the crystal habit retrieval from far infrared spectral radiance measurements
Abstract. To generate reliable climate predictions, global models need accurate estimates of all the energy fluxes contributing to the Earth Radiation Budget (ERB). Clouds in general, and more specifically ice clouds play a key role in the determination of the ERB, as they may exert either a feedback or a forcing action, depending on their optical and microphysical properties and physical state (solid/liquid). To date, accurate statistics and climatologies of cloud parameters are not available. The ice cloud composition in terms of ice crystal shape (or habit) is one of the parameters with the largest uncertainty. This is because crystal shape estimates are generally hard to extract from space measurements. Satellite spectral radiance measurements available to date, are actually limited to the middle-infrared spectral region, therefore their information on ice cloud crystal habits is not sufficient to attempt a retrieval.
The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) experiment, foreseen to be the 9th Earth Explorer mission of the European Space Agency, will measure, for the first time spectrally resolved from space, the entire upwelling spectrum emitted by the Earth, from 100 to 1600 cm-1. The far-infrared portion of the Earth spectrum, especially from 200 to 600 cm-1, is very sensitive to cloud ice crystal shapes, thus FORUM measurements could also represent an opportunity to study the ice cloud composition in terms of ice crystal habit mixtures.
To investigate this possibility, we developed an accurate and advanced scheme allowing to model ice cloud optical properties also in cases of clouds composed of mixed ice crystal habits. This feature is in fact necessary, because also in situ measurements acquired over the years point out that the shape of ice cloud crystals varies depending on the crystal size range. In our model, the resulting cloud optical properties are also determined by the input habit fractions. Thus, the retrieval of these fractions from spectral radiance measurements can be attempted. Using 375 different cloudy scenarios, we assess the performance of our retrieval scheme in the determination of crystal habit mixtures starting from FORUM simulated measurements. The most relevant error components affecting the retrieved cloud parameters are not very large and are of random nature, thus FORUM measurements will allow to set up an accurate climatology of cloud parameters.
To provide an example of the benefit that one could get out of the habit mixture retrievals, we also show the improved accuracy of the thermal outgoing fluxes calculations as compared to using assumed mixtures.
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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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RC1: 'Comment on egusphere-2023-2260', Anonymous Referee #1, 07 Nov 2023
This paper presents an inversion algorithm for retrieving the shapes of ice cloud particles using far infrared spectral radiance measurements. The algorithm's performance is evaluated using simulated measurements from the new far infrared sensor, Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM), in various scenarios: tropics, mid-latitudes, and polar regions. This study is a continuation of a previous paper by Di Natale and Palchetti (2022) that focuses on the algorithm's development and validation.
Overall, the results indicate a successful convergence of the inversion algorithm in tropical and mid-latitude scenarios. However, some challenges were encountered in the polar scenario, particularly when considering retrieval affected by FORUM measurement noise. Notably, there were significant differences in the simulated outgoing longwave radiation when using pre-defined fixed shapes compared to retrieved shapes. These findings have implications for improving ice cloud parameterization and enhancing our understanding of ice particle habits in different locations.
While the paper is logically structured and well-organized, I have a few suggestions to enhance its clarity:
- Figure 1: Why does the difference between the radiance computed with the rigorous approach and the approximation increase with OD before OD < 2.0 but decrease when OD > 2.0? The author should provide an explanation for this trend.
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Section 2: I am wondering which database of the optical properties of ice crystals was used in this study. Yang et al. (2013) was continuously updated based on the improvement of computational techniques. For example, Bi and Yang (2017) updated the database based on the invariant imbedding T-matrix method (Bi and Yang, 2014) and improved ray tracing technique for absorbing particles.
- Section 5, Lines 399-410: The author conducted test retrievals using the pre-defined habits of King et al. 2004 and the retrieved habits in this study, assuming that the atmospheric, surface, and cloud parameters were known. However, if the cloud parameters were also influenced by the habits in the retrieval when considering simultaneous retrieval of cloud parameters, I wonder if the value of 2.7 W/ would be lower. Additionally, it is possible that the difference caused by the habits would be partially compensated by adjusting the cloud parameters.
- In Figure 1, the y-axis tick labels of "0,001" should be "0.001". This typo also exists in Figures 4, 6, 7, and 8.
- Line 278: "the a priori..." should be corrected.
- Line 324: "...a part 4 out..." should be corrected.
- Line 319: "Fig.s 3 and 4..." should be corrected.
- Line 410: “(0.6+-0.4)W/m^2 Wild et al. (2013)” should be corrected.
References:
Di Natale, G., & Palchetti, L. (2022). Sensitivity studies toward the retrieval of ice crystal habit distributions inside cirrus clouds from upwelling far infrared spectral radiance observations. Journal of Quantitative Spectroscopy and Radiative Transfer, 282, 108120. https://doi.org/10.1016/j.jqsrt.2022.108120
Bi, L., P. Yang, (2017). Improved ice particle optical property simulations in the ultraviolet to far-infrared regime. Journal of Quantitative Spectroscopy and Radiative Transfer, 189, 228-237.
Bi, L., P. Yang ( 2014). Accurate simulation of the optical properties of atmospheric ice crystals with invariant imbedding T-matrix method. Journal of Quantitative Spectroscopy and Radiative Transfer, 138,17-35.
King MD, Platnick S, Yang P, Arnold GT, Gray MA, Riedi JC, et al. Remote sens- ing of liquid water and ice cloud optical thickness and effective radius in the arctic: application of airborne multispectral MAS data. J Atmos Oceanic Technol 2004;21:857–75. doi: 10.1175/1520-0426(2004)021 < 0857:RSOLWA > 2.0. CO;2 .
Citation: https://doi.org/10.5194/egusphere-2023-2260-RC1 -
AC1: 'Reply on RC1', Gianluca Di Natale, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2260/egusphere-2023-2260-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2023-2260', Anonymous Referee #2, 07 Feb 2024
Summary
This paper presents a sophisticated scheme for modeling ice cloud optical properties by incorporating mixed ice crystal habits, leveraging the upcoming FORUM experiment data. It tests the retrieval scheme on simulated measurements, demonstrating promising performance and identifying the main error components affecting cloud parameter retrievals.
Strengths- Innovative Approach: Develops a new methodology for retrieving ice crystal habit mixtures from spectral radiance measurements, filling a significant gap in atmospheric science.
- Utilization of FORUM Data: The study is forward-looking, leveraging future FORUM experiment measurements to improve ice cloud parameterization, which is crucial for climate modeling.
- Ill-posed problem: The concern regarding the manuscript's handling of the ill-posed problem, particularly the issue of solution uniqueness, is a critical aspect that warrants further clarification and enhancement in the manuscript. The ill-posed nature of the problem stems from the inversion process involved in retrieving ice crystal habit fractions from spectral radiance measurements, which, in some scenarios, is shown to be particularly challenging due to the possibility of multiple solutions yielding almost the same minimum of the cost function. I suggest to the authors to expand this point. This will enhance the manuscript strength, providing a more comprehensive understanding of the ill-posed nature of the retrieval problem.
- Limited Validation: The methodology is tested only on simulated data, lacking validation against actual satellite measurements, which raises questions about its real-world applicability.
- Complexity and Accessibility: The complex methodology and reliance on specific satellite data may limit its accessibility and applicability by the broader research community.
- Clarify Methodological Assumptions and Limitations: The manuscript would benefit from a more detailed discussion of the assumptions underlying the SACR code and the optimal estimation approach used. Addressing the potential limitations these assumptions may pose to the generalizability of the findings will strengthen the manuscript. For instance, how might different atmospheric conditions or cloud compositions affect the retrieval accuracy?
- Strengthen the Literature Review: A more thorough review of the current state of research in ice cloud characterization and the retrieval of cloud properties from spectral radiance measurements could provide a stronger foundation for the study. Highlighting the novelty of the approach in the context of existing methodologies will help to underscore the contribution of this work to the field.
- Graphical Representations: Some figures are dense and may be simplified for better clarity and comprehension.
Given its innovative approach and potential impact on climate modeling, I recommend this paper for acceptance with minor revisions. Addressing the validation with actual satellite data and simplifying complex explanations and figures would significantly enhance the paper's value and readability.
Citation: https://doi.org/10.5194/egusphere-2023-2260-RC2 -
AC2: 'Reply on RC2', Gianluca Di Natale, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2260/egusphere-2023-2260-AC2-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2260', Anonymous Referee #1, 07 Nov 2023
This paper presents an inversion algorithm for retrieving the shapes of ice cloud particles using far infrared spectral radiance measurements. The algorithm's performance is evaluated using simulated measurements from the new far infrared sensor, Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM), in various scenarios: tropics, mid-latitudes, and polar regions. This study is a continuation of a previous paper by Di Natale and Palchetti (2022) that focuses on the algorithm's development and validation.
Overall, the results indicate a successful convergence of the inversion algorithm in tropical and mid-latitude scenarios. However, some challenges were encountered in the polar scenario, particularly when considering retrieval affected by FORUM measurement noise. Notably, there were significant differences in the simulated outgoing longwave radiation when using pre-defined fixed shapes compared to retrieved shapes. These findings have implications for improving ice cloud parameterization and enhancing our understanding of ice particle habits in different locations.
While the paper is logically structured and well-organized, I have a few suggestions to enhance its clarity:
- Figure 1: Why does the difference between the radiance computed with the rigorous approach and the approximation increase with OD before OD < 2.0 but decrease when OD > 2.0? The author should provide an explanation for this trend.
-
Section 2: I am wondering which database of the optical properties of ice crystals was used in this study. Yang et al. (2013) was continuously updated based on the improvement of computational techniques. For example, Bi and Yang (2017) updated the database based on the invariant imbedding T-matrix method (Bi and Yang, 2014) and improved ray tracing technique for absorbing particles.
- Section 5, Lines 399-410: The author conducted test retrievals using the pre-defined habits of King et al. 2004 and the retrieved habits in this study, assuming that the atmospheric, surface, and cloud parameters were known. However, if the cloud parameters were also influenced by the habits in the retrieval when considering simultaneous retrieval of cloud parameters, I wonder if the value of 2.7 W/ would be lower. Additionally, it is possible that the difference caused by the habits would be partially compensated by adjusting the cloud parameters.
- In Figure 1, the y-axis tick labels of "0,001" should be "0.001". This typo also exists in Figures 4, 6, 7, and 8.
- Line 278: "the a priori..." should be corrected.
- Line 324: "...a part 4 out..." should be corrected.
- Line 319: "Fig.s 3 and 4..." should be corrected.
- Line 410: “(0.6+-0.4)W/m^2 Wild et al. (2013)” should be corrected.
References:
Di Natale, G., & Palchetti, L. (2022). Sensitivity studies toward the retrieval of ice crystal habit distributions inside cirrus clouds from upwelling far infrared spectral radiance observations. Journal of Quantitative Spectroscopy and Radiative Transfer, 282, 108120. https://doi.org/10.1016/j.jqsrt.2022.108120
Bi, L., P. Yang, (2017). Improved ice particle optical property simulations in the ultraviolet to far-infrared regime. Journal of Quantitative Spectroscopy and Radiative Transfer, 189, 228-237.
Bi, L., P. Yang ( 2014). Accurate simulation of the optical properties of atmospheric ice crystals with invariant imbedding T-matrix method. Journal of Quantitative Spectroscopy and Radiative Transfer, 138,17-35.
King MD, Platnick S, Yang P, Arnold GT, Gray MA, Riedi JC, et al. Remote sens- ing of liquid water and ice cloud optical thickness and effective radius in the arctic: application of airborne multispectral MAS data. J Atmos Oceanic Technol 2004;21:857–75. doi: 10.1175/1520-0426(2004)021 < 0857:RSOLWA > 2.0. CO;2 .
Citation: https://doi.org/10.5194/egusphere-2023-2260-RC1 -
AC1: 'Reply on RC1', Gianluca Di Natale, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2260/egusphere-2023-2260-AC1-supplement.pdf
-
RC2: 'Comment on egusphere-2023-2260', Anonymous Referee #2, 07 Feb 2024
Summary
This paper presents a sophisticated scheme for modeling ice cloud optical properties by incorporating mixed ice crystal habits, leveraging the upcoming FORUM experiment data. It tests the retrieval scheme on simulated measurements, demonstrating promising performance and identifying the main error components affecting cloud parameter retrievals.
Strengths- Innovative Approach: Develops a new methodology for retrieving ice crystal habit mixtures from spectral radiance measurements, filling a significant gap in atmospheric science.
- Utilization of FORUM Data: The study is forward-looking, leveraging future FORUM experiment measurements to improve ice cloud parameterization, which is crucial for climate modeling.
- Ill-posed problem: The concern regarding the manuscript's handling of the ill-posed problem, particularly the issue of solution uniqueness, is a critical aspect that warrants further clarification and enhancement in the manuscript. The ill-posed nature of the problem stems from the inversion process involved in retrieving ice crystal habit fractions from spectral radiance measurements, which, in some scenarios, is shown to be particularly challenging due to the possibility of multiple solutions yielding almost the same minimum of the cost function. I suggest to the authors to expand this point. This will enhance the manuscript strength, providing a more comprehensive understanding of the ill-posed nature of the retrieval problem.
- Limited Validation: The methodology is tested only on simulated data, lacking validation against actual satellite measurements, which raises questions about its real-world applicability.
- Complexity and Accessibility: The complex methodology and reliance on specific satellite data may limit its accessibility and applicability by the broader research community.
- Clarify Methodological Assumptions and Limitations: The manuscript would benefit from a more detailed discussion of the assumptions underlying the SACR code and the optimal estimation approach used. Addressing the potential limitations these assumptions may pose to the generalizability of the findings will strengthen the manuscript. For instance, how might different atmospheric conditions or cloud compositions affect the retrieval accuracy?
- Strengthen the Literature Review: A more thorough review of the current state of research in ice cloud characterization and the retrieval of cloud properties from spectral radiance measurements could provide a stronger foundation for the study. Highlighting the novelty of the approach in the context of existing methodologies will help to underscore the contribution of this work to the field.
- Graphical Representations: Some figures are dense and may be simplified for better clarity and comprehension.
Given its innovative approach and potential impact on climate modeling, I recommend this paper for acceptance with minor revisions. Addressing the validation with actual satellite data and simplifying complex explanations and figures would significantly enhance the paper's value and readability.
Citation: https://doi.org/10.5194/egusphere-2023-2260-RC2 -
AC2: 'Reply on RC2', Gianluca Di Natale, 22 Mar 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-2260/egusphere-2023-2260-AC2-supplement.pdf
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Gianluca Di Natale
Marco Ridolfi
Luca Palchetti
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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