the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Feb 2022
A study of Mie scattering modelling for mixed phase Polar Stratospheric Clouds
Abstract. Mie scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Backscattering and extinction can be computed once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of Supercooled Ternary Solution (STS) droplets, solid Nitric Acid Trihydrate (NAT) and possibly ice particles, making questionable the use of Mie theory with a single refractive index and with the underlying assumption of spherical scatterers. Here we consider a set of fifteen coincident measurements of PSC above McMurdo Station, Antarctica, by ground-based lidar and balloon-borne Optical Particle Counters (OPC), and in situ observations taken by a laser backscattersonde and an OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows to test the performances of Mie theory under fairly reasonable corrections when aspherical scatterers are present.
Here we consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if above it. The refractive indexes are assumed known from literature. Moreover, the Mie result for solid particles are reduced by a factor C < 1, which takes into account the backscattering depression expected from the asphericity. Finally, we consider the fraction X of the backscattering from the aspherical part of the PSD as polarized, and the remaining (1-X) as depolarized. The three parameters Rth, C and X of our model are chosen to provide the best match with the observed optical backscattering and depolarization. The comparison of the calculations with the measures is satisfactory for the backscattering but not for the depolarization, and possible causes are discussed. The results of this work help to understand the limits of the application of Mie theory in modeling the optical response of particles of different composition and morphology.
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Interactive discussion
Status: closed
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RC1: 'Comment on amt-2022-28', Anonymous Referee #1, 26 Feb 2022
The paper describes an effort of using the Mie theory to model lidar backscattering and depolarization of Polar Stratospheric Clouds. Two empirical parameters are introduced to describe depolarization (X) and depression factor of solid particle backscattering (C). The input to the Mie code include in-situ measured size distributions, while the refractive index is assumed from the literature. The authors found that they can match the backscattering pretty well but failed to reproduce the depolarization factor. They claim the novelty of this paper is to test the range of Mie theory in modeling lidar data.
I cannot support the publication of this paper because the optical models lacks the basic physics consideration of light scattering. Light scattering by aspherical particles is essentially different from spherical particles. The most prominent difference is the depolarization property. The Mie (spherical) particles cannot depolarize lidar signal, if we ignore multiple scattering. It makes no sense to the reviewer that using an empirical factor X to model depolarization of spherical particles (see Eqs. 12 and 13). In addition, they use an empirical factor C to adjust the backscattering of spherical particles to aspherical particles, though it is more tolerable than the depolarization but still undesirable because of its empirical nature. There are two main causes of lidar depolarizations, nonsphericity and multipole scattering, neither is considered by this optical model so it is no surprise that you cannot mimic the behavior of depolarizations.
I encourage the author to examine their work under the light of assuming true nonspherical particle contribution instead of just trying to fit the data with some arbitrary empirical parameters. In the end the empirical parameters does not help the community gain any knowledge about the microphysical and optical properties of PSC.
Citation: https://doi.org/10.5194/amt-2022-28-RC1 - AC1: 'Reply on RC1', Francesco Cairo, 07 Mar 2022
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RC2: 'Comment on amt-2022-28', Anonymous Referee #2, 31 Mar 2022
This paper describes a simplified method for calculating the optical properties of mixed-phase polar stratospheric clouds (PSCs), i.e., those composed of both spherical supercooled ternary solution (STS) droplets and non-spherical nitric acid trihydrate (NAT) or ice particles. It is assumed that the backscatter and depolarization of NAT and ice particles can be modeled by applying empirically derived factors (C = backscatter depression, and X = polarized fraction) to Mie scattering results for equivalent-size spheres. The Mie calculations are based on monomodal or bimodal lognormal fits to size distributions measured on balloon flights in the Antarctic and Arctic, with particles smaller (larger) than a threshold radius (Rth) assumed to be STS (NAT or ice). Best-fit values/ranges for C, X, and Rth are determined by comparing the calculations to concomitant lidar and backscatter-sonde measurements of backscatter and depolarization.
I don’t think this study makes a significant contribution to the literature. The method does a very poor job of modeling the measured depolarization. The authors cite a number of prior papers supporting this negative finding, which makes it seem that this was a foregone conclusion. There is a decent match between modeled and measured particulate backscatter, but this parameter provides much less information about PSC formation and evolution than depolarization does.
In my opinion, the authors should redo the study using a different approach in which the scattering and depolarization of the presumed non-spherical particles larger than Rth are computed using readily available tables of T-matrix results for randomly oriented spheroids with different aspect ratios. They should attempt to determine a value/range of aspect ratios that best fit the observations, which would be a much more valuable contribution to the literature than the present paper.
I must also comment that the use of the English language could have been improved in many spots in the paper. The paper could also have benefited from closer proofreading, e.g., 3 formulae are presented on both page 3 and page 4, but there are 4 formula numbers on each page.Citation: https://doi.org/10.5194/amt-2022-28-RC2 -
AC2: 'Reply on RC2', Francesco Cairo, 13 Apr 2022
The authors thank the reviewer for the comments. In particular, they are ready to follow the reviewer's suggestion on the use of T-matrix codes to derive a range of R_th and, in this proposed perspective, of the particulate aspect ratio, which best accord with the backscattering and depolarization observations. This will be done in an upcoming redaction of the work, or in a new submission, following the instructions of the editors.
Citation: https://doi.org/10.5194/amt-2022-28-AC2
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AC2: 'Reply on RC2', Francesco Cairo, 13 Apr 2022
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EC1: 'Comment on amt-2022-28', Omar Torres, 13 Apr 2022
I was waiting for the closing of the open discussion phase to proced with my decision. Given the two reviewers' unambiguous opinion on the the paper's lack of merit for publication, at this point I recommend the authors to expand their work according to the given recommendations and submit a new manuscript. Since the required additional work will take some time to be completed, I will not be recommending the submission of a revised version of the current manuscript.
Citation: https://doi.org/10.5194/amt-2022-28-EC1 -
EC2: 'Comment on amt-2022-28', Omar Torres, 20 Apr 2022
The paper is not accepted for publication. Following the review process, the authors are ecouraged to prepare a new manuscript to be processed as a new submission.
Citation: https://doi.org/10.5194/amt-2022-28-EC2
Interactive discussion
Status: closed
-
RC1: 'Comment on amt-2022-28', Anonymous Referee #1, 26 Feb 2022
The paper describes an effort of using the Mie theory to model lidar backscattering and depolarization of Polar Stratospheric Clouds. Two empirical parameters are introduced to describe depolarization (X) and depression factor of solid particle backscattering (C). The input to the Mie code include in-situ measured size distributions, while the refractive index is assumed from the literature. The authors found that they can match the backscattering pretty well but failed to reproduce the depolarization factor. They claim the novelty of this paper is to test the range of Mie theory in modeling lidar data.
I cannot support the publication of this paper because the optical models lacks the basic physics consideration of light scattering. Light scattering by aspherical particles is essentially different from spherical particles. The most prominent difference is the depolarization property. The Mie (spherical) particles cannot depolarize lidar signal, if we ignore multiple scattering. It makes no sense to the reviewer that using an empirical factor X to model depolarization of spherical particles (see Eqs. 12 and 13). In addition, they use an empirical factor C to adjust the backscattering of spherical particles to aspherical particles, though it is more tolerable than the depolarization but still undesirable because of its empirical nature. There are two main causes of lidar depolarizations, nonsphericity and multipole scattering, neither is considered by this optical model so it is no surprise that you cannot mimic the behavior of depolarizations.
I encourage the author to examine their work under the light of assuming true nonspherical particle contribution instead of just trying to fit the data with some arbitrary empirical parameters. In the end the empirical parameters does not help the community gain any knowledge about the microphysical and optical properties of PSC.
Citation: https://doi.org/10.5194/amt-2022-28-RC1 - AC1: 'Reply on RC1', Francesco Cairo, 07 Mar 2022
-
RC2: 'Comment on amt-2022-28', Anonymous Referee #2, 31 Mar 2022
This paper describes a simplified method for calculating the optical properties of mixed-phase polar stratospheric clouds (PSCs), i.e., those composed of both spherical supercooled ternary solution (STS) droplets and non-spherical nitric acid trihydrate (NAT) or ice particles. It is assumed that the backscatter and depolarization of NAT and ice particles can be modeled by applying empirically derived factors (C = backscatter depression, and X = polarized fraction) to Mie scattering results for equivalent-size spheres. The Mie calculations are based on monomodal or bimodal lognormal fits to size distributions measured on balloon flights in the Antarctic and Arctic, with particles smaller (larger) than a threshold radius (Rth) assumed to be STS (NAT or ice). Best-fit values/ranges for C, X, and Rth are determined by comparing the calculations to concomitant lidar and backscatter-sonde measurements of backscatter and depolarization.
I don’t think this study makes a significant contribution to the literature. The method does a very poor job of modeling the measured depolarization. The authors cite a number of prior papers supporting this negative finding, which makes it seem that this was a foregone conclusion. There is a decent match between modeled and measured particulate backscatter, but this parameter provides much less information about PSC formation and evolution than depolarization does.
In my opinion, the authors should redo the study using a different approach in which the scattering and depolarization of the presumed non-spherical particles larger than Rth are computed using readily available tables of T-matrix results for randomly oriented spheroids with different aspect ratios. They should attempt to determine a value/range of aspect ratios that best fit the observations, which would be a much more valuable contribution to the literature than the present paper.
I must also comment that the use of the English language could have been improved in many spots in the paper. The paper could also have benefited from closer proofreading, e.g., 3 formulae are presented on both page 3 and page 4, but there are 4 formula numbers on each page.Citation: https://doi.org/10.5194/amt-2022-28-RC2 -
AC2: 'Reply on RC2', Francesco Cairo, 13 Apr 2022
The authors thank the reviewer for the comments. In particular, they are ready to follow the reviewer's suggestion on the use of T-matrix codes to derive a range of R_th and, in this proposed perspective, of the particulate aspect ratio, which best accord with the backscattering and depolarization observations. This will be done in an upcoming redaction of the work, or in a new submission, following the instructions of the editors.
Citation: https://doi.org/10.5194/amt-2022-28-AC2
-
AC2: 'Reply on RC2', Francesco Cairo, 13 Apr 2022
-
EC1: 'Comment on amt-2022-28', Omar Torres, 13 Apr 2022
I was waiting for the closing of the open discussion phase to proced with my decision. Given the two reviewers' unambiguous opinion on the the paper's lack of merit for publication, at this point I recommend the authors to expand their work according to the given recommendations and submit a new manuscript. Since the required additional work will take some time to be completed, I will not be recommending the submission of a revised version of the current manuscript.
Citation: https://doi.org/10.5194/amt-2022-28-EC1 -
EC2: 'Comment on amt-2022-28', Omar Torres, 20 Apr 2022
The paper is not accepted for publication. Following the review process, the authors are ecouraged to prepare a new manuscript to be processed as a new submission.
Citation: https://doi.org/10.5194/amt-2022-28-EC2
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