the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Multiple-scattering effects on single-wavelength lidar sounding of multi-layered clouds
Valery Shcherbakov
Frédéric Szczap
Guillaume Mioche
Céline Cornet
Abstract. We performed Monte Carlo simulations of single-wavelength lidar signals from multi-layered clouds with special attention focused on multiple-scattering (MS) effect in regions of the cloud-free molecular atmosphere, i.e. between layers or outside a cloud system. Despite the fact that the strength of lidar signals from molecular atmosphere is much lower compared to the in-cloud intervals, studies of MS effects in such regions are of interest from scientific and practical points of view.
The MS effect on lidar signals is always decreasing with the increasing distance from the cloud far edge. The decreasing is the direct consequence of the fact that the forward peak of particles phase functions is much larger than the receiver field of view (RFOV). Therefore, the photons scattered within the forward peak escape the sampling volume formed by the RFOV, i.e. the escape effect. We demonstrated that the escape effect is an inherent part of MS properties within the free atmosphere beyond the cloud far edge.
In the cases of the ground-based lidar, the MS contribution is lower than 5 % within the regions of the cloud-free molecular atmosphere having the distance from the cloud far edge about 1 km or higher. In the cases of the space-borne lidar, the decreasing rate of the MS contribution is so slow that the threshold of 5 % can hardly be reached. In addition, the effect of non-uniform beam filling is extremely strong. Therefore, practitioners should employ with proper precautions lidar data from regions below the cloud base when treating data of a space-borne lidar.
In the case of two-layered cloud, the distance of 1 km is sufficiently large that the scattered photons emerging from the first layer do not affect signals from the second layer when we are dealing with the ground-based lidar. In contrast, signals from the near edge of the second cloud layer are severely affected by the photons emerging from the first layer in the case of a space-borne lidar.
We evaluated the Eloranta model (EM) in extreme conditions and showed its good performance in the cases of ground-based and space-borne lidars. At the same time, we revealed the shortcoming that can affect practical applications of the EM. Namely, values of the key parameters, i.e. the ratios of phase functions in the backscatter direction for the nth-order-scattered photon and a singly scattered photon depend not only on the particles phase function, but also on the distance from a lidar to the cloud and the receiver field of view. Those ratios vary within a quite large range and the MS contribution to lidar signals can be largely overestimated or underestimated if erroneous values of the ratios are assigned to the EM.
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Valery Shcherbakov et al.
Status: open (extended)
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RC1: 'Comment on amt-2023-109', Anonymous Referee #1, 11 Jul 2023
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General comments: The authors numerically simulated single-wavelength lidar multiple scattering signals by the Monte Carlo method, and focuses on the impacts of cloud multiple scattering on the lidar return signals in the cloud-free molecular atmosphere between cloud layers or outside a cloud layer. The author reported some interesting results, for example, the ratio of multiple scattering contributions to single scattering signals in the molecular atmosphere near the cloud edge is even larger than the ratio in the cloud (i.e., stepwise jump phenomena), and the multiple scattering effects are decreasing with the increase of the distance from the cloud edge (i.e., escape effects), and so on. It is worth publishing. However, there are several shortcomings in this paper. First, why did the authors set the altitude of the cloud base to 8km? It is too high for water clouds. Why is the extinction coefficient of the water cloud set to 1.0 km^{-1}? It is too small. It is not representative for water clouds. The authors should give a reasonable explanation for this. Secondly, the simulation results are particularly noisy. Is it convincing? Thirdly, there is logical confusion in the interpretation of the simulation results in Section 3.1.1, which is misleading. For example, the authors stated that the stepwise jump phenomenon is simply caused by the stepwise jump in the phase function at scattering angles close to 180 degrees. In my opinion, other factors (such as the molecular extinction coefficient) also might have effects on this. The validation results in Fig 3a and 3b can only show that the free atmospheric signal is mainly affected by the forward scattering of clouds. The author mentioned pulse stretching many times and emphasized that their explanations are different from pulse stretching explanations. Can the author specifically point out the difference between the two? Finally, this paper is rather lengthy, and the simulation description is repeated many times. It is suggested that the author refine the text to improve the reading experience.
Specific comments:
Line 62-66 in Section 1: “results of MC simulations published in the literature evidenced the following. As it is expected, lidar signals from regions of the cloud-free molecular atmosphere … are affected by the scattered light emerging from clouds.” Has the literature analyzed this multiple scattering effect in detail? If so, what is the difference between the results discussed by the authors and them? The authors should make a detailed discussion on this, which is the main contribution of this paper.
Section 2.1: Subscript “MS” is used in several physical quantities and has different physical meanings, which are easy to cause misunderstanding. For example, it denotes total order-scattering in the lidar signal S_{MS}, while the contribution of single scattering is excluded in the ratio R_{MSto1}. In contrast, the ratio R’_{MS} contains the contribution of single scattering.
Section 2.1: I suggest providing a simple schematic diagram to describe the lidar sounding of the cloud-free atmosphere outside the clouds. It will be especially helpful for understanding formulas (especially Eq. (8-14)).
Line 92-95 in Section 2.2: As seen from Fig. 1, the phase function f_{ch2} is the same as the f_{ch1} except for the backscattering directions. Does this mean that f_{ch2} does not meet the normalization condition? If so, it would significantly increase the backscattering contribution of lidar signals, and the difference between Figs. 3a and 3b will be easily understood. See the comment “Line 265-293 in Section 3.1.1”. The author should briefly elaborate on it.
Line 209, 215, and 386-387: Why is the extinction coefficient of the water cloud set to 1.0 km^{-1}? It is too small. It is not representative for water clouds. Why did the authors set the altitude of the cloud bottom to 8km? It is too high for water clouds. The authors should give a reasonable explanation for this. Alternatively, the authors at least emphasize the scope of application of the conclusions made in this paper in the abstract or conclusion.
Fig. 2 in Section 3.1: The simulation results are particularly noisy in Fig. 2. Is it convincing? Can the authors reproduce the simulation results using other Monte Carlo programs?
Line 261-262 in Section 3.1.1: “It means that only the range 𝑑 ∈]3., 3.02] km of 𝑅2𝑡𝑜1(𝑑) can be somewhat affected by the pulse-length stretching.” Why is only the range in [3,3.02] affected by a little pulse-length stretching? According to Miller and Stephens, 1999, if multiple scattering (except the exact forward scattering with zero scattering angle) occurs, the half of light path length will be greater than the sounding ranges under the single scattering approximation, and pulse stretching will occur. From this point of view, the multiple scattering signals in the cloud-free atmosphere can be considered as caused by pulse stretching, but the influence degree is different.
Line 262-264 in Section 3.1.1: “Thus, it is safe to assume that the stepwise jump of 𝑅𝑀𝑆𝑡𝑜1(𝑑) and 𝑅2𝑡𝑜1(𝑑) is due to the stepwise jump in phase-function properties at angles close to 180° (the phase function of particles within the cloud and the Rayleigh scattering within the free atmosphere).” It is too vague to be misleading. Do the authors mean that the stepwise jump of 𝑅_{𝑀𝑆𝑡𝑜1} is caused by the stepwise jump in the phase function of the water cloud within the scattering angles close to 180 degrees or the significant difference between the phase function of the water cloud and the Rayleigh scattering at scattering angles close to 180 degrees? I prefer to think of it as the latter. However, I think other factors (such as the molecular extinction coefficient) also have effects on this.
Line 265-293 in Section 3.1.1: “That assumption is confirmed by the plots in Figs. 3a and b”. It's not convincing. It can only show that the phase function of the water cloud at scattering angles close to 180 degrees can affect the lidar multiple scattering signals in the cloud, but not in the cloud-free atmosphere. Therefore, it can show that the free atmospheric signal is mainly affected by the forward scattering of clouds. However, it is hard to demonstrate that the stepwise jump pattern occurring in the cloud-free atmosphere is due to the stepwise jump in phase-function properties at angles close to 180°.
In my opinion, it can be more clearly explained by the small-angle forward scattering theory, i.e., the lidar signal is contributed by the light that experiences a series of forward scattering from the emitter, then a single backward scattering, and finally a series of forward scattering back to the receiver. The forward scattering is roughly the same for both the cloud and the free atmosphere. They all occur in the cloud. The difference is the backscattering events, one occurring in the cloud and one occurring in the free atmosphere. Therefore, I claim that both the molecular extinction coefficients and phase function are the main reason for the stepwise jump in the 𝑅_{𝑀𝑆𝑡𝑜1}.
In addition, the author mentioned pulse stretching many times and emphasized that their explanations are different from pulse stretching explanations. Can the author specifically point out the difference between the two?
Line 294-302 in Section 3.1.1: The explanation of the “escape effect” is incomplete. In addition to the large forward diffraction ring relative to the field of view, the particularly small cloud extinction coefficient should also be a major reason. This is because if the extinction coefficient is large, the average free path length of light is short, and it is difficult to escape from the sampling volume even after many times of scattering. That's why I asked the authors to explain why they chose this special case.
Line 309-314 in Section 3.1.1: “the pulse stretching is the cause of the jump” This judgment seems to be too subjective. Although the field of view is large, the extinction coefficient is too small and thus the free path length is large, so the light may not be able to experience enough scattering to produce obvious pulse stretching.
Line 408-410 in Section 4.1.1: “Thus, the stepwise jump in those cases is not only due to the stepwise jump in phase-function properties for angles close to 180°. We can suggest that the range 𝑑 ∈]3., 3.1] km is somewhat affected by the pulse stretching.” This judgment seems to be too subjective. Can the authors specifically show the difference between the two explanations? It is the same question in the comment “Line 265-293 in Section 3.1.1”
Line 430 Section 4.1.2: “The NUBF effect is so high in such conditions that it has to be shown in terms of lidar signals.” It's expected, I think, and it's easy to predict. Can the author provide the lidar single scattering signal results in the case of a three-dimensional cloud field? This may be more interesting.
Technical corrections:
Line 52 in Section 1: “Using some cases as examples, good performance of approximate models was underlined by their authors.” It is too vague to provide any useful information.
Line 75 in Section 1: “because multiple integrals are in its core. Therefore, it is an easy matter to develop the corresponding code.” It seems that there is no logical relationship between the two, so it is suggested to modify it clearly.
Line 156 in Section 2.2: “matrixes” can be corrected to “matrices”.
Line 191 in Section 2.2: “The Gaussian component 𝐺2(𝜃) is large” need be corrected to “The width of …”.
Line 261 in Section 3.1.1: “the distance from the lidar to particles of the cloud layer is quite low” It is “short” not “low”.
Line 337 in Section 3.1.2: “when and 𝜀𝑝 ≤ 0.2 km-1.” The word “and” may need to be deleted.
Citation: https://doi.org/10.5194/amt-2023-109-RC1
Valery Shcherbakov et al.
Valery Shcherbakov et al.
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