the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Nov 2023
Microphysical modelling of aerosol scavenging by different types of clouds. Description and validation of the approach
Abstract. With dry deposition and below-cloud scavenging, in-cloud scavenging is one of the three components of aerosol transfer from the atmosphere to the ground. There is no experimental validation of in-cloud particle scavenging models for all cloud types that is not impacted by uncertainties concerning below-cloud scavenging. In this article, the choice was made to start with a recognised and validated microphysical cloud formation model (DESCAM) to extract a scheme of aerosol scavenging by clouds, valid for different cloud types. The resulting model works for the two most extreme precipitation clouds: from cumulonimbus to stratus. It is based on data accessible a priori from Numerical Weather Prediction (NWP) outputs, i.e., the intensity of the rain and the relative humidity in the cloud. The diagnostic of the altitude of the cloud base proves to be a key parameter, and accuracy in this regard is vital. This new in-cloud scavenging scheme can be used by long-distance (> 100 km) Atmospheric Transport Models (ATMs) or Global Climate Models (GCMs).
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RC1: 'Comment on egusphere-2023-2105', Anonymous Referee #1, 14 Jan 2024
This article presents in-cloud scavenging coefficients that are function of rain rate for different types of clouds (cumulonimbus and stratus). Their suggested formulas can be a good reference for simplified models. I have several questions that need to be answered or clarified before publication.
1. They said that their approach is a theoretical approach, but I think there are many parameterizations and assumptions used to build their in-cloud scavenging coefficients. Also, their formulas are based on only one set of specific representation of cloud microphysics. For example, in equation (14), they adopted some collection efficiencies that were previously developed or parameterized by someone else. What if different collection efficiencies are used? Will the in-cloud scavenging coefficients be significantly different? I wonder how general the formulas they proposed in this study are.
2. Are solid hydrometeors taken into account in-cloud scavenging? For the cumulonimbus case, I believe there are some ice or snow in the upper levels, and they certainly contribute aerosol removal. At the cloud base, there are no solid hydrometeors? all melted? In this warm temperature case, there might no snow. However, let's assume temperature is low enough, and only snow is the precipitating hydrometeors (no rain). In this case, one cannot use their formulas that only consider rain. Do you have plan to include snow?
3. They explained why status clouds are more efficient in removing aerosols even though the rain rates are the same for status and cumulonimbus. Besides the explanation using equations, can you provide more physically based interpretation?
4. I wonder if aerosol removal by falling hydrometeors within clouds can be called in-cloud scavenging. In-cloud scavenging usually indicates the removal by activation or collection. Based on their approach that examines the mass change of aerosols at the cloud base, I wonder if it is okay to include the effect of wash out in the in-cloud scavenging process.
5. Can scavenging coefficients be the same for mass concentration and number concentration of aerosols in equation (1)?
6. I recommend improving their English writing through the manuscript.Citation: https://doi.org/10.5194/egusphere-2023-2105-RC1 -
RC2: 'Comment on egusphere-2023-2105', Anonymous Referee #2, 12 Feb 2024
I - General comments
This manuscript introduces a new parameterization for the in-cloud scavenging of atmospheric particles. The proposed scavenging coefficient is derived from the analysis of simulation results of the DESCAM cloud-resolving model. The derived parameterization, representative at the cloud scale, can be adjusted to treat both stratus and cumulonimbus clouds. Given the considerable uncertainties that remain in the scavenging process in clouds and the way in which its impact on pollutant concentrations can be represented, this is clearly a subject of scientific interest and within the scope of atmospheric chemistry and physics. The general presentation of this interesting work is clear and straightforward, and the results are quite convincing. I think it would be interesting from time to time to expand on the discussion of the limitations associated with the choices made. I also think that some careful proofreading and changes to certain turns of phrase would be useful to improve the accuracy of the text.
II - Specific Comments
* Introduction :
l40: The reference cited, Petroff et al. (2008), relates only to dry deposition, whereas the sentence seems intended to cover all the deposition processes. Probably that some other relevant publications could be added here (e.g. Modeling the Processing of Aerosol and Trace Gases in Clouds and Fogs, Barbara Ervens, Chemical Reviews 2015 115 (10), 4157-4198, DOI: 10.1021/cr5005887)
l51: The reference cited, Querel et al (2021), is focused on operational atmospheric transport model devoted to radionuclides dispersion and certainly not summarised "all these models".
Definition and theoretical context :
l67: It might be useful to remember that the equivalence of the scavenging coefficient for number and mass is based on the assumption of homogeneous particle densities and morphology (and potentially other properties) for the size class under consideration. The scavenging coefficient should otherwise be distinguished.
l90-92 and equation 5: For greater clarity, I suggest specifying that the cloud volume nu_Vdrop is a function of D_drop.
l140-150 : Insofar as the equations provided are not explicit, I am not fully convinced that this is the most relevant way to present the processes considered in DESCAM? A simple text could probably be just as effective? Nevertheless if the equations are kept, several typos need to be corrected :
- equation 9 : subscript Köhler is not introduced in Figure 2.
- equation 11 : a "d" should be removed I guess, the subscript "fra" is not introduced.Equation 14 : Variables of the equation are not introduced.
* Applications
l217-218 : All the DESCAM simulations appear to have been done with a kappa value representative of ammonium sulphate. But further on (l408) a comparison is proposed with previous works for caesium-137. Is the kappa value chosen representative of caesium-137?
Equation 15: If the scavenging coefficient proposed is integrated over the entire aerosol distribution, the dependence to "d_ap" should be removed.
l253 : Could the authors precise how the thresholds are chosen? It appears they differ between the cumulonimbus and the stratus simulation.
l268: Could the need for a bijective relationship between the scavenging coefficient and a set of meteorological parameters be discussed? Is this just a practical choice, or is there a fundamental reason?
Figures 8 and 10: The figure 6 shows rain rates that do not exceed 50 mm/h, but Figures 8 and 10 show values beyond 60 mm/h. Both correspond to the same simulation?
Figures 15, 16 and 17: The figure 12 shows rain rates that do not exceed 1.5 mm/h, but Figures 15, 16 and 17 show values beyond 2 mm/h. Both correspond to the same simulation?
section "Comparison with the literature"
I suggest to change the title of this section or to split this section in two. The second part of this section is interesting but has nothing to do with a comparison with literature?l428-437 : Is all this paragraph really useful to explain that the direct comparison of the parameterization derived for cumulonimbus is probably not really relevant?
l438-471 : It seems to me that the general aim of this paragraph could be largely clarified. As it stands, the main part of the paragraph is an explanation of the lower scavenging coefficient derived for cumulonimbus on the basis of a scavenging coefficient depending only on rain intensity. From my point of view, this is interesting, but not directly the final objective which is to propose a common parameterization for cumulonimbus and stratus. This objective might be the title of the last section for instance?
l453 : The variable s_v,w is not introduced?
l468-471 : The way in which the correction coefficient is finally determined is not clear to me as it stands. Could the authors provide more details on this?
Figure 20 : The differences with Figure 18 are not obvious. It would be interesting to trace the previous version of the parameterizations here.
III - Technical corrections
l32 : improving -> improve / impacting -> impactl43 : The reference to Laguionie et al. (2014) seems missing.
Figure 2 : The variables written in red and blue are very difficult to read. Another color should be considered.
l172: In the legend some exponents appear as subscripts.
l174: A semi-column is missing and one should be replaced by colon.
Equation 15: What is the meaning of the vertical bar?
Figure 7 : It seems the colors are not fully consistent (orange vs red) between the figure and the legend?
l296-297 : 0.003 cm-3 instead of 0.003 m-3
l366 : "their drop velocities" <-> "their velocities"
l392 : "status" <-> "stratus"
l408 : Section 2.1 should be section 2.3
l414: Querel et al. (2017) is not referenced?
l423: Querel et al. (2015) is not referenced?
Citation: https://doi.org/10.5194/egusphere-2023-2105-RC2
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