Microphysics of radiation fog and estimation of fog deposition velocity for atmospheric dispersion applications

Abida, Rachid; Nelli, Narendra; Francis, Diana; Masson, Olivier; Fonseca, Ricardo; Bosc, Emmanuel; Temimi, Marouane

doi:https://doi.org/10.5194/egusphere-2023-956

Preprints

https://doi.org/10.5194/egusphere-2023-956

Preprints

25 Jul 2023

| 25 Jul 2023

Microphysics of radiation fog and estimation of fog deposition velocity for atmospheric dispersion applications

Rachid Abida, Narendra Nelli, Diana Francis, Olivier Masson, Ricardo Fonseca, Emmanuel Bosc, and Marouane Temimi

Abstract. Fog augments the wet deposition of airborne particles entrained in its hydrometeors. This article aims to characterize fog deposition processes around the Barakah nuclear power plant (BNPP), in the United Arab Emirates (UAE), and assess the potential impact of fog on the deposition rate of radionuclides in case of an accidental release. To this end, the microphysics of twelve radiation fog events, typical in such arid climate, were measured during the winter seasons of 2021 and 2022 using a fog monitor that was deployed at the BNPP. The impact of fog deposition on the settling of radionuclides is investigated based on model simulations using the Weather Research Forecasting (WRF) model with the MYJ PBL scheme and FLEXPART.

All fog events are found to share a common feature of a bimodal distribution in droplet number concentration (N_c), with modes at 4.5 μm and 23.16 μm. It was pointed out that despite the high proportion of smaller droplets in the fog associated with the fine mode, the greatest contribution to the liquid water content (LWC) comes essentially from medium to large droplets between 10 and 35 μm. The deposition flux of fog water at the site and the fog droplet deposition velocity were estimated using an Eddy Covariance (EC) onsite. Typical mean values for fog droplet deposition velocity are found to range between 2.11 and 7.87 cm s^-1. The modeling results show that fog deposition contributed by 30–40 % to the total ground deposition of ¹³⁷Cs, highlighting the importance of incorporating fog deposition as an additional scavenging mechanism in dispersion modeling under foggy conditions.

Received: 09 May 2023 – Discussion started: 25 Jul 2023

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Rachid Abida, Narendra Nelli, Diana Francis, Olivier Masson, Ricardo Fonseca, Emmanuel Bosc, and Marouane Temimi

Status: closed

RC1:
'Comment on egusphere-2023-956', Anonymous Referee #3, 10 Aug 2023

This manuscript is a detailed description of studies of fog physics in the Abu Dhabi region in the winter seasons 2021 and 2022. Twelve events were analyzed, while not all instrumentation was operative during all 12 events. Nevertheless, the data sets bear a lot of information. The manuscript is long, providing much information about fog physics and meteorological conditions. Much of the description provided does not seem focused in terms of the scientific goal of the manuscript.
This reviewer has 2 major concern:
(1) How was the ultrasonic anemometer located with respect to the FM120 fog spectrometer? The photographs in Figure 1 suggest that these 2 instruments were not co-located. That would lead to the conclusions that all eddy covariance (EC) data with respect to turbulent LWC and fog droplet fluxes were void. Authors need to either present convincing arguments that the positions of the 2 instruments were close enough to each other (please provide exact details) for EC application, or to delete all information and data from the manuscript that refers to fog EC.
(2) Please provide evidence that a data collection rate of 1 Hz (line 145) is sufficient to compute reasonable fog LWC fluxes.
Further comments:
In the abstract, please explain the acronyms MYJ PBL scheme and FLEXPART upon their first mention
line 27: The precision of the number 23.16 is too high.
line 32: The precision of the numbers 2.11 and 7.87 is too high. Please use only 2 significant digits. In this case: 2.1 and 7.9.
line 163, Table 1: Replace Luft by Lufft
Line 214: Re-type forty-five
Lines 282 – 285: The described procedure lets this reviewer suspect that there might be a circular conclusion. Please provide evidence showing that this is not the case.
Fig. 2: In the graphical representation, data of individual fog events overlap each other. Please separate individual events from each other.
Fig. 5: Please be more specific on the shading. It is not clear what it exactly means.
Line 514 – 516: Agreed in principle. However, the reasoning is still speculative and must be classified as such in the manuscript.
Fig. 6a: The rH not even reaching 90 % in one of the events (no. 3?) needs explanation. Could it be a measuring artefact? If yes, what would that mean for the data of the other events?
Fig. 6a: In view of this reviewer, it is not useful to compute and to show composite data as presented here, because the timing of the individual fog events differs largely.
Fig. 8: It is not clear what “turbulent vertical velocity” should be. It would be much netter to show the turbulent LWC fluxes or fog droplet number fluxes.
Fig. 9 shows that during a significant portion of the fog duration, the turbulent LWC fluxes are upward. Although this is briefly mentioned in a side comment in lines 620 and 621, and although this phenomenon has been observed by several authors at other locations, this phenomenon and the potential causes should be discussed in much more detail.

Citation: https://doi.org/10.5194/egusphere-2023-956-RC1
- AC1: 'Reply on RC1', Diana Francis, 20 Oct 2023
  
  Kindly find our point-by-point response attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-956-AC1
RC2:
'Comment on egusphere-2023-956', Anonymous Referee #1, 21 Aug 2023

Please find the attachment.

Citation: https://doi.org/10.5194/egusphere-2023-956-RC2
- AC2: 'Reply on RC2', Diana Francis, 20 Oct 2023
  
  Kindly find our point-by-point response attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-956-AC2
RC3:
'Comment on egusphere-2023-956', Anonymous Referee #2, 08 Sep 2023

Introduction
In this paper the authors investigate a series of fog events near the nuclear power station Barakah, situated close to the red see in an arid environment. The investigation is performed using quite a number of different meteorological instruments on-site and numerical simulations, employing and testing various paramterisations for the planetary boundary layer physics. The final goal is to show whether and how much fog would change the contamination of soil nearby if there would be a nuclear accident with release of radioactive matter. Indeed it is found, that fog changes the deposition pattern of the radioactive material. It is said that the results depend on the type of the ground and that therefore the present results are peculiar for the considered site. It is recommended that fog deposition is considered as an additional scavenging mechanism in dispersion models.
The paper is well written and the reader has the nice feeling of a fluent read. My impression is that there are no technical flaws or unjustified statements. Most statements are put into context by mentioning results from similar studies. To my view, this paper can be published after some minor corrections which I think would improve an already quite good manuscript.
These are my minor comments:
Line 47 ff: Please rewrite "Masson et al. (2015) showed that cloud water was relevant to detect 134Cs (.....) on a longer time scale than both in aerosol and in rainwater." I don't understand this sentence. Additionally, check whether it is 134Cs or rather 137Cs.
Line 55: "the number of studies on fog in arid and semi-arid regions has caught recent attention". Please rewrite, as it says that the number of something has caught attention which is surely not meant.
Line 137: I am not happy with the expression "to measure fog microphysics". Perhaps better "to observe and quantify microphysical processes".
Line 146: Has MVD already been spelled out?
Line 147: "2 counts cm-3": Unclear. Either it must be 2 counts per ccm and unit time, or you must add over which time the counting had been integrated.
Line 156/7: Incomplete sentence "For a full description of the operational method and measurement uncertainties (..)."
Line 172: Please find a better expression than "construction structures".
Line 214: Correct "Forty_-five".
Line 223: Please rewrite "the cloud water-rain and the cloud ice-snow are treated for temperatures above and below 0°C."
Line 399: Not sure whether WESTAR was defined somehere.
Line 403/4: "it is observed that similar to the horizontal visibility, the LWC and number concentration also increase abruptly". This should be rewritten since it indicates that visibility would increase which it doesn't.
Figure 2 should be replaced. The presentation of the data is misleading. The problem is that the data from independent days are shown in the form of a contour plot (or something similar) which results in smooth transitions between these days. Instead, we have 12 independent time series and I strongly suggest to replace the plot with one showing simply the 12 timeseries as 12 single curves.
Line 416 and several other instances: You write "bin size" but you mean "size bin" which are two different things. Additionally, I do not remember whether you defined before what "size" is. Is it the droplet diameter or radius? Please check.
Line 429: Delete "While" and start the sentence "The second mode...".
Lines 432/3: Please check the numbers 4.5 vs 5.5 micron. As I understood, there is only one small mode, that should be characterised by only one value.
Line 437: I suggest to replace "discrepancies" with "differences". Discrepancies implies to me that there is something inconsistent, but that seems not implied here.
Line 474: It seems that onset/temination times and durations are inconsistent here and in table 3.
Line 477: "droplet sizes between 20 and 30". Add units.
Figure 3: x-axis label should be radius or diamenter, but not bin.
Line 528: Check "number concentration were observed Figs. 2a-c". I suggest also that you would better write "number concentrations were measured".
Line 534 and Figure A2: Use "backscatter profiles" instead of "backscattered profiles" (The profiles themselves are not backscattered).
Lines 579/80: Please rewrite "Settling velocities calculated every 5 min and values with visibility greater than 1 km are discarded (Fig. 7b)." I don't understand what you mean.
LIne 588: What is an "increasing relationship"?
Line 681: Here you should repeat or mention the PBL schemes that you use, for the convenience of the reader. A few words on each PBL schemes particular properties and abilities would be helpful as well.
Line 697: Replace "ribbon" with "band".
Line 707: Please use a simple dot or an "x" instead of a star for multiplication. In the figures it is a "x" which is ok.
Line 734: write "long" instead of "longer".
Lines 773 ff: Please rewrite "the actual impact of fog on radionuclide deposition can vary widely depending on the specific situation as well as more on the solubility and chemical form of the radionuclide-labeled particles." I don't understand the second half of the sentence beginning from "as well as...".
Line 807: What is a "size distribution of the mean number of droplets"?

Citation: https://doi.org/10.5194/egusphere-2023-956-RC3
- AC3: 'Reply on RC3', Diana Francis, 20 Oct 2023
  
  Kindly find our point-by-point response attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-956-AC3

Status: closed

RC1:
'Comment on egusphere-2023-956', Anonymous Referee #3, 10 Aug 2023

This manuscript is a detailed description of studies of fog physics in the Abu Dhabi region in the winter seasons 2021 and 2022. Twelve events were analyzed, while not all instrumentation was operative during all 12 events. Nevertheless, the data sets bear a lot of information. The manuscript is long, providing much information about fog physics and meteorological conditions. Much of the description provided does not seem focused in terms of the scientific goal of the manuscript.
This reviewer has 2 major concern:
(1) How was the ultrasonic anemometer located with respect to the FM120 fog spectrometer? The photographs in Figure 1 suggest that these 2 instruments were not co-located. That would lead to the conclusions that all eddy covariance (EC) data with respect to turbulent LWC and fog droplet fluxes were void. Authors need to either present convincing arguments that the positions of the 2 instruments were close enough to each other (please provide exact details) for EC application, or to delete all information and data from the manuscript that refers to fog EC.
(2) Please provide evidence that a data collection rate of 1 Hz (line 145) is sufficient to compute reasonable fog LWC fluxes.
Further comments:
In the abstract, please explain the acronyms MYJ PBL scheme and FLEXPART upon their first mention
line 27: The precision of the number 23.16 is too high.
line 32: The precision of the numbers 2.11 and 7.87 is too high. Please use only 2 significant digits. In this case: 2.1 and 7.9.
line 163, Table 1: Replace Luft by Lufft
Line 214: Re-type forty-five
Lines 282 – 285: The described procedure lets this reviewer suspect that there might be a circular conclusion. Please provide evidence showing that this is not the case.
Fig. 2: In the graphical representation, data of individual fog events overlap each other. Please separate individual events from each other.
Fig. 5: Please be more specific on the shading. It is not clear what it exactly means.
Line 514 – 516: Agreed in principle. However, the reasoning is still speculative and must be classified as such in the manuscript.
Fig. 6a: The rH not even reaching 90 % in one of the events (no. 3?) needs explanation. Could it be a measuring artefact? If yes, what would that mean for the data of the other events?
Fig. 6a: In view of this reviewer, it is not useful to compute and to show composite data as presented here, because the timing of the individual fog events differs largely.
Fig. 8: It is not clear what “turbulent vertical velocity” should be. It would be much netter to show the turbulent LWC fluxes or fog droplet number fluxes.
Fig. 9 shows that during a significant portion of the fog duration, the turbulent LWC fluxes are upward. Although this is briefly mentioned in a side comment in lines 620 and 621, and although this phenomenon has been observed by several authors at other locations, this phenomenon and the potential causes should be discussed in much more detail.

Citation: https://doi.org/10.5194/egusphere-2023-956-RC1
- AC1: 'Reply on RC1', Diana Francis, 20 Oct 2023
  
  Kindly find our point-by-point response attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-956-AC1
RC2:
'Comment on egusphere-2023-956', Anonymous Referee #1, 21 Aug 2023

Please find the attachment.

Citation: https://doi.org/10.5194/egusphere-2023-956-RC2
- AC2: 'Reply on RC2', Diana Francis, 20 Oct 2023
  
  Kindly find our point-by-point response attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-956-AC2
RC3:
'Comment on egusphere-2023-956', Anonymous Referee #2, 08 Sep 2023

Introduction
In this paper the authors investigate a series of fog events near the nuclear power station Barakah, situated close to the red see in an arid environment. The investigation is performed using quite a number of different meteorological instruments on-site and numerical simulations, employing and testing various paramterisations for the planetary boundary layer physics. The final goal is to show whether and how much fog would change the contamination of soil nearby if there would be a nuclear accident with release of radioactive matter. Indeed it is found, that fog changes the deposition pattern of the radioactive material. It is said that the results depend on the type of the ground and that therefore the present results are peculiar for the considered site. It is recommended that fog deposition is considered as an additional scavenging mechanism in dispersion models.
The paper is well written and the reader has the nice feeling of a fluent read. My impression is that there are no technical flaws or unjustified statements. Most statements are put into context by mentioning results from similar studies. To my view, this paper can be published after some minor corrections which I think would improve an already quite good manuscript.
These are my minor comments:
Line 47 ff: Please rewrite "Masson et al. (2015) showed that cloud water was relevant to detect 134Cs (.....) on a longer time scale than both in aerosol and in rainwater." I don't understand this sentence. Additionally, check whether it is 134Cs or rather 137Cs.
Line 55: "the number of studies on fog in arid and semi-arid regions has caught recent attention". Please rewrite, as it says that the number of something has caught attention which is surely not meant.
Line 137: I am not happy with the expression "to measure fog microphysics". Perhaps better "to observe and quantify microphysical processes".
Line 146: Has MVD already been spelled out?
Line 147: "2 counts cm-3": Unclear. Either it must be 2 counts per ccm and unit time, or you must add over which time the counting had been integrated.
Line 156/7: Incomplete sentence "For a full description of the operational method and measurement uncertainties (..)."
Line 172: Please find a better expression than "construction structures".
Line 214: Correct "Forty_-five".
Line 223: Please rewrite "the cloud water-rain and the cloud ice-snow are treated for temperatures above and below 0°C."
Line 399: Not sure whether WESTAR was defined somehere.
Line 403/4: "it is observed that similar to the horizontal visibility, the LWC and number concentration also increase abruptly". This should be rewritten since it indicates that visibility would increase which it doesn't.
Figure 2 should be replaced. The presentation of the data is misleading. The problem is that the data from independent days are shown in the form of a contour plot (or something similar) which results in smooth transitions between these days. Instead, we have 12 independent time series and I strongly suggest to replace the plot with one showing simply the 12 timeseries as 12 single curves.
Line 416 and several other instances: You write "bin size" but you mean "size bin" which are two different things. Additionally, I do not remember whether you defined before what "size" is. Is it the droplet diameter or radius? Please check.
Line 429: Delete "While" and start the sentence "The second mode...".
Lines 432/3: Please check the numbers 4.5 vs 5.5 micron. As I understood, there is only one small mode, that should be characterised by only one value.
Line 437: I suggest to replace "discrepancies" with "differences". Discrepancies implies to me that there is something inconsistent, but that seems not implied here.
Line 474: It seems that onset/temination times and durations are inconsistent here and in table 3.
Line 477: "droplet sizes between 20 and 30". Add units.
Figure 3: x-axis label should be radius or diamenter, but not bin.
Line 528: Check "number concentration were observed Figs. 2a-c". I suggest also that you would better write "number concentrations were measured".
Line 534 and Figure A2: Use "backscatter profiles" instead of "backscattered profiles" (The profiles themselves are not backscattered).
Lines 579/80: Please rewrite "Settling velocities calculated every 5 min and values with visibility greater than 1 km are discarded (Fig. 7b)." I don't understand what you mean.
LIne 588: What is an "increasing relationship"?
Line 681: Here you should repeat or mention the PBL schemes that you use, for the convenience of the reader. A few words on each PBL schemes particular properties and abilities would be helpful as well.
Line 697: Replace "ribbon" with "band".
Line 707: Please use a simple dot or an "x" instead of a star for multiplication. In the figures it is a "x" which is ok.
Line 734: write "long" instead of "longer".
Lines 773 ff: Please rewrite "the actual impact of fog on radionuclide deposition can vary widely depending on the specific situation as well as more on the solubility and chemical form of the radionuclide-labeled particles." I don't understand the second half of the sentence beginning from "as well as...".
Line 807: What is a "size distribution of the mean number of droplets"?

Citation: https://doi.org/10.5194/egusphere-2023-956-RC3
- AC3: 'Reply on RC3', Diana Francis, 20 Oct 2023
  
  Kindly find our point-by-point response attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-956-AC3

Rachid Abida, Narendra Nelli, Diana Francis, Olivier Masson, Ricardo Fonseca, Emmanuel Bosc, and Marouane Temimi

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Short summary

This study is the first application of the Eddy Covariance (EC) framework to measure the fog droplet deposition velocity in a hyperarid coastal site. The average deposition velocity of fog droplets is around 3 cm s^-1. The ratio of the time-integrated ground deposition of ¹³⁷Cs under foggy conditions to that under clear sky conditions, showed that the fog contributed to the total ground deposition of ¹³⁷Cs by up to 40 %.


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