the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Dec 2024
High sensitivity of simulated fog properties to parameterized aerosol activation in case studies from ParisFog
Abstract. Aerosols influence fog properties such as visibility and lifetime by affecting fog droplet number concentrations (Nd). Numerical weather prediction (NWP) models often represent aerosol-fog interactions using highly simplified approaches. Incorporating prognostic size-resolved aerosol microphysics from climate models could allow them to simulate Nd and aerosol-fog interactions without incurring excessive computational expense. However, microphysics code designed for coarse spatial resolution may struggle with sub-kilometer-scale grid spacings. Here we test the ability of the UK Met Office Unified Model to simulate aerosol and fog properties during case studies from the ParisFog field campaign in 2011. We examine the sensitivity of fog properties to variations in Nd caused by modifications to simulated aerosol activation.
Our model with 500 m horizontal resolution and interactive aerosol and cloud microphysics significantly underpredicts Nd, although only slightly underestimates the cloud condensation nuclei concentration. With an updated version of the Abdul-Razzak and Ghan (2000) activation scheme, we produce Nd that are more consistent with those predicted by a cloud parcel model under fog-like conditions. We activate droplets only by adiabatic cooling. We incorporate more realistic hygroscopicities for sulfate and organic aerosols and explore the sensitivity of simulated Nd to unresolved updrafts. We find that both Nd and simulated fog liquid water content are very sensitive to the updated activation scheme but remain unaffected by the update to hygroscopicities. Our improvements offer insights into the physical processes regulating Nd in stable conditions, potentially laying foundations for improved operational fog forecasts that incorporate interactive aerosol simulations or aerosol climatologies.
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RC1: 'Comment on egusphere-2024-3376', Anonymous Referee #3, 27 Dec 2024
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Accurately forecasting fog is currently a challenge for many operational numerical weather prediction models, with important implications in both weather and climate prediction. It has been shown in previous work (eg. Boutle et al 2018) that improper aerosol activation can lead to poor fog forecasts. This paper presents a detailed analysis of fog properties in model simulations as compared to observations for 11 fog events, and discusses the impact of aerosol activation as well as several modifications to the aerosol activation scheme. It is shown that the model, with 500m grid spacing, is capable of producing fog with reasonable droplet concentrations (Nd), and that Nd is sensitive to the aerosol activation parameterization. The authors include a discussion of the limitations of their study. Recommendation: Accept with minor revisions
General Comments:
It may be helpful to the reader to include a table or graphic of the various schemes used in the various model components. I found the model description long and somewhat confusing, due to the 3 different model components, each using different parameterizations (just a suggestion).
Given that the Mod-ARG scheme significantly improves the results, how important are the multiple aerosol modes?
The role of sedimentation is only very briefly discussed, but could be important in removing droplets and aerosols. Was there any evaluation of the surface deposition due to sedimentation? Was there precipitation in any of the cases?
It is somewhat surprising to see a lack of sensitivity to aerosol hygroscopicity. Perhaps, as the authors suggest, that is due to the type of fog that is studied here.
This study included a complex chemistry scheme, which is probably too expensive for typical weather and climate forecasting. Could the authors comment on the minimum requirements needed in NWP to produce reasonable fog simulations?
The paper is long. I would encourage the authors to consider ways to shorten the text.
Specific comments, typos, etc:
Line 51: thermodynamic is misspelled.
Table 1: Welas 2020? Or WELAS-2000 as in lines 137 and 138?
Line 166: The author should be van Weverberg, I believe.
Fig. 5 (and others): Please clarify if the model profile is a point value or an average over multiple points?
Fig. 12 caption refers to S9, but it looks like that should be S10.
Line 496: Are the 2 numbers referring to the two events on the 16th? Please clarify.
Line 503: Subfigure is misspelled.
Line 506: November is misspelled.
Line 602: “subfigures (a,b,c)” should be subfigures (a,d,g).
Fig. 15 (b, e, h): Retrievals is misspelled.
In the supplemental material:
Fig. S2: Add to the caption the meaning of the shading (showing the observed foggy periods, presumably)
Figures S5 and S6 appear to be out of order.
Fig. S8, line 2 of the caption: Omit either “boxes” or “gridboxes”.
Fig. S12: The legend in the figure doesn’t match the experiment description in the caption.
Citation: https://doi.org/10.5194/egusphere-2024-3376-RC1 -
RC2: 'Comment on egusphere-2024-3376', Anonymous Referee #1, 06 Jan 2025
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Review of “High sensitivity of simulated fog properties to parameterized aerosol activation in case studies from ParisFog” by Ghosh et al.
This study presents simulations of multiple fog cases observed during the ParisFog field campaign. Updates to the aerosol activation scheme are shown to improve the simulation of droplet number concentration. These updates include more realistic hygroscopicities and updated parameters that control activation rate. These updated parameters are found to have a much larger impact than the hygroscopicities. The improved droplet concentration prediction does not necessarily improve the liquid water prediction. The improved Abdul-Razzak-Ghan scheme could potentially be used by others. The manuscript is long and could possibly be shortened. But otherwise, I don’t have any major concerns with the study. It is a straightforward comparison of simulations with observations. The comparison is particularly comprehensive in this manuscript.
Specific Comments:
- It would be useful to mention at some point how droplet sedimentation to the surface is handled (if at all).
- I don’t think that section 5.5 adds much to the manuscript. Consider removing it.
- Figure 1 should have panel labels since they are referenced in the caption.
- I believe that units should have spaces between components, e.g. on Line 265 ms-1 technically reads as per millisecond, not meters per second. Use m s-1 Same for gm-3.
- Figure 3 – presumably the temperatures are cloud top temperatures?
- Line 671 – I wouldn’t state that LWC can be realistically simulated “most of the time”. Even previously in the conclusions the authors only state “some of the time”.
- A few typos/extra words to note: Lines 51, 135 (“the point at which point” is awkward), 503, 662 (grammatical parallelism).
Citation: https://doi.org/10.5194/egusphere-2024-3376-RC2
Data sets
High sensitivity of simulated fog properties to parameterized aerosol activation in case studies from ParisFog Pratapaditya Ghosh, Ian Boutle, Paul Field, Adrian Hill, Anthony Jones, Marie Mazoyer, Katherine J. Evans, Salil Mahjan, Hyun-Gyu Kang, Min Xu, Wei Zhang, Noah Asch, and Hamish Gordon https://doi.org/10.5281/zenodo.14004871
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