the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Oct 2024
Counteracting Influences of Gravitational Settling Modulate Aerosol Impacts on Cloud Base Lowering Fog Characteristics
Abstract. One common process for marine fog formation is cloud base lowering (CBL), which is frequently observed, for example, off the coast of California and in Canada's Grand Banks, as well as other foggy ocean regions. While previous studies have extensively examined the meteorological controls on CBL fog, its microphysical characteristics have received comparatively less attention. We employ PAFOG, a single-column model, to investigate the interplay among aerosols, microphysics, and CBL fog evolution under diverse meteorological conditions. We find that lower aerosol concentrations make fog formation more probable, but that if fog does form, fog water concentrations are lower. Particularly at low aerosol concentration, lower aerosol concentrations lead to earlier fog formation due to faster gravitational settling of larger droplets, which serves to flux moisture downward. Faster gravitational settling (among other mechanisms at low aerosol concentration) also suppresses entrainment at cloud top which aids in keeping the liquid water path high. However, faster gravitational settling also limits the fog water concentration through faster liquid deposition to the surface. It is these counteracting influences of gravitational settling that appear to cause both prolonged fog duration and suppressed fog water concentration. The relative strength of these counteracting influences depends on the environmental conditions.
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RC1: 'Comment on egusphere-2024-3214', Anonymous Referee #1, 27 Nov 2024
Counteracting Influences of Gravitational Settling Modulate Aerosol Impacts on Cloud Base Lowering Fog Characteristics by N. H. Pope and A. L. Igel.
General:
This paper presents the interesting results of a single a single-column model, PAFOG model used to examine the results of aerosols and microphysics on marine, cloud-based lowering which has been associated with a portion of the process that can lead to marine fog. MISTRA, a microphysics scheme was included. An idealized case was presented in which a persistent stratus deck lowers at its base, becomes fog, and then rises back up over the course of 24 hours. Model variables included aerosol concentrations, gravitational settling, cloud top entrainment, vertical moisture flux, liquid water path, surface water deposition and fog water concentration. Meteorological variables include subsidence velocity above the boundary layer, geostrophic wind speed, air-sea temperature difference and rate of change of the sea surface temperature.
Presenting the results of this model and interpreting what this means is a very reasonable and useful thing to do. This is especially helpful as all marine fog studies are incomplete due to the impossibility of covering all of the possibilities/variables or even most of the possibilities/variables, so it is useful to see what the aerosols might do by themselves even if it is with a numerical model that has limitations and there are no direct data comparisons.
The writing is well done and the figures are excellent.
I have made only small suggestions. I would also ask that the remind the readers throughout the manuscript that they are referring to the model/simulations to distinguish when the actual marine conditions are being discussed.
Specific comments:
Line 23-25: Notably, the California coast and the Grand Banks represent two types of regions highly susceptible to marine fog, characterized by cold coastal upwelling (California) or cold protected waters with nearby western ocean boundary currents (Grand Banks).
Suggest something as: Notably, the California coast and the Grand Banks represent two types of regions susceptible to marine fog with cloud base lowering. The northern California case has a summer concentration of fog occurrence related to the wind driven cold water upwelling along the coast. Atlantic Canada over the shelf (including the Grand Banks) has greater fog occurrence that includes factors of warm surface air blowing over colder water and synoptic scale forcing.
L29: Since stratus lowering is one of the most common fog…
Change to: Since stratus lowering is a fog formation mechanism along the California coast and Atlantic Canada.
Note: There is no direct statistical study to show that stratus is most common, or even that it is common in other world marine fog areas – just some studies to show that it does occur.
L38-39 Although the sea surface is cool, studies have noted that the air is nonetheless cooler than the sea surface during CBL events.
Comment: State what studies show this. It would be more correct to say: Some studies (cite) show that the air is cooler than the sea surface during CBL events.
Note: That cooler air during CBL events in Atlantic Canada is contrary to that stated earlier in this paper that it is warm air traveling over cooler water to the north that forms fog over the Grand Banks or any part of Atlantic Canada.
L39-40: The sea surface fluxes both sensible and latent heat into the boundary layer, counteracting moisture loss due to entrainment and supporting turbulent mixing
Change to: Off northern California, the sea surface fluxes of both sensible and latent heat into the boundary layer counteract moisture loss due to entrainment and supporting turbulent mixing.
L41-42: Though differential advection can play a role in the setup of a CBL fog case (Wagh et al., 2021), it is typically conceived of as a Lagrangian process occurring within a column
Change to: Though differential advection can play a role in the setup of a CBL fog case (Wagh et al., 2021), it is has been conceived of as a Lagrangian process occurring within a column…
Note: these are only a few case studies and are not a statistically supported analysis of what is typical.
L42-44: As long as the conditions of a subsidence-capped boundary layer and net cooling of the boundary layer persist, the cloud base will tend to lower.
Comment: Delete this sentence or provide citations that support this.
L64-65: While it is understood that aerosols can impact fog formed through cloud base lowering and that the response of CBL fog to aerosols is probably different to that of radiation fog, the relationship between aerosol and CBL fog is not well understood.
Comment: The focus of this manuscript is on aerosol and CBL. The comment about radiation fog is extraneous. Suggest removing it.
L67-68: our aim is examine the microphysical processes of marine CBL fogs as well as the relationship between microphysics and meteorology.
Comment: “Meteorology” is too broad . Only a few general aspects of meteorology in the form of forcing are considered in this manuscript. Better: Our aim is to examine the microphysical processes of marine CBL fogs as well as the relationship between microphysics and some general meteorological forcing conditions.
Line 84: campaign in the Grand Banks, PAFOG coupled with ERA5 advection terms performed well (Chen et al., 2021)
Comment: Change “Grand Banks” to on the “NW edge of the Grand Banks adjacent to Newfoundland”.
Line 96-98: The initialization run begins at 1400 UTC because initial tests showed this time to be associated with the daily maximum of cloud base height.
Comment: Change to: The initialization run begins at 1400 UTC because initial tests showed this time to be associated with the simulated daily maximum of cloud base height.
Line 82-83: “ the 2018 C-FOG campaign in the Grand Banks, ”
Change to: “the 2018 C-FOG campaign around the SE Newfoundland coast and the NW edge of the Grand Banks,”
Line 98-99: The initialization run begins at 1400 UTC because initial tests showed this time to be associated with the daily maximum of cloud base height.
Comment: Change to: The initialization run begins at 1400 UTC because initial tests showed this time to be associated with the simulated daily maximum of cloud base height.
Line 122-124: Three of the parameters modify the meteorological background in which our fog case is forming. The first of these "back ground" variables is the subsidence velocity above the boundary layer (wsub) from 2.5 mm/s to 3.5 mm/s (Wood and Bretherton (2004)). The second is the geostrophic wind (Ug) which is varied from 5.0 m/s to 15.0 m/s.
Comment: What marine area do these parameters apply to? The world? Are these related to N. California or the Grand Banks? Please cite the supports for this.
Line 126-127: In addition to these two parameters, we vary the rate of change of the sea surface temperature (dTsurf) from-2 to +2 K/day. This change mimics the effects of advection of the fog over a sea surface temperature gradient.
Comment: What marine area does this apply to? Please note how the advection of fog over a sea surface temperature gradient was constructed.
Line 133, Table 1.
Comment: What areas does this data represent and how was it constructed?
Line 135-184: 3.1 Overall Response.
Comment: Please note these results are the simulated responses using the values in Table 1.
Line 142. Fig. 2 Caption: CLWC, 2 times, should be CWC.
Line 143. liquid water path of fog is typically enhanced by higher Na.
Change to: liquid water path of fog is correlated with higher Na.
Line 143- 144: Figure 2 also elucidates the existence of "outlier" simulations. A subset of simulations with low Na and a cooling surface formed dense but atypically shallow fog layers.
Change to: In the lower right of Figure 2 are outlier simulations which are a subset of simulations with low Na and a cooling surface that formed dense but atypically shallow fog layers. (Yes?)
Line 145-147: These simulations all ended with a much thinner boundary layer than they began with. The height of the inversion lowers by at least 100 m, with the lowering taking place after nightfall. Once fog forms in these simulations, it does not dissipate prior to simulation end.
Change to: All of these simulations ended with a much thinner boundary layer in which the height of the inversion lowered by at least 100 m and occurred after nightfall. Once fog formed in these simulations, it did not dissipate prior to the simulation ending.
Line 149: Despite being physically reasonable the gap between
Change to: Despite being physically reasonable, the gap between
Line 153-4: Mean fog duration is calculated with non foggy simulations included with a duration of zero.
Comment: The intent of this sentence is not clear.
Figure 3 Caption:
Comment: Explain what (A), (B), (C) mean in the caption. It looks like the variables are the same in (A) and (B). Is (C) average duration difference of the fog that forms relative to the aerosol "base case"?
Line 185.
Comment: Add the highlighted to sentence:
First, why is it that increasing aerosol leads to increased liquid water path and density of fog at the surface, but decreases the likelihood of fog formation and duration of fog in the model?
Line 188.
Comment: Add the highlighted to sentence and explain what thermodynamic represents:
Furthermore, are the model processes that drive the response of fog to aerosol primarily microphysical or thermodynamic?
Line 195: Add the highlighted to the following:
between fog density and duration with respect to aerosol that differs starkly from the relationship when only external meteorological factors are varied.
Line194-5: This is not a surprise, as we would expect meteorological conditions that produce longer-lasting fog to tend towards producing denser fog as well.
Comment: Is there any marine observational evidence of a relationship between fog duration and density?
Line 197-198: This contrast suggests that the processes controlling the duration-density relationship for varying aerosol concentration are microphysical rather than thermodynamic.
Comment: Explain what is meant by “thermodynamic”.
Line 203+, footnote. If we think of the diurnal cycle as forcing fog onset and dissipation..
Comment: This footnote is not clear and should be removed.
After Line 203, Figure 4 Caption: CLWC should be changed to CWC
Line 204-5. In fact, there is essentially no relationship between fog duration and density as meteorological conditions are varied for very low Na.
Comment: This should be changed to: In fact, there is essentially no relationship between fog duration and density when meteorological conditions are varied with very low Na.
Line 205+, Figure 5 Caption: CLWC should be changed to CWC. Figure labels (A) and (B) are not used. “R2 values are shown for each Na” should be changed to “ R2 values are shown for each Na listed in figure (A)”. Change “trend lines do a good job of describing” to “trend lines represent well”
Line 207, 5b should be 5A
Line 207-208: Here, the x-axis shows the maximum thickness of subsaturated but still cloudy air at the bottom of the cloud during fog.
Comment: I think that this sentence should be rewritten or needs more explanation. Nominally, the fog layer is saturated and extends down to the surface of the ocean or land.
Line 208, thickness of subsaturated but still cloudy air,
Comment: should “cloudy air” be “misty air”?
Line 217: Such considerations then can plausibly explain why fog has a longer duration at low aerosol concentrations
Change to: Such considerations then can plausibly explain why model fog has a longer duration at low aerosol concentrations
Lines 230-245;
Comment: add the word “model” to word “fog” in this area.
Line 235: Our simulations suggest that this is not typically the case.
Comment: remove “typically”.
Line 239: In summary, we see here that meteorological conditions..
Comment: change to “In summary, we see here that model meteorological conditions…”
Line 247: through a layer of sub-saturated but cloudy air
Comment: change “cloudy” to “misty”
Line 257: when the aerosol concentration is low
Comment: after low, add “ )”
Line 258-261: The dominant mechanism by which aerosol concentration impacts entrainment is not important to this study, but the point remains that lower aerosol concentrations and the resulting larger and less-numerous droplets decrease the rate of entrainment and can lead to increased liquid water path.
Comment: Change to: Thus, lower aerosol concentrations and the resulting larger and less-numerous droplets decrease the rate of entrainment and can lead to increased liquid water path.
Line 279: In summary so far, we’ve seen that microphysical changes..,
Change to: In summary so far, we’ve seen that model microphysical changes..
Line 321-322:
Comment: change 1st 2 sentences of paragraph to:
Our study investigates the sensitivity of marine fog formed through cloud base lowering to aerosols under different meteorological conditions using PAFOG, a single column model coupled with a 2-D spectral bin microphysics scheme, MISTRA.
Line 328: a substantial layer of subsaturated but cloudy air”
Comment: Suggest change “to “a substantial layer of subsaturated air with visibilities in the range of mist.”
Line 320-356: Conclusion
Comment: The conclusion should include a statement summarizing the limitations of the PAROG model and what this possibly means to the key results.
Lines 345-355:
Comment: I don’t think that this is especially useful and should be removed.
Citation: https://doi.org/10.5194/egusphere-2024-3214-RC1 -
RC2: 'Comment on egusphere-2024-3214', Anonymous Referee #2, 13 Dec 2024
This paper describes the sensitivity of cloud-base lowering fog to microphysical characteristics, particularly the underlying aerosol concentrations. It is very well written and clearly illustrated with relevant figures, leading to simple and well evidenced conclusions. I have no hesitation in recommending for publication. I have only one suggestion that I think would enhance the paper for the reader.
Sect 3 - what is not mentioned here is the influence of radiative changes due to the different aerosol concentrations. With lower aerosol numbers, the larger droplets will also have larger effective radii. This makes the fog layer less absorbing to radiation (short-wave and long-wave), which in turn will make it less emissive. Less absorption in the short wave will tend to promote fog development (less dissipation by the SW heating), while less absorption & emission in the long wave will tend to inhibit fog development (less radiative cooling driving turbulence development at the fog top, see e.g. Boutle et al 2018). In focussing solely on microphysical and thermodynamic mechanisms, these possible effects are not discussed. I think at the very least these mechanisms should be mentioned, but it would also be useful to try to quantify how important radiative effects are on the fog development - are they negligible compared to the microphysical and thermodynamical controls (a general result), or are the radiative effects large but the SW and LW effects just happen to cancel in the current simulations making them unimportant?
L80-85 - could also be worth mentioning the Demistify intercomparison (Boutle et al 2022) here for some background on PAFOG comparison to other models.
Refs:
Boutle, I., Price, J., Kudzotsa, I., Kokkola, H. and Romakkaniemi, S. (2018). Aerosol-fog interaction and the transition to well-mixed radiation fog. Atmos. Chem. Phys., 18, 7827-7840, doi:10.5194/acp-18-7827-2018
Boutle, I., Angevine, W., Bao, J.-W., Bergot, T., Bhattacharya, R., Bott, A., Duconge, L., Forbes, R., Goecke, T., Grell, E., Hill, A., Igel, A., Kudzotsa, I., Lac, C., Maronga, B., Romakkaniemi, S., Schmidli, J., Schwenkel, J., Steeneveld, G.-J. and Vie, B. (2022). Demistify: a large-eddy simulation (LES) and single-column model (SCM) intercomparison of radiation fog. Atmos. Chem. Phys., 22, 319-333, doi:10.5194/acp-22-319-2022
Citation: https://doi.org/10.5194/egusphere-2024-3214-RC2
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