the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jul 2024
Valley floor inclination affecting valley winds and transport of passive tracers in idealised simulations
Abstract. In mountainous regions, diurnal thermally-driven winds impact daily weather and air-quality. This study investigates how the inclination of idealised valleys affects these winds and the transport of passive tracers using high-resolution numerical simulations with the Weather Research and Forecasting (WRF) model. We explore a range of valley inclinations between 0 and 2.28 degrees, bridging the gap between previous studies on flat and moderately inclined (up to 0.86 degrees) idealised valleys and steeper (2–5 degrees) real Himalayan valleys. We find that in the inclined valleys during the daytime the up-valley winds penetrate deeper into the valleys and are strengthen, up to a critical angle beyond which the winds weaken. Flat-floored valleys exhibit the strongest night-time down-valley winds overall, but surface-based down-valley winds are more prominent in inclined valleys. Steeper valleys enhance the vertical transport of passive tracers, resulting in ventilation at higher altitudes compared to the flat-floored valley. Despite stronger overall tracer outflow in the flat valley, this occurs at lower altitudes, leading most of the ventilated tracers being accumulated in the lowest few kilometers of the atmosphere. Consequently, steeper valleys are more efficient in ventilating tracers to the upper troposphere, which would for example lead to higher potential for long-range transport. These findings underscore the critical role of valley geometry in shaping wind patterns and pollutant transport, providing valuable insights for improving transport modeling in mountainous regions.
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RC1: 'Comment on egusphere-2024-1900', Anonymous Referee #1, 06 Aug 2024
Abstract - What are the “surface-based down-valley winds”? Is a surface-based wind an anabatic/katabatic wind? Please clarify this concept.
L35/36- It is unclear to the reviewer why a steeper slope angle would decrease the mass flux resulting in a weaker flow magnitude. The buoyancy force that drives the anabatic/katabatic winds is proportional to sin(a), where a is the slope angle, increasing as the angle increases. Please clarify.
L41/43- the description of the air volume heating above the valley and plain is not clear to me. Can it be rephrased to be clearer?
L211- “a typical isentropic structure associated with up-valley winds”, is the word isentropic correctly used? It seems that an isentropic process is a (idealised) thermodynamic process both anabatic and reversible. One might think that in this real situation, the dynamics are neither anabolic nor reversible, so one wonders whether the term chosen is correct. Please clarify the choice and add references in this field on the use of the term.
Figure 3a- It seems curious that in the presence of a flat valley and no atmospheric wind, a current is generated entering the valley (see -10km<x<50km). How can this phenomenon be explained?
L209/218- The description of Figure 3 can be made clearer, it is a bit hard to follow.
L229- It is not clear to the reviewer by what mechanism the air heats up more in I1, I2 configurations while it is much less hot and the flow seems less intense for I3. This seems contradictory, what explanation do the authors give?
L230- The recirculation cells are not visible from the plot display. Please show the cells if you wish to discuss them.
L240- Please specify to which “similar isentrope structure” you are referring, and check if isentrope structure is a good terminology, used in litterature for these types of phenomena. Reference needed.
L241/242- “This suggests that the buoyancy-driven anabatic winds are contributing in the daytime to the up-valley winds near the surface.” If the reviewer has correctly understood, the convection is the only force triggering the flow as the horizontal wind components are set to zero. If not, this point can made clearer.
Section 3.1.2- It seems that these results do not add anything to the discussion. This section can be removed or the authors can highlight the information added to the discussion in this section.
L302/303- Also here, it seems very strange that the down-slope flow develops where no slope is present. How this can be justified?
L303- What is the accuracy of the WRF model? The error in temperature can be around 0.5K, what about the velocity magnitude? is it possible that a velocity of 0.5m/s is within the numerical error of the simulation?
Section 3.1.1- Authors may consider shortening this section and making it more concise, as done for section 3.1.3.
L356- The formula for computing this quantity can be reported for clarity. Is this quantity made non-dimensional with respect to the total volume? It seems strange that the FLAT case exhibits the largest values.
Figure 6- what is the formula used to compute this quantity’ Please report it for completeness.
L402/following- This is a very interesting finding that can be expanded. I think that authors can report Eq. 5 and show some more analyses to gain a deeper knowledge of this mechanism, which is a bit counterintuitive.
Figure 7,8,9- Please add the expression of the formulas used to compute the quantity plotted.
Figure 10- Percentages are not all readable.
Section 4- The reviewer believes that the manuscript is very long and prolix in some parts. The analyses are extensively discussed and, in this section, information that has already been read previously in the text is reported again, adding a further very long and verbose discussion. It is recommended that the manuscript be shortened by making it more concise and focused on points of interest. With this aim, this section can be integrated into the previous section 3, or it should be greatly shortened by eliminating all previously reported information.
L653- As already mentioned, this is a very interesting point that deserves some more analysis and a dedicated point in the conclusion, in the opinion of the reviewer.
L16- “Diurnal valley and slope winds” is a bit confusing, did you mean “diurnal and slop winds”.
Citation: https://doi.org/10.5194/egusphere-2024-1900-RC1 -
RC2: 'Comment on egusphere-2024-1900', Anonymous Referee #2, 09 Sep 2024
General note: I find Introduction, Results, and Discussion to be too lengthy. However, this may be influenced by my bias towards terse writing and shorter texts, so keep that in mind! I understand that it might be necessary for the potential readers. That is why I tried to point out how to simplify the sentences.
Abstract
3: We explore a range of the valley inclinations…
3-4: We explore a range of valley inclinations from 0 to 2.28 degrees.
5: the sentence could be written better. A suggestion:
We find that during daytime in the inclined valleys, up-valley winds penetrate deeper [into the valleys] and become stronger, up to a critical inclination [angle] beyond which the winds weaken.
- The text within [ ] is implied and I think the sentence works without it as well.
- additional comma after inclined valleys adds to readability
- “inclination” instead of “angle” - angle might be a bit ambigious since it might refer to the angle of the wind. Changing the word to “inclination” more clearly refers to the valley inclination. The sentence also works without “angle”, but it can be added.
- “daytime” without “the” - you are not referring to the specific daytime that was mentioned before or that awaits the reader in the paper.
7-9: Flat-floored valleys exhibit the strongest night-time down-valley winds overall, but surface-based down-valley winds are more prominent in inclined valleys. - ?
9: flat-floored valleys - consistency with the previous.
11: at ventilating tracers
11: which would, for example, lead to… - “for example” is parenthetical
Introduction
25-36 (second paragraph): is the explanation of valley winds necessary? Vergeiner and Dreiseitl (1987) is cited 4 times. Line 34-36 is fine, but before it, it seems too lenghtly. Of course, if this kind of explanation is required for the readers of this paper, discard this comment.
36:”hence result as weaker flow magnitude and flow depth.” - suggestion: “resulting in a weaker flow magnitude and shallower flow depth.”
97: “… ends up”.
Methods
Generally, I find no objections to the Methods section, except a few suggestions below.
115: provide a reference for the local time (LT = UTC + ?). Also, write that, e.g. 18:00, refers to LT.
165-167: instead of 0 km < y < 10 km, etc., just: 0-10 km, 20-30 km, 40-50 km? Or $y\in<0,10>. Same for x. Just a suggestion.
193: inclination, meaning they…
195-196: Unclear sentence, missing 2 commas. Suggestion: For the case SLOPE, the flux components are the same as those shown by F1-F3, but they account for the entire domain in the x-direction due to the homogenity of the slope.
Results
Most of the results clear, with occassional dips in clarity. I find it a bit lenghty though, like the introduction.
3.1.1: inconsistent use of CBLv and CBL_v (same for p). Please correct.
256: Here, the term \textit{main cell circulation} refers to…
293: surface-based
Section 3.1.4. - I have comments and suggestions.
First, the clarity of the following lines should be improved:
- 336: “valley-volume-averaged” would clarify it as a compound-adjective
- 338: valley-averaged
- 342: A diurnal cylce?
- 342: down-valley wind at night…
- 345: up-valley winds occur in case I1…
- 345: I3 which aligns with Fig.3
- 348: values in Fig 5., it would appear that the case FLAT clearly has the strongest…
- 349-351: This difference is due to the down-valley-directed return flow, which is mostly outside the valley volume in case FLAT (Section 3.1.1) and, therefore, does not affect the valley volume average as much as in the inclined cases.
- 370: on the second day (Fig 5b).
Then, Figure 5 could use a bolder horizontal line at 0. Also, what about making the y-limits identical: -1.0 to 3.0.? It would make the figures a pair. It was hard to read the figure as presented due to the y-limits that kept mixing in my head. Additionally, section 3.1.4 jumps between 5a and 5b, so this different y-limits really get annoying. Consider making the captions at least 1 fontsize larger.
Finally, when I, as a reader, am first introduced to fig 5, you make me guess: why is only the upslope component shown in b? I expect to analyse the 5a, since it is a more conventional average, and then for you to lead me into 5b and reveal the reason behind this choice. However, I am not able to comprehend the message of this section without reading it VERY carefully and multiple times. If you spent more time with 5a, and then wrote: “(…) values in Fig 5., it would appear that the case FLAT clearly has the strongest (…)”, you would naturally lead me, the reader, to the reasoning and make your case better and my reading easier.
I have made suggestions for clarity, but consider restructuring this paragraph and modyfing figure 5.
375-380: could use a more clearer phrasing. Instead of commenting everything, I rewrote it:
“””
In the volume-averaged along-slope wind, case SLOPE shows much weaker winds compared to case I2 (Fig. 5a). This is because its return flow is entirely located within the analysis volume (Section 3.1.1), which significantly reduces the volume-averaged up-slope wind. The 2-km-deep analysis volume in case SLOPE was chosen to correspond to the ridge height in the valley simulations. However, when averaging only the positive along-slope winds, the flow strength in case SLOPE is comparable to that in case I2 (Fig. 5b). In the morning, the winds in case SLOPE are slightly stronger than those in the valley simulations.
“””
387: For a detailed explanation how the plotted variable in Fig 6 is derived, refer to Appendix A.
Figure 6: again, confused me a bit due to the unequal y-ranges. How about minusing the 6c and then making an equal y-range?
484: In the second day, the ventilation…
494-500: simplify the phrases and remove unnecessary words (In case SLOPE, nearly all the transport), brake up the sentence about the first and second days, change “drastic” to “significant”
515: are weak, so the…
Discussion
562: In Wagner et al (2015a), the ridges…
591: inclination, and …
598: ,but
Conclusions
651: allows
657-660: Use “Steeper valley floors” OR “inclined valleys” instead of “steeper valleys”? Sure, it is implied, but if someone just reads the conclusion, it might be ambigious since you are using 2 terms.
All in all, I find the paper interesting, particularly in the way it presents the tracer export/outflow as an interplay between the cross-valley winds, valley volume effect, and buoyancy forcing due to heating.
Citation: https://doi.org/10.5194/egusphere-2024-1900-RC2 -
AC1: 'Comment on egusphere-2024-1900', Johannes Mikkola, 04 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1900/egusphere-2024-1900-AC1-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2024-1900', Anonymous Referee #1, 06 Aug 2024
Abstract - What are the “surface-based down-valley winds”? Is a surface-based wind an anabatic/katabatic wind? Please clarify this concept.
L35/36- It is unclear to the reviewer why a steeper slope angle would decrease the mass flux resulting in a weaker flow magnitude. The buoyancy force that drives the anabatic/katabatic winds is proportional to sin(a), where a is the slope angle, increasing as the angle increases. Please clarify.
L41/43- the description of the air volume heating above the valley and plain is not clear to me. Can it be rephrased to be clearer?
L211- “a typical isentropic structure associated with up-valley winds”, is the word isentropic correctly used? It seems that an isentropic process is a (idealised) thermodynamic process both anabatic and reversible. One might think that in this real situation, the dynamics are neither anabolic nor reversible, so one wonders whether the term chosen is correct. Please clarify the choice and add references in this field on the use of the term.
Figure 3a- It seems curious that in the presence of a flat valley and no atmospheric wind, a current is generated entering the valley (see -10km<x<50km). How can this phenomenon be explained?
L209/218- The description of Figure 3 can be made clearer, it is a bit hard to follow.
L229- It is not clear to the reviewer by what mechanism the air heats up more in I1, I2 configurations while it is much less hot and the flow seems less intense for I3. This seems contradictory, what explanation do the authors give?
L230- The recirculation cells are not visible from the plot display. Please show the cells if you wish to discuss them.
L240- Please specify to which “similar isentrope structure” you are referring, and check if isentrope structure is a good terminology, used in litterature for these types of phenomena. Reference needed.
L241/242- “This suggests that the buoyancy-driven anabatic winds are contributing in the daytime to the up-valley winds near the surface.” If the reviewer has correctly understood, the convection is the only force triggering the flow as the horizontal wind components are set to zero. If not, this point can made clearer.
Section 3.1.2- It seems that these results do not add anything to the discussion. This section can be removed or the authors can highlight the information added to the discussion in this section.
L302/303- Also here, it seems very strange that the down-slope flow develops where no slope is present. How this can be justified?
L303- What is the accuracy of the WRF model? The error in temperature can be around 0.5K, what about the velocity magnitude? is it possible that a velocity of 0.5m/s is within the numerical error of the simulation?
Section 3.1.1- Authors may consider shortening this section and making it more concise, as done for section 3.1.3.
L356- The formula for computing this quantity can be reported for clarity. Is this quantity made non-dimensional with respect to the total volume? It seems strange that the FLAT case exhibits the largest values.
Figure 6- what is the formula used to compute this quantity’ Please report it for completeness.
L402/following- This is a very interesting finding that can be expanded. I think that authors can report Eq. 5 and show some more analyses to gain a deeper knowledge of this mechanism, which is a bit counterintuitive.
Figure 7,8,9- Please add the expression of the formulas used to compute the quantity plotted.
Figure 10- Percentages are not all readable.
Section 4- The reviewer believes that the manuscript is very long and prolix in some parts. The analyses are extensively discussed and, in this section, information that has already been read previously in the text is reported again, adding a further very long and verbose discussion. It is recommended that the manuscript be shortened by making it more concise and focused on points of interest. With this aim, this section can be integrated into the previous section 3, or it should be greatly shortened by eliminating all previously reported information.
L653- As already mentioned, this is a very interesting point that deserves some more analysis and a dedicated point in the conclusion, in the opinion of the reviewer.
L16- “Diurnal valley and slope winds” is a bit confusing, did you mean “diurnal and slop winds”.
Citation: https://doi.org/10.5194/egusphere-2024-1900-RC1 -
RC2: 'Comment on egusphere-2024-1900', Anonymous Referee #2, 09 Sep 2024
General note: I find Introduction, Results, and Discussion to be too lengthy. However, this may be influenced by my bias towards terse writing and shorter texts, so keep that in mind! I understand that it might be necessary for the potential readers. That is why I tried to point out how to simplify the sentences.
Abstract
3: We explore a range of the valley inclinations…
3-4: We explore a range of valley inclinations from 0 to 2.28 degrees.
5: the sentence could be written better. A suggestion:
We find that during daytime in the inclined valleys, up-valley winds penetrate deeper [into the valleys] and become stronger, up to a critical inclination [angle] beyond which the winds weaken.
- The text within [ ] is implied and I think the sentence works without it as well.
- additional comma after inclined valleys adds to readability
- “inclination” instead of “angle” - angle might be a bit ambigious since it might refer to the angle of the wind. Changing the word to “inclination” more clearly refers to the valley inclination. The sentence also works without “angle”, but it can be added.
- “daytime” without “the” - you are not referring to the specific daytime that was mentioned before or that awaits the reader in the paper.
7-9: Flat-floored valleys exhibit the strongest night-time down-valley winds overall, but surface-based down-valley winds are more prominent in inclined valleys. - ?
9: flat-floored valleys - consistency with the previous.
11: at ventilating tracers
11: which would, for example, lead to… - “for example” is parenthetical
Introduction
25-36 (second paragraph): is the explanation of valley winds necessary? Vergeiner and Dreiseitl (1987) is cited 4 times. Line 34-36 is fine, but before it, it seems too lenghtly. Of course, if this kind of explanation is required for the readers of this paper, discard this comment.
36:”hence result as weaker flow magnitude and flow depth.” - suggestion: “resulting in a weaker flow magnitude and shallower flow depth.”
97: “… ends up”.
Methods
Generally, I find no objections to the Methods section, except a few suggestions below.
115: provide a reference for the local time (LT = UTC + ?). Also, write that, e.g. 18:00, refers to LT.
165-167: instead of 0 km < y < 10 km, etc., just: 0-10 km, 20-30 km, 40-50 km? Or $y\in<0,10>. Same for x. Just a suggestion.
193: inclination, meaning they…
195-196: Unclear sentence, missing 2 commas. Suggestion: For the case SLOPE, the flux components are the same as those shown by F1-F3, but they account for the entire domain in the x-direction due to the homogenity of the slope.
Results
Most of the results clear, with occassional dips in clarity. I find it a bit lenghty though, like the introduction.
3.1.1: inconsistent use of CBLv and CBL_v (same for p). Please correct.
256: Here, the term \textit{main cell circulation} refers to…
293: surface-based
Section 3.1.4. - I have comments and suggestions.
First, the clarity of the following lines should be improved:
- 336: “valley-volume-averaged” would clarify it as a compound-adjective
- 338: valley-averaged
- 342: A diurnal cylce?
- 342: down-valley wind at night…
- 345: up-valley winds occur in case I1…
- 345: I3 which aligns with Fig.3
- 348: values in Fig 5., it would appear that the case FLAT clearly has the strongest…
- 349-351: This difference is due to the down-valley-directed return flow, which is mostly outside the valley volume in case FLAT (Section 3.1.1) and, therefore, does not affect the valley volume average as much as in the inclined cases.
- 370: on the second day (Fig 5b).
Then, Figure 5 could use a bolder horizontal line at 0. Also, what about making the y-limits identical: -1.0 to 3.0.? It would make the figures a pair. It was hard to read the figure as presented due to the y-limits that kept mixing in my head. Additionally, section 3.1.4 jumps between 5a and 5b, so this different y-limits really get annoying. Consider making the captions at least 1 fontsize larger.
Finally, when I, as a reader, am first introduced to fig 5, you make me guess: why is only the upslope component shown in b? I expect to analyse the 5a, since it is a more conventional average, and then for you to lead me into 5b and reveal the reason behind this choice. However, I am not able to comprehend the message of this section without reading it VERY carefully and multiple times. If you spent more time with 5a, and then wrote: “(…) values in Fig 5., it would appear that the case FLAT clearly has the strongest (…)”, you would naturally lead me, the reader, to the reasoning and make your case better and my reading easier.
I have made suggestions for clarity, but consider restructuring this paragraph and modyfing figure 5.
375-380: could use a more clearer phrasing. Instead of commenting everything, I rewrote it:
“””
In the volume-averaged along-slope wind, case SLOPE shows much weaker winds compared to case I2 (Fig. 5a). This is because its return flow is entirely located within the analysis volume (Section 3.1.1), which significantly reduces the volume-averaged up-slope wind. The 2-km-deep analysis volume in case SLOPE was chosen to correspond to the ridge height in the valley simulations. However, when averaging only the positive along-slope winds, the flow strength in case SLOPE is comparable to that in case I2 (Fig. 5b). In the morning, the winds in case SLOPE are slightly stronger than those in the valley simulations.
“””
387: For a detailed explanation how the plotted variable in Fig 6 is derived, refer to Appendix A.
Figure 6: again, confused me a bit due to the unequal y-ranges. How about minusing the 6c and then making an equal y-range?
484: In the second day, the ventilation…
494-500: simplify the phrases and remove unnecessary words (In case SLOPE, nearly all the transport), brake up the sentence about the first and second days, change “drastic” to “significant”
515: are weak, so the…
Discussion
562: In Wagner et al (2015a), the ridges…
591: inclination, and …
598: ,but
Conclusions
651: allows
657-660: Use “Steeper valley floors” OR “inclined valleys” instead of “steeper valleys”? Sure, it is implied, but if someone just reads the conclusion, it might be ambigious since you are using 2 terms.
All in all, I find the paper interesting, particularly in the way it presents the tracer export/outflow as an interplay between the cross-valley winds, valley volume effect, and buoyancy forcing due to heating.
Citation: https://doi.org/10.5194/egusphere-2024-1900-RC2 -
AC1: 'Comment on egusphere-2024-1900', Johannes Mikkola, 04 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1900/egusphere-2024-1900-AC1-supplement.pdf
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