the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Sep 2023
Rainfall enhancement downwind of hills due to standing waves on the melting-level and the extreme rainfall of December 2015 in the Lake District of northwest England
Abstract. Flow over orography can be investigated through stationary gravity waves, i.e. those whose speed exactly opposes, and therefore cancels, that of the airstream in which they are embedded. They give rise to persistent zones of ascent and descent, which modulate precipitation patterns and contribute to large accumulations, e.g. through the well-known seeder-feeder mechanism. It is shown here that opposite, stationary waves on the melting-level focus rain, potentially multiplying intensity downwind of hills by a factor of rain fall speed divided by snow fall speed, and that the effect is maximised when the vertical profile near the melting-level is isothermal. A 2D diagnostic model based on linear gravity wave theory is used to investigate the record-breaking rainfall of December 2015 in the Lake District of northwest England. The pattern of vertical velocity is shown to have a good, qualitative fit to that of the Met Office’s operational, high-resolution UKV model averaged over 24 hours, suggesting that orographically excited standing waves were the principal cause of the rain. Precipitation trajectories suggest that a persistent, downstream, elevated wave caused by the Isle of Man maintained a spray of seeding ice particles directed towards the Lake District; that these grew whilst suspended in strong upslope flow before being focussed by the undulating melting-level into intense shafts of rain.
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RC1: 'Comment on egusphere-2023-1973', Anonymous Referee #1, 27 Oct 2023
General comments:
The author introduces an interesting thought experiment regarding the role of orographically-induced gravity waves on precipitation distribution across the Isle of Man and the Lake District of northwest England. Unfortunately, the data presented in support of these ideas is insufficient to justify publication in a peer-reviewed journal. The author relies on relatively coarse operational model output that does not include any moisture variables. Additionally, verification of this model output is virtually absent, which significantly limits its credibility, especially when discussing orographic precipitation processes. A separate, higher resolution gravity wave model is also employed, whose results seem to be consistent with the operational model output. However, this does not mitigate the aforementioned lack of model validation. It also does not mitigate the absence of moisture variables in the analysis dataset. The author has to make numerous assumptions in his analysis to get around this limitation. I find many, if not most of these assumptions to be dubious.
Ultimately, the author makes assertions that are not supported with credible evidence. As a result, my recommendation is to reject the manuscript for publication.
Specific comments:
1. sections 2 and 3: The author goes into great detail about how gravity waves and an oscillating melting layer can impact the spatial distribution of precipitation across orography (i.e., Figs. 1-2; Equs. 1-4). He calls the areas of precipitation where the "trajectory" lines are closer together more "intense" than the areas where the trajectory lines are farther apart. If precipitation intensity is based on an areal integration, this may be an appropriate interpretation. However, precipitation intensity is typically based on precipitation rate, which is a mass flux for a given vertical column. The only way that precipitation intensity can be enhanced is by adding mass to the volume of hydrometeors through cloud microphysical processes. The author is not really addressing precipitation enhancement; rather, he is addressing precipitation redistribution.
2. sections 2 and 3: Does the author have any observational evidence to support the notion that a melting layer can oscillate as he hypothesizes? For example, are there any radar studies that show a bright band that oscillates in such a manner?
3. L274-279: This paragraph outlines the very limited nature of the data available to the author. This data was in the form of 24-hour mean fields from a single operational model simulation. The data was limited to horizontal and vertical winds and temperature. No moisture variables were available (i.e., water vapor, precipitating ice and/or liquid water). While the horizontal resolution was 1.5 km, there were only 14 vertical levels (~600 m resolution near the melting level). No attempt at validating the model output is evident. This dataset is clearly insufficient for addressing the processes discussed by the author.
4. L333-338: This paragraph makes a sweeping assertion: "However, the generally good correspondence between this output and the UKV, despite all the caveats, along with the remarkable steadiness of rain rate at Honister, strongly suggests that gravity waves were the main driver for vertical velocity over the period of extreme rainfall and therefore for the rain itself." The operational model output and corresponding gravity wave model output do not provide sufficient evidence to support this assertion. In particular, the lack of moisture variables eliminates evidence of possible alternative explanations based on cloud microphysics. The author does not present evidence about the depth of precipitation and whether there are hydrometeors aloft that could be influenced by the vertical velocity patterns described. For all we know, the precipitation could be very shallow in nature.
5. L461-463: The author states: "Of course, augmentation of rainfall by wash-out of droplets from the feeder cloud, essentially a cloud physics problem, is not included in this, neither is the growth by riming whilst ice particles are in near suspension over the windward slopes as
supercooled cloud droplets rise around them." This apparent "disclaimer" does not make any attempt to diminish the significance of these processes. It is quite possible that these processes are the dominant factors in the precipitation distribution associated with the case.6. L494-496: The author reasserts an unsupported conclusion that the rainfall in the case study was "principally driven by gravity wave motions".
Citation: https://doi.org/10.5194/egusphere-2023-1973-RC1 -
AC1: 'Reply on RC1', Edward Carroll, 28 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1973/egusphere-2023-1973-AC1-supplement.pdf
- AC2: 'Reply on RC1', Edward Carroll, 11 Nov 2023
-
AC1: 'Reply on RC1', Edward Carroll, 28 Oct 2023
-
RC2: 'Comment on egusphere-2023-1973', Anonymous Referee #2, 27 Dec 2023
Review of ‘Rainfall enhancement downwind of hills due to standing waves on the melting-level and the extreme rainfall of December 2015 in the Lake District of northwest England’ by Carroll.
Overview
This paper describes a technique whereby undulations in the melting layer and gravity-wave-induced-winds can modulate the precipitation occurring over complex terrain. It then describes an extreme rain event over the Lake District of northwest England, and applies this theory as a possible mechanism for precipitation enhancement during this event. The paper needs significant revision, as described by my Major and Minor comments, before it’s acceptable for publication.
Major Comments
Introduction:
Several key references, and some description therein, is missing in the introduction’s description of the state of knowledge about orographic precipitation, its relationship to standing gravity waves, and the undulations of the melting layer. Regarding the first two, I refer the author to two widely cited review papers on orographic precipitation, Roe 2005 and Houze 2012, and two book chapters, Colle et al. 2013 and Stoelinga et al. 2013 (both are chapters from the same book). Regarding the melting layer and its variations in height with respect to the terrain, Minder et al. 2011 thoroughly explores the contribution of three mechanisms, two of which are described in this paper (albeit through a somewhat different lens). Although Minder et al. (2011) focuses on an idealized case where the snow line intersects the terrain, the discussion of mechanisms that modulate the altitude of the melting layer are highly relevant to this paper. Significantly more attention to prior literature and discussions about the mechanisms at play is necessary for this paper to adequately address its contribution.
Section 2 and associated Appendix:
These sections need considerable rewriting and reorganization to more clearly state why each equation is shown, how it is derived, and critically what assumptions are made in its derivation. In addition, much of the language surrounding the equations is vague and/or conversational; this section should be explicit and extremely plain with its language, for clarity.
Paper structure
The paper begins with its derivation of its precipitation trajectory mechanisms, followed by some discussion of those mechanisms, and then goes into a case study. This structure seemed back-to-front to me, and the story would have been significantly more clear had the paper been structured as follows: Following the introduction, the data and methods for the case study analysis should be clearly laid out including a description of the two models (UKV and GWM) and any other data used in the paper as well as a description of the particle trajectory software/process used (perhaps this is part of the GWM but it is not clear). Then, the case study could be described, and used to motivate the derivation of undulating melting layer+GW bunching enhancement. The last results section should then apply the enhancement to the case study (pages ~17-21 of the current paper) with some discussion of the value added of the new method, and the paper can then end with a conclusions section.
Figure use and reference
In general, the figures should be referred to at specific sections of the text when they are discussed.
Minor Comments
- 25: I suggest adding ‘mechanisms’ to the text ‘One of the first to be described…’ so that it reads ‘One of the first mechanisms to be described…’
- 26: I suggest adding ‘moist’ to ‘... replenished by the ascent of air…’ so that it reads ‘... replenished by the ascent of moist air…’
- Figure 1: I suggest you add a vector indicating the wind is blowing from the left.
- 57-58: This sentence requires considerable assumptions, e.g. that the evaporation doesn’t change across the interface, etc. More attention should be given to the assumptions made prior to each assertion.
- 70: Is ws assumed to be negative or does the negative sign before ws in the equation capture the downward direction (i.e., the sign conventions used are not clear)?
- Figure 2: I believe this figure is intended as a toy schematic for teasing apart the mechanisms, but this is not clearly described, and as such it is simplified to the point of being incorrect.
- 81-83: These three sentences need some revision for clarity.
- 96-98: This should refer back to Figure 1.
- 104: Stout et al. 1997 should also be cited here.
- 129: ‘So the magnitude of modulation…’ this is extremely conversational and needs to be revised for clarity.
- 146-149: Where has this analysis been done? Is this testing not shown?
- 220-224: This section of text poorly described and needs expansion for clarity.
- 251: This is, I believe, the first introduction of the UKV, and it needs to be defined.
- 254: A map of the analyzed rainfall should be included as one of the figures for the case study.
- 274-279: This text which describes the UKV model should be moved into a section where data and methods are described (adjacent to the GWM description). Why were moist variable data unavailable?
- 293-294: ‘Note that only the region…’ is not a strong start to a new section. Transitions should be used to make the paper more readable.
- 300: Figures 5, 6, and 7 were not referenced before this reference to Figure 8.
- 308: This satellite imagery is not shown.
- 326-331: These experiments are presumably not shown; ‘not shown’ should be explicitly stated.
- 336-338: Since these are 24-hour averages, an alternative hypothesis would be that any diabetic/other effects that generated vertical velocity and precipitation occurred randomly through the domain at shorter time intervals and thus when averaged, their signal was largely removed.
- 370-372: This sentence needs revision for clarity.
- 381: principal->principle
- 413-416: The paper should refer to its equations for the enhancement calculation.
- 526-545: This section of the appendix provides an example of what I’m suggesting in my Major Comment regarding Section 2 and the Appendix: This section describes an expression for the slope of an isotherm, but does not motivate this derivation by noting that it will be applied for a specific isotherm, the melting level. It also needs a bit more thorough defining and discussion for each equation shown (and for any inferences made between equations).
References
Colle, Brian A., Ronald B. Smith, and Douglas A. Wesley. "Theory, observations, and predictions of orographic precipitation." Mountain Weather Research and Forecasting: Recent Progress and Current Challenges (2013): 291-344
Houze Jr, Robert A. "Orographic effects on precipitating clouds." Reviews of Geophysics 50.1 (2012)
Minder, Justin R., Dale R. Durran, and Gerard H. Roe. "Mesoscale controls on the mountainside snow line." Journal of the atmospheric sciences 68.9 (2011): 2107-2127.
Roe, Gerard H. "Orographic precipitation." Annu. Rev. Earth Planet. Sci. 33 (2005): 645-671
Stoelinga, Mark T., et al. "Microphysical processes within winter orographic cloud and precipitation systems." Mountain Weather Research and Forecasting: Recent Progress and Current Challenges (2013): 345-408
Stout, J. E., and G. S. Janowitz. "Particle trajectories above sinusoidal terrain." Quarterly Journal of the Royal Meteorological Society 123.543 (1997): 1829-1840
Citation: https://doi.org/10.5194/egusphere-2023-1973-RC2 -
AC3: 'Reply on RC2', Edward Carroll, 22 Jan 2024
Thanks very much for your careful reading of the paper and for the many useful suggestions you made for its improvement. Your critique has prompted me to account for diabatic processes such as snow melt explicitly in the analytical treatment. This leaves equations 3 and 4 in the original manuscript unchanged, representing the isothermal limiting case.
I attach a PDF of individual responses to your points extracted from the account I submitted to the editors to accompany the revised manuscript.
Status: closed
-
RC1: 'Comment on egusphere-2023-1973', Anonymous Referee #1, 27 Oct 2023
General comments:
The author introduces an interesting thought experiment regarding the role of orographically-induced gravity waves on precipitation distribution across the Isle of Man and the Lake District of northwest England. Unfortunately, the data presented in support of these ideas is insufficient to justify publication in a peer-reviewed journal. The author relies on relatively coarse operational model output that does not include any moisture variables. Additionally, verification of this model output is virtually absent, which significantly limits its credibility, especially when discussing orographic precipitation processes. A separate, higher resolution gravity wave model is also employed, whose results seem to be consistent with the operational model output. However, this does not mitigate the aforementioned lack of model validation. It also does not mitigate the absence of moisture variables in the analysis dataset. The author has to make numerous assumptions in his analysis to get around this limitation. I find many, if not most of these assumptions to be dubious.
Ultimately, the author makes assertions that are not supported with credible evidence. As a result, my recommendation is to reject the manuscript for publication.
Specific comments:
1. sections 2 and 3: The author goes into great detail about how gravity waves and an oscillating melting layer can impact the spatial distribution of precipitation across orography (i.e., Figs. 1-2; Equs. 1-4). He calls the areas of precipitation where the "trajectory" lines are closer together more "intense" than the areas where the trajectory lines are farther apart. If precipitation intensity is based on an areal integration, this may be an appropriate interpretation. However, precipitation intensity is typically based on precipitation rate, which is a mass flux for a given vertical column. The only way that precipitation intensity can be enhanced is by adding mass to the volume of hydrometeors through cloud microphysical processes. The author is not really addressing precipitation enhancement; rather, he is addressing precipitation redistribution.
2. sections 2 and 3: Does the author have any observational evidence to support the notion that a melting layer can oscillate as he hypothesizes? For example, are there any radar studies that show a bright band that oscillates in such a manner?
3. L274-279: This paragraph outlines the very limited nature of the data available to the author. This data was in the form of 24-hour mean fields from a single operational model simulation. The data was limited to horizontal and vertical winds and temperature. No moisture variables were available (i.e., water vapor, precipitating ice and/or liquid water). While the horizontal resolution was 1.5 km, there were only 14 vertical levels (~600 m resolution near the melting level). No attempt at validating the model output is evident. This dataset is clearly insufficient for addressing the processes discussed by the author.
4. L333-338: This paragraph makes a sweeping assertion: "However, the generally good correspondence between this output and the UKV, despite all the caveats, along with the remarkable steadiness of rain rate at Honister, strongly suggests that gravity waves were the main driver for vertical velocity over the period of extreme rainfall and therefore for the rain itself." The operational model output and corresponding gravity wave model output do not provide sufficient evidence to support this assertion. In particular, the lack of moisture variables eliminates evidence of possible alternative explanations based on cloud microphysics. The author does not present evidence about the depth of precipitation and whether there are hydrometeors aloft that could be influenced by the vertical velocity patterns described. For all we know, the precipitation could be very shallow in nature.
5. L461-463: The author states: "Of course, augmentation of rainfall by wash-out of droplets from the feeder cloud, essentially a cloud physics problem, is not included in this, neither is the growth by riming whilst ice particles are in near suspension over the windward slopes as
supercooled cloud droplets rise around them." This apparent "disclaimer" does not make any attempt to diminish the significance of these processes. It is quite possible that these processes are the dominant factors in the precipitation distribution associated with the case.6. L494-496: The author reasserts an unsupported conclusion that the rainfall in the case study was "principally driven by gravity wave motions".
Citation: https://doi.org/10.5194/egusphere-2023-1973-RC1 -
AC1: 'Reply on RC1', Edward Carroll, 28 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1973/egusphere-2023-1973-AC1-supplement.pdf
- AC2: 'Reply on RC1', Edward Carroll, 11 Nov 2023
-
AC1: 'Reply on RC1', Edward Carroll, 28 Oct 2023
-
RC2: 'Comment on egusphere-2023-1973', Anonymous Referee #2, 27 Dec 2023
Review of ‘Rainfall enhancement downwind of hills due to standing waves on the melting-level and the extreme rainfall of December 2015 in the Lake District of northwest England’ by Carroll.
Overview
This paper describes a technique whereby undulations in the melting layer and gravity-wave-induced-winds can modulate the precipitation occurring over complex terrain. It then describes an extreme rain event over the Lake District of northwest England, and applies this theory as a possible mechanism for precipitation enhancement during this event. The paper needs significant revision, as described by my Major and Minor comments, before it’s acceptable for publication.
Major Comments
Introduction:
Several key references, and some description therein, is missing in the introduction’s description of the state of knowledge about orographic precipitation, its relationship to standing gravity waves, and the undulations of the melting layer. Regarding the first two, I refer the author to two widely cited review papers on orographic precipitation, Roe 2005 and Houze 2012, and two book chapters, Colle et al. 2013 and Stoelinga et al. 2013 (both are chapters from the same book). Regarding the melting layer and its variations in height with respect to the terrain, Minder et al. 2011 thoroughly explores the contribution of three mechanisms, two of which are described in this paper (albeit through a somewhat different lens). Although Minder et al. (2011) focuses on an idealized case where the snow line intersects the terrain, the discussion of mechanisms that modulate the altitude of the melting layer are highly relevant to this paper. Significantly more attention to prior literature and discussions about the mechanisms at play is necessary for this paper to adequately address its contribution.
Section 2 and associated Appendix:
These sections need considerable rewriting and reorganization to more clearly state why each equation is shown, how it is derived, and critically what assumptions are made in its derivation. In addition, much of the language surrounding the equations is vague and/or conversational; this section should be explicit and extremely plain with its language, for clarity.
Paper structure
The paper begins with its derivation of its precipitation trajectory mechanisms, followed by some discussion of those mechanisms, and then goes into a case study. This structure seemed back-to-front to me, and the story would have been significantly more clear had the paper been structured as follows: Following the introduction, the data and methods for the case study analysis should be clearly laid out including a description of the two models (UKV and GWM) and any other data used in the paper as well as a description of the particle trajectory software/process used (perhaps this is part of the GWM but it is not clear). Then, the case study could be described, and used to motivate the derivation of undulating melting layer+GW bunching enhancement. The last results section should then apply the enhancement to the case study (pages ~17-21 of the current paper) with some discussion of the value added of the new method, and the paper can then end with a conclusions section.
Figure use and reference
In general, the figures should be referred to at specific sections of the text when they are discussed.
Minor Comments
- 25: I suggest adding ‘mechanisms’ to the text ‘One of the first to be described…’ so that it reads ‘One of the first mechanisms to be described…’
- 26: I suggest adding ‘moist’ to ‘... replenished by the ascent of air…’ so that it reads ‘... replenished by the ascent of moist air…’
- Figure 1: I suggest you add a vector indicating the wind is blowing from the left.
- 57-58: This sentence requires considerable assumptions, e.g. that the evaporation doesn’t change across the interface, etc. More attention should be given to the assumptions made prior to each assertion.
- 70: Is ws assumed to be negative or does the negative sign before ws in the equation capture the downward direction (i.e., the sign conventions used are not clear)?
- Figure 2: I believe this figure is intended as a toy schematic for teasing apart the mechanisms, but this is not clearly described, and as such it is simplified to the point of being incorrect.
- 81-83: These three sentences need some revision for clarity.
- 96-98: This should refer back to Figure 1.
- 104: Stout et al. 1997 should also be cited here.
- 129: ‘So the magnitude of modulation…’ this is extremely conversational and needs to be revised for clarity.
- 146-149: Where has this analysis been done? Is this testing not shown?
- 220-224: This section of text poorly described and needs expansion for clarity.
- 251: This is, I believe, the first introduction of the UKV, and it needs to be defined.
- 254: A map of the analyzed rainfall should be included as one of the figures for the case study.
- 274-279: This text which describes the UKV model should be moved into a section where data and methods are described (adjacent to the GWM description). Why were moist variable data unavailable?
- 293-294: ‘Note that only the region…’ is not a strong start to a new section. Transitions should be used to make the paper more readable.
- 300: Figures 5, 6, and 7 were not referenced before this reference to Figure 8.
- 308: This satellite imagery is not shown.
- 326-331: These experiments are presumably not shown; ‘not shown’ should be explicitly stated.
- 336-338: Since these are 24-hour averages, an alternative hypothesis would be that any diabetic/other effects that generated vertical velocity and precipitation occurred randomly through the domain at shorter time intervals and thus when averaged, their signal was largely removed.
- 370-372: This sentence needs revision for clarity.
- 381: principal->principle
- 413-416: The paper should refer to its equations for the enhancement calculation.
- 526-545: This section of the appendix provides an example of what I’m suggesting in my Major Comment regarding Section 2 and the Appendix: This section describes an expression for the slope of an isotherm, but does not motivate this derivation by noting that it will be applied for a specific isotherm, the melting level. It also needs a bit more thorough defining and discussion for each equation shown (and for any inferences made between equations).
References
Colle, Brian A., Ronald B. Smith, and Douglas A. Wesley. "Theory, observations, and predictions of orographic precipitation." Mountain Weather Research and Forecasting: Recent Progress and Current Challenges (2013): 291-344
Houze Jr, Robert A. "Orographic effects on precipitating clouds." Reviews of Geophysics 50.1 (2012)
Minder, Justin R., Dale R. Durran, and Gerard H. Roe. "Mesoscale controls on the mountainside snow line." Journal of the atmospheric sciences 68.9 (2011): 2107-2127.
Roe, Gerard H. "Orographic precipitation." Annu. Rev. Earth Planet. Sci. 33 (2005): 645-671
Stoelinga, Mark T., et al. "Microphysical processes within winter orographic cloud and precipitation systems." Mountain Weather Research and Forecasting: Recent Progress and Current Challenges (2013): 345-408
Stout, J. E., and G. S. Janowitz. "Particle trajectories above sinusoidal terrain." Quarterly Journal of the Royal Meteorological Society 123.543 (1997): 1829-1840
Citation: https://doi.org/10.5194/egusphere-2023-1973-RC2 -
AC3: 'Reply on RC2', Edward Carroll, 22 Jan 2024
Thanks very much for your careful reading of the paper and for the many useful suggestions you made for its improvement. Your critique has prompted me to account for diabatic processes such as snow melt explicitly in the analytical treatment. This leaves equations 3 and 4 in the original manuscript unchanged, representing the isothermal limiting case.
I attach a PDF of individual responses to your points extracted from the account I submitted to the editors to accompany the revised manuscript.
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