Near-surface snow particle dynamics from particle tracking velocimetry and turbulence measurements during alpine blowing snow storms

Aksamit, Nikolas O.; Pomeroy, John W.

doi:https://doi.org/10.5194/tc-10-3043-2016

Articles | Volume 10, issue 6

https://doi.org/10.5194/tc-10-3043-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/tc-10-3043-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 10, issue 6

Research article

|

16 Dec 2016

Research article |

| 16 Dec 2016

Near-surface snow particle dynamics from particle tracking velocimetry and turbulence measurements during alpine blowing snow storms

Nikolas O. Aksamit and John W. Pomeroy

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Final revised paper (published on 16 Dec 2016)
Supplement to the final revised paper
Preprint (discussion started on 03 May 2016)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Comments to authors', Anonymous Referee #1, 23 Jun 2016
- AC1: 'Referee #1 Comment Response', Nikolas Aksamit, 27 Jul 2016
RC2: 'Near-Surface Snow Particle Dynamics from Particle Tracking Velocimetry and Turbulence Measurements during Alpine Blowing Snow Storms by N. O. Aksamit and J. W. Pomeroy', Anonymous Referee #2, 29 Jun 2016
- AC2: 'Referee #2 Comment Response', Nikolas Aksamit, 27 Jul 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Nikolas Aksamit on behalf of the Authors (31 Aug 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Sep 2016) by Guillaume Chambon

RR by Anonymous Referee #1 (07 Oct 2016)

Suggestions for revision or reasons for rejection

Comments to authors

In general, the manuscript has been largely improved with including the new data and expanding the data analysis part; total volume is nearly the twice as the previous one at this stage. On the other hand, I have got a feeling that it became too wordy. I hope the authors eliminate the repetitions and the redundant part, and make the manuscript more compact.
Further, I am still concerned about the surface topography. In fact, 5 mm rise over 130 mm is not enormous and is inevitable in nature, I understand. However, as you may see, particle motions are very sensitive on the surface feature. Thus, I am afraid that the particle speed in Fig. 6 and the horizontal flux in Fig.7 are strongly affected, even though the terrain-following coordinates are adapted. Such as, the particle located at x=60 mm and z=10 mm has a history which passed over the snow surface at x=0 mm where the snow surface is 5 mm lower. It should be carefully taken into account in the analysis. Although it is not shown in revised version, in Fig. 3 (g) of the previous manuscript, particle flux showed the maximum at 2 to 7 mm high. This is presumably caused by the effect of topography change.
Bagnold (1941) used the term ‘creep’ to describe grains rolling and jostling along the surface. Thus I do have slightly an aversion to call the particle motion less than 4mm as creep. Probably “reputation” that means “move slowly” introduced by Ungar and Haff (1987) is more suitable. At any rate, 4 mm is several tens times larger than the mean particle diameter and, needless to say, large number of saltation particles are involved there.
Further, although it is not related to the evaluation of the contents directly, the authors need to reply to the reviewer’s comment line by line. ‘Refer to the response to other reviewer’ is not an appropriate attitude.
As a whole, I do believe this manuscript is approaching to the publishing stage. However, I am glad if the authors can clarify and improve some of the points listed below before the publication.
Specific comments are listed below.
Page 4, line 23: I do not see where the potential for adapting the continuum model to the blowing snow phenomena is discussed in the manuscript.

Figure 5: (a) to (c) are not shown in the figure. The scale of the vertical of upper two figures needs to be set equal.

Page 9, line 4: We cannot expect the steady-wind condition in nature, except for the specific situation like in Antarctica.

Page 10, line 6: Why don’t you say “the focal point was not recognized in this study”?

Figure 6: Please check the figure caption. d) - f) are for the particle velocity gradient and g) – i) are for the particle slip velocity.

Page 15, line 18: I don’t see how you can expect the dense surface flow with profiles in Figs. 6 and 7. The particle number flux increases with approaching the bed surface, but no specific layers with maximum flux are recognized. In actual, as pointed previously, in Fig. 3 (g) of the previous manuscript, particle flux showed the maximum at 2 to 7 mm high. However, this is presumably induced by the topography change.

Figure 8: In the figure caption, no explanation that this is for just “descending particles” is shown.

Page 17, line 11: Shear stress due to the wind can be estimated from the ultra sonic anemometer data. Thus, it will be informative to compare with the particle momentum near the surface quantitatively.

Page 17, line 23: The definition of three height bands should be defined earlier, when the data of near-surface and high bands were introduced.

Page 24, line 27: Descriptions of the article number concentrations will be useful to discuss whether it will be comparable with the ordinary fluidized-bed and be applicable the physical properties of the bed.

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RR by Florence Naaim-Bouvet (04 Nov 2016)

Suggestions for revision or reasons for rejection

The reviewer really appreciates efforts made by the authors to improve the manuscript by adding new references, new graphs and data allowing generalization of the results. The main points raised have been answered.
Discussions concerning wind characteristics and more particularly the quadrant hole analysis are a prime example: conclusions are now convincing but lead to new comments. Why did the authors choose a fifteen minute average to calculate the mean wind speed and the friction velocity and what influence does this choice have on the quadrant hole analysis (Figure 4).
My main other comments are still in regard to the limit between creep and saltation. The authors cannot hide behind the arguments: “the arbitrary height is not a threshold, it was chosen to illuminate the subtle difference in particle transport and the continuum of motion…”. This argument can be admitted for 3.3 Turbulent flow and Figures 9. But in order to calculate the ratio of the sum of momentum in the creeping population to that lost to the surface from impacting high-energy particles capable of splash, the authors need to be very precise as to the method used and to explain as clearly and in as much details as possible the reasons leading to the values 3 and 4 mm. In such a way, similar experiments with different hardness can be conducted in the future and compared with the results presented in this paper. P28 it is written that this choice is made according the high and low-energy saltating grain populations theory proposed by Ho et al, 2014. I would like to recall the conclusions drawn by Ho et al., 2014:
As a result, two different populations can be identified but using a criterion that differs from that of Bagnold:
(i) the low energetic saltating particles lying below 2zf and hardly influenced by change of the flow strength
(ii) the highly energetic ones saltating above 2zf sensitive to increase of the flow intensity.

On what figures or zf values do the authors base the choice of the threshold 3 or 4 mm ?
Moreover the method used for calculating Crp/Spl is a little ambiguous for the reader and need to be clarified. The low-energy population at the surface is identified as the difference of the two collections of particles vector (P 29 line 5). As far I understand, this implies that the high-energy population (particles capable of splash) is identified to be twice of particles vector for upper region height bands. If not the total impacting high-energy snow grain momentum will be wrong. But later (p35 line 24) the authors wrote « the momentum present in the slow surface transport was often larger than that found in the upper region high energy grains as can be seen by crp/Spl>4.” It is really confusing because it seems in this sentence that only particles in the upper region have been taken into account. For clarity it will be helpful to give an example showing a horizontal particle velocity histogram for near surface and upper regions and the corresponding histogram for creeping population and particles capable of splash.

Referee Report: PDF

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ED: Publish subject to minor revisions (Editor review) (05 Nov 2016) by Guillaume Chambon

AR by Nikolas Aksamit on behalf of the Authors (15 Nov 2016) Author's response Manuscript

ED: Publish as is (15 Nov 2016) by Guillaume Chambon

Short summary

The first implementation of particle tracking velocimetry in outdoor alpine blowing snow has both provided new insight on intermittent snow particle transport initiation and entrainment in the dense near-surface "creep" layer whilst also confirming some wind tunnel observations. Environmental PTV has shown to be a viable avenue for furthering our understanding of the coupling of the atmospheric boundary layer turbulence and blowing snow transport.