the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Sep 2024
An innovative tool for measuring Sunlight propagation through different snowpacks
Abstract. Sunlight penetration in the snowpack plays a fundamental role in many environmental processes, including the local radiative energy balance, snow hydrology and snow microbiology and can potentially contribute to climate change. In addition, many photochemical reactions typically occur in the snowpack driven by solar radiation. Although a few measurements have been attempted in the past decades with several approaches, light penetration through the snowpack is currently almost only modelled numerically, frequently using severe assumptions and several parameters not always easy to be fixed. The lack of experimental data and dedicated studies leave a remarkable scientific gap in the snow research. In this paper, we propose a novel sensor, specifically designed and custom-made, to assess sunlight propagation through the snowpack in three different spectral bands. The probe has been designed to be very compact and lightweight and therefore easily transportable. We measure at different depths in the snowpack the scattered light propagating horizontally with respect to the surface with high spatial resolution (3 mm). Measurements were performed over the past two years across multiple sites with different altitudes and geographic exposure, different illumination conditions and snowpack characteristics. Data are compared to numerical simulations from the “Snow, Ice and Aerosol Radiative” (SNICAR) code, exploited here to extract the information about the light propagation at different depths. This approach provides important constraints to properly model the snowpack characteristics, allowing us to extrapolate this information to the UV radiation range. Nevertheless, in some cases the comparison between our measurement and model run suggest a more complex light penetration depending on the snowpack peculiar characteristics that SNICAR numerical simulations cannot capture. We believe that our tight experimental approach will strongly contribute to a better understanding of the radiative transfer process inside the snow layers, as well as to a quantitative description of all those chemical, physical and biological processes that occur in the uppermost layers of the snowpack.
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RC1: 'Comment on egusphere-2024-2057', Anonymous Referee #1, 30 Sep 2024
Major comment:
The authors have apparently put a lot of effort into developing this instrument, but I am skeptical of its utility. The results they obtain are puzzling, and they admit this, but they do not attempt to explain the puzzling results. Instead, they dismiss them by saying (three times) that an attempted explanation is “outside the scope of this work” (lines 421, 461, and 470). In another place (line 529), they say the same thing with different wording: “need a more accurate study of this peculiar behavior”.
The instrument measures radiation intensity coming horizontally at various depths in a snowpack, in three spectral channels: blue, green, red. Here is what is puzzling:
(a) The expected behavior would be a simple exponential decay with depth, but many of the experimental plots deviate wildly from exponential (Figure 6).
(b) The logarithmic slope should be consistent with the spectral absorption coefficient of ice, i.e. steepest for red, less steep for green, and shallowest for blue, because the absorption coefficient of ice increases with wavelength across the visible spectrum. These diverse slopes are shown for example in Figure 9a of Picard et al. (2016, cited in the manuscript). Instead, Teruzzi’s Figure 5 shows that for four of the six snow samples (2, 3, 4, and 5b) it is the green channel that has the steepest slope, not the red channel. For Site 5a the three channels all have nearly the same slope, but green is still the steepest. For Site 1, green has the shallowest slope. In none of the samples does green sit in the middle, where it belongs. [Other parts of the paper focus on the green channel alone, which therefore is probably a poor choice.]
Other comments:
Lines 106, 208. The authors say that their probe is designed to study only the upper few centimeters of the snowpack. Why is their interest so limited? Plants and animals are sensitive to light conditions much deeper in the snowpack.
Figure 1 does not include double-headed arrows giving the dimensions, but the caption does say that the probe has a cross-section of 4 cm by 4 cm. This is surprisingly large; Picard’s rod had a much smaller diameter of 1 cm. I think 4 mm should be adequate to transmit the signal, for example by a fiber-optic guide. Teruzzi’s probe is intended to measure fluxes of sunlight at vertical resolution of a few mm, but the large size of the probe is surely altering the light field.
Table 3 gives differences in albedo for different grain size, but calls these differences “discrepancies”. But different albedo spectra are to be expected for different grain sizes. The table should be deleted.
Figure 9. I see no value to this figure.
Figure 10. For photochemistry, one should compute not up-flux plus down-flux, but rather actinic flux (i.e. not cosine-weighted). Also, is the “flux” in this figure limited to 1-3 microns, or is it a broadband flux? Or a UV flux?
Figure A3. The vertical axis label says W/m2, but the caption says “at 300 nm”. So either a wavelength-interval should be stated, or the label should instead be W m-2nm-1.
Citation: https://doi.org/10.5194/egusphere-2024-2057-RC1 -
RC2: 'Comment on egusphere-2024-2057', Anonymous Referee #2, 19 Oct 2024
The manuscript “An innovative tool for measuring sunlight propagation through different snowpacks” describes a novel instrument for measuring horizonal irradiance in the upper snowpack for three spectral bands. The upper few centimeters of snow are particularly important for optical processes in snow and sea-ice (albedo, transmittance of thin snow layers), but are challenging to measure. Improved observational tools are therefore highly welcome. The instrument has been laboriously designed and tested in the laboratory and on multiple field sites with large variability, yielding an interesting data set. However, the data presentation and interpretation in the manuscript can been greatly improved, which will help readers relate the instrument to existing observations and more convincingly show its usefulness. The horizontal irradiance must be compared to existing apparent optical properties such as downwelling, upwelling or scalar irradiance. The manuscript should also be edited more carefully (see comments below).
Major comments
The probe in this study does not measure a standard apparent optical property, which is done for practical reasons but needs a good theoretical justification to tie it to existing knowledge and observations. The description of why horizonal irradiance flux measurements can be related to more common optical properties such as albedo, transmittance, and downwelling and upwelling flux is quite hard to follow. The first part of Section 2.3 makes an argument based on an analytical expression from Kokhanovsky. However, the whole argument builds on the assumption that we are “deep enough where direct sunlight vanishes and light propagation is purely diffusive”, which is not where this probe is being used. A radiative transfer model is introduced to fill the gap, but the horizonal flux is related directly to input parameters of the model. It would be much more convincing if the modelled horizontal flux is compared directly with downwelling/upwelling flux and the scalar irradiance for the numerical simulation. This can be easily presented with scatter plots.
The experimental study on “probe spurious effects” only really looks at the best case-scenario where the sun (lamp in the set-up) is directly above. In most snow-covered regions, the sun will usually be relatively close to the horizon (i.e. in polar regions or in winter). In this case, there might be significant effects from reflection or self-shading by the probe of scattered light (not direct sunlight). This could have been tested in the lab by moving the horizontal position of the lamp. This should be address as a possible flaw in the text. Furthermore, the experimental results in figure A2 needs to be discussed and quantitative error estimates (e.g. mean relative error, mean bias) should be given. There seems to be a bias in the green channel that looks a bit concerning.
In the results section, considerably more attention needs to be given to evaluating the field measurements plotted in figure 5. Are they realistic? It is not obvious why green light should attenuate faster than the other channels. Usually, red light is attenuated faster. Measurements at stations #3 and #4 could potentially be explained by vertically varying snow properties. However, the SNICAR-model seems to be using a two-stream approach (correct me if I’m wrong), which may not be suitable in the upper centimeters of the snowpack.
It is unclear how sections 4.2 and 4.3 are connected to the rest of the manuscript, as they seem like mostly a modelling exercise. It would be more interesting if albedo is directly related to the measurements, instead of doing the “detour” through snow properties. The sensitivity analysis for grain shape and size should be brief as it is not central to the study. I don’t find believable that UV information can be accurately extrapolated with these measurements, and I would remove this part for conciseness. The modelling closure experiment (snow properties->SNICAR->observations) should be in one subsection.
The text throughout most of the manuscript would be greatly improved with more careful editing. The topic of each paragraph is not immediately clear, and information that logically belongs in one paragraph is found in another. There is often excessive information included in different sections, which is repeated elsewhere. Finally, there is an excessive use of adjectives and adverbs throughout.
Minor comments
Figures 5, 6, 7, 10, and A2 need more labels on the y-axis, for instance 0.1, 0.2, 0.5, 1 and so on.
Figure 5: If the purpose is to normalize it to surface measurements, this should be stated clearly in the plot, e.g. by plotting the “Percent of surface flux”. The figures also contain a lot of unnecessary empty space.
The abstract should be split into two or three paragraphs for better readability.
The introduction includes considerable details on different topics related to light in snow. Not all of these details are relevant for the rest of the manuscript, and this section can be trimmed down to improve readability. However, the explanation of numerical models and earlier fieldwork studies on snow optics is good and describes well the motivation for the study.
Snow on top of sea ice is also crucial for the radiation budget of sea ice and marine ecosystems beneath sea ice, in particular in the Arctic Ocean. This could be mentioned in the introduction (several relevant papers are already included as references).
L216: “G light” should be “G band”.
Citation: https://doi.org/10.5194/egusphere-2024-2057-RC2
Status: closed
-
RC1: 'Comment on egusphere-2024-2057', Anonymous Referee #1, 30 Sep 2024
Major comment:
The authors have apparently put a lot of effort into developing this instrument, but I am skeptical of its utility. The results they obtain are puzzling, and they admit this, but they do not attempt to explain the puzzling results. Instead, they dismiss them by saying (three times) that an attempted explanation is “outside the scope of this work” (lines 421, 461, and 470). In another place (line 529), they say the same thing with different wording: “need a more accurate study of this peculiar behavior”.
The instrument measures radiation intensity coming horizontally at various depths in a snowpack, in three spectral channels: blue, green, red. Here is what is puzzling:
(a) The expected behavior would be a simple exponential decay with depth, but many of the experimental plots deviate wildly from exponential (Figure 6).
(b) The logarithmic slope should be consistent with the spectral absorption coefficient of ice, i.e. steepest for red, less steep for green, and shallowest for blue, because the absorption coefficient of ice increases with wavelength across the visible spectrum. These diverse slopes are shown for example in Figure 9a of Picard et al. (2016, cited in the manuscript). Instead, Teruzzi’s Figure 5 shows that for four of the six snow samples (2, 3, 4, and 5b) it is the green channel that has the steepest slope, not the red channel. For Site 5a the three channels all have nearly the same slope, but green is still the steepest. For Site 1, green has the shallowest slope. In none of the samples does green sit in the middle, where it belongs. [Other parts of the paper focus on the green channel alone, which therefore is probably a poor choice.]
Other comments:
Lines 106, 208. The authors say that their probe is designed to study only the upper few centimeters of the snowpack. Why is their interest so limited? Plants and animals are sensitive to light conditions much deeper in the snowpack.
Figure 1 does not include double-headed arrows giving the dimensions, but the caption does say that the probe has a cross-section of 4 cm by 4 cm. This is surprisingly large; Picard’s rod had a much smaller diameter of 1 cm. I think 4 mm should be adequate to transmit the signal, for example by a fiber-optic guide. Teruzzi’s probe is intended to measure fluxes of sunlight at vertical resolution of a few mm, but the large size of the probe is surely altering the light field.
Table 3 gives differences in albedo for different grain size, but calls these differences “discrepancies”. But different albedo spectra are to be expected for different grain sizes. The table should be deleted.
Figure 9. I see no value to this figure.
Figure 10. For photochemistry, one should compute not up-flux plus down-flux, but rather actinic flux (i.e. not cosine-weighted). Also, is the “flux” in this figure limited to 1-3 microns, or is it a broadband flux? Or a UV flux?
Figure A3. The vertical axis label says W/m2, but the caption says “at 300 nm”. So either a wavelength-interval should be stated, or the label should instead be W m-2nm-1.
Citation: https://doi.org/10.5194/egusphere-2024-2057-RC1 -
RC2: 'Comment on egusphere-2024-2057', Anonymous Referee #2, 19 Oct 2024
The manuscript “An innovative tool for measuring sunlight propagation through different snowpacks” describes a novel instrument for measuring horizonal irradiance in the upper snowpack for three spectral bands. The upper few centimeters of snow are particularly important for optical processes in snow and sea-ice (albedo, transmittance of thin snow layers), but are challenging to measure. Improved observational tools are therefore highly welcome. The instrument has been laboriously designed and tested in the laboratory and on multiple field sites with large variability, yielding an interesting data set. However, the data presentation and interpretation in the manuscript can been greatly improved, which will help readers relate the instrument to existing observations and more convincingly show its usefulness. The horizontal irradiance must be compared to existing apparent optical properties such as downwelling, upwelling or scalar irradiance. The manuscript should also be edited more carefully (see comments below).
Major comments
The probe in this study does not measure a standard apparent optical property, which is done for practical reasons but needs a good theoretical justification to tie it to existing knowledge and observations. The description of why horizonal irradiance flux measurements can be related to more common optical properties such as albedo, transmittance, and downwelling and upwelling flux is quite hard to follow. The first part of Section 2.3 makes an argument based on an analytical expression from Kokhanovsky. However, the whole argument builds on the assumption that we are “deep enough where direct sunlight vanishes and light propagation is purely diffusive”, which is not where this probe is being used. A radiative transfer model is introduced to fill the gap, but the horizonal flux is related directly to input parameters of the model. It would be much more convincing if the modelled horizontal flux is compared directly with downwelling/upwelling flux and the scalar irradiance for the numerical simulation. This can be easily presented with scatter plots.
The experimental study on “probe spurious effects” only really looks at the best case-scenario where the sun (lamp in the set-up) is directly above. In most snow-covered regions, the sun will usually be relatively close to the horizon (i.e. in polar regions or in winter). In this case, there might be significant effects from reflection or self-shading by the probe of scattered light (not direct sunlight). This could have been tested in the lab by moving the horizontal position of the lamp. This should be address as a possible flaw in the text. Furthermore, the experimental results in figure A2 needs to be discussed and quantitative error estimates (e.g. mean relative error, mean bias) should be given. There seems to be a bias in the green channel that looks a bit concerning.
In the results section, considerably more attention needs to be given to evaluating the field measurements plotted in figure 5. Are they realistic? It is not obvious why green light should attenuate faster than the other channels. Usually, red light is attenuated faster. Measurements at stations #3 and #4 could potentially be explained by vertically varying snow properties. However, the SNICAR-model seems to be using a two-stream approach (correct me if I’m wrong), which may not be suitable in the upper centimeters of the snowpack.
It is unclear how sections 4.2 and 4.3 are connected to the rest of the manuscript, as they seem like mostly a modelling exercise. It would be more interesting if albedo is directly related to the measurements, instead of doing the “detour” through snow properties. The sensitivity analysis for grain shape and size should be brief as it is not central to the study. I don’t find believable that UV information can be accurately extrapolated with these measurements, and I would remove this part for conciseness. The modelling closure experiment (snow properties->SNICAR->observations) should be in one subsection.
The text throughout most of the manuscript would be greatly improved with more careful editing. The topic of each paragraph is not immediately clear, and information that logically belongs in one paragraph is found in another. There is often excessive information included in different sections, which is repeated elsewhere. Finally, there is an excessive use of adjectives and adverbs throughout.
Minor comments
Figures 5, 6, 7, 10, and A2 need more labels on the y-axis, for instance 0.1, 0.2, 0.5, 1 and so on.
Figure 5: If the purpose is to normalize it to surface measurements, this should be stated clearly in the plot, e.g. by plotting the “Percent of surface flux”. The figures also contain a lot of unnecessary empty space.
The abstract should be split into two or three paragraphs for better readability.
The introduction includes considerable details on different topics related to light in snow. Not all of these details are relevant for the rest of the manuscript, and this section can be trimmed down to improve readability. However, the explanation of numerical models and earlier fieldwork studies on snow optics is good and describes well the motivation for the study.
Snow on top of sea ice is also crucial for the radiation budget of sea ice and marine ecosystems beneath sea ice, in particular in the Arctic Ocean. This could be mentioned in the introduction (several relevant papers are already included as references).
L216: “G light” should be “G band”.
Citation: https://doi.org/10.5194/egusphere-2024-2057-RC2
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