the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2024
Brief communication: Not as dirty as they look, flawed airborne and satellite snow spectra
Abstract. Key to the success of spaceborne missions is understanding snowmelt in our warming climate, having implications for nearly 2 billion people. An obstacle is that surface reflectance products over snow show an erroneous hook that often shows sharp decreases in the visible wavelengths. This hook is sometimes mistaken for soot or dust but can result from three artifacts: 1) a background reflectance that is too dark; 2) an assumption of level terrain; 3) or differences in optical constants of ice. Sensor calibration and directional effects may also contribute. Solutions currently being implemented address these artifacts.
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RC1: 'Comment on egusphere-2024-1681', Anonymous Referee #1, 21 Aug 2024
The manuscript by Bair et al. is focused on the erroneous representation of snow reflectance spectra in airborne and satellite data. While I find this topic very interesting and current, the content of the manuscript is quite redundant with another manuscript currently under evaluation in this same journal (https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1020/) from the same group. I have already reviewed that manuscript, so I address the authors to my comments (RC1).Â
In general, I would highly suggest to merge these two submissions during the revision of Bohn et al. 2024. I leave to the Editor the decision on this point. If the authors prefer to keep this manuscript as a separate submission, a major review is needed before publication. Hereafter, I will detail my major concerns.The authors build their argument on one (n=1) spectrum from PRISMA showed in Figure 1. Furthermore, this spectrum is derived from a atmospheric-topographic correction that itself can introduce erroneous hook in snow reflectance. At least, I would ask the authors to provide a comparison with standard L2(C-D) products from PRISMA. A recent paper (Di Mauro et al. 2024) provided an evaluation of PRISMA reflectance and radiance  products for different snow conditions. Same holds for Ravasio et al. (2024). In that cases, no clear hook is displayed in snow reflectance spectra. Which PRISMA processor has been used for generating the plot in Figure 1? When data have been downloaded from the ASI portal? In fact, several improvements have been made in the latest PRISMA processor (v_4_1_0_02_05). For example, Kokhanovsky et al. (2022) is based on an earlier version of the processor, and a downward hook is sometimes displayed in an area with expected clean snow (i.e. upper portion of the Nansen Ice Shelf, Antarctica). If the authors want to show that the hook is widespread, they should provide more evidence (e.g. different snow types, different latitudes, different sensors, etc.). Furthermore, they should provide evidence that the snow was clean (low concentration of impurities) at the ground.
Further information on the properties of snow at the surface is needed. I see that they reference to Townsend et al. (2023) dataset, and I learnt about the SISTER initiative. This should be described in detail in this manuscript as well. How many pixels have been averaged? Was snow flat in that area? During which period field data have been collected? Which spectrometer and protocol have been used for field spectroscopy measurements?
In the title, I read that the manuscript is about satellite and airborne sensor. Throughout the manuscript those airborne sensors are not detailed. Can you provide evidence of hooking from airborne sensors (e.g. AVIRIS, APEX etc.)?
Further still on the title: If the hook is located below 500nm, likely snow will not "look" dirtier, at least from a correct RGB representation.Â
In line 31, I read: "Standard surface reflectance products are rife with hooking errors", but no references either evidence of this hooking errors is detailed. I strongly encourage the authors to go more in detail on this error. Please, see my comments to Bohn et al. 2024 on this topic.
Line 94-95: these conclusions strongly overlaps with Section 5.1 ("the blue hook") in Bohn et al. 2024.
References:
Bohn, Niklas and Bair, Edward H. and Brodrick, Philip G. and Carmon, Nimrod and Green, Robert O. and Painter, Thomas H. and Thompson, David R., The Pitfalls of Ignoring Topography in Snow Retrievals: A Case Study with Emit. Available at SSRN: https://ssrn.com/abstract=4671920 or http://dx.doi.org/10.2139/ssrn.4671920
Di Mauro, B., Cogliati, S., Bohn, N., Traversa, G., Garzonio, R., Tagliabue, G., et al. (2024). Evaluation of PRISMA products over snow in the Alps and Antarctica. Earth and Space Science, 11, e2023EA003482. https://doi.org/10.1029/2023EA003482
Kokhanovsky A, Di Mauro B and Colombo R (2022) Snow surface properties derived from PRISMA satellite data over the Nansen Ice Shelf (East Antarctica). Front. Environ. Sci. 10:904585. doi: 10.3389/fenvs.2022.904585
Ravasio, C., Garzonio, R., Di Mauro, B., Matta, E., Giardino, C., Pepe, M., et al. (2024). Retrieval of snow liquid water content from radiative transfer model, field data and PRISMA satellite data. Remote Sensing of Environment, 311, 114268. https://doi.org/https://doi.org/10.1016/j.rse.2024.114268
Citation: https://doi.org/10.5194/egusphere-2024-1681-RC1 -
AC1: 'Reply on RC1', Edward Bair, 07 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1681/egusphere-2024-1681-AC1-supplement.pdf
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AC1: 'Reply on RC1', Edward Bair, 07 Oct 2024
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RC2: 'Comment on egusphere-2024-1681', Christopher Donahue, 02 Sep 2024
This brief communication documents the root cause of an erroneous "hook" observed in the visible wavelengths of measured snow reflectance spectra from airborne and satellite imaging spectrometers, which is often mistaken for dirty snow. This phenomenon has been documented in recent papers and is something I have observed and documented in aerial Airborne Coastal Observatory data collected over rugged terrain using ATCOR4 atmospheric/topographic correction (Donahue et al., 2023).
To my knowledge, this is the first paper that specifically investigates the cause of this issue for aerial and satellite platforms and breaks it down into multiple possible components. Specifically, I find the results shown in Figure 2 to be a valuable contribution and visualization for the community. Given the numerous current and forthcoming spaceborne imaging spectrometer missions, this is a timely communication that will help raise awareness of and provide solutions for this commonly observed artifact. The communication is well-written, and the modeling methods are sound. I recommend publication following consideration of the following comments.
- In cases where hooking is caused by the atmospheric correction algorithm, a few more details are needed to describe how the background snow reflectance spectra is used to correct downwelling and upwelling radiation. It is noted that the background snow spectrum is spectrally varying, while the dark reflectance is spectrally constant. Given this, I would expect to see differences in Figure 2 beyond 900 nm for the two error cases (dashed lines) when compared to the two unflawed spectra (solid lines), but the spectra appear to overlap each other. I would also expect possible differences into the SWIR region which is not shown in the figure. Does the commonly used constant background reflectance cause artifacts in other regions of the spectra that could be a concern? Also, how does one select an appropriate background spectrum?
- NASA's goal, as stated in the introduction, is to accurately measure/model absorption within 10%. How much error could this hooking artifact introduce to a broadband albedo measurement? A brief quantitative assessment of this impact would increase the impact of the brief communication. Since solar irradiance is lower in the 350-450 nm range—where the hook is steepest—the resulting broadband albedo error, when convolved with spectral irradiance, would be smaller compared to error in longer visible bands.
- I appreciate the inclusion of the ice optical property case for completeness; however, it’s important to note that this issue is not the result of a flawed airborne or satellite measurement, nor is it due to atmospheric or topographic correction. This should be acknowledged in the manuscript.
- Need to define mu in equation 2
- Consider adding subsection numbering to section 3 for each case.
References
Donahue, C. P., Menounos, B., Viner, N., Skiles, S. M., Beffort, S., Denouden, T., ... & Heathfield, D. (2023). Bridging the gap between airborne and spaceborne imaging spectroscopy for mountain glacier surface property retrievals. Remote Sensing of Environment, 299, 113849.
Citation: https://doi.org/10.5194/egusphere-2024-1681-RC2 -
AC2: 'Reply on RC2', Edward Bair, 07 Oct 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-1681/egusphere-2024-1681-AC2-supplement.pdf
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