Preprints
https://doi.org/10.5194/tc-2021-361
https://doi.org/10.5194/tc-2021-361

  30 Nov 2021

30 Nov 2021

Review status: this preprint is currently under review for the journal TC.

Divergence of apparent and intrinsic snow albedo over a season at a sub-alpine site with implications for remote sensing

Edward Hamilton Bair1, Jeff Dozier2, Charles Stern3, Adam LeWinter4, Karl Rittger5,1, Alexandria Savagian6, Timbo Stillinger1, and Robert Davis4 Edward Hamilton Bair et al.
  • 1Earth Research Institute, University of California, Santa Barbara, CA USA 93106
  • 2Bren School of Environmental Science & Management, University of California, Santa Barbara, CA USA 93106
  • 3Lamont-Doherty Earth Observatory, Palisades, NY USA 10964
  • 4Cold Regions Research and Engineering Laboratory, Hanover, NH USA 03755
  • 5Institute of Arctic and Alpine Research, University of Colorado, Boulder, Boulder, CO 80309
  • 6Bowdoin College, Brunswick, ME USA 04011

Abstract. Intrinsic albedo is the bihemispherical reflectance of a substance with a smooth surface. Conversely, the apparent albedo is the bihemispherical reflectance of the same substance with a rough surface. For snow, the surface is often rough, and these two optical quantities have different uses: intrinsic albedo is used in scattering equations whereas apparent albedo should be used in energy balance models. Complementing numerous studies devoted to surface roughness and its effect on snow reflectance, this work analyzes a timeseries of intrinsic and apparent snow albedos over a season at a sub-alpine site using an automated terrestrial laser scanner to map the snow surface topography. An updated albedo model accounts for shade, and in situ albedo measurements from a field spectrometer are compared to those from a spaceborne multispectral sensor. A spectral unmixing approach using a shade endmember (to address the common problem of unknown surface topography) produces grain size and impurity solutions; the modeled shade fraction is compared to the intrinsic and apparent albedo difference. As expected and consistent with other studies, the results show that intrinsic albedo is consistently greater than apparent albedo. Both albedos decrease rapidly as ablation hollows form during melt, combining effects of impurities on the surface and increasing roughness. Intrinsic broadband albedos average 7 % greater than apparent albedos, with the difference being about 6 % in the near-infrared or 3–4 % if the average (planar) topography is known and corrected. Field measurements of spectral surface reflectance confirm that multispectral sensors see the apparent albedo but lack the spectral resolution to distinguish between darkening from ablation hollows versus low concentrations of impurities. In contrast, measurements from the field spectrometer have sufficient resolution to discern darkening from the two sources. Based on these results, conclusions are: 1) impurity estimates from multispectral sensors are only reliable for relatively dirty snow with high snow fraction; 2) a shade endmember must be used in spectral mixture models, even for in situ spectroscopic measurements; and 3) snow albedo models should produce apparent albedos by accounting for the shade fraction. The conclusion re-iterates that albedo is the most practical snow reflectance quantity for remote sensing.

Edward Hamilton Bair et al.

Status: open (until 11 Feb 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Edward Hamilton Bair et al.

Edward Hamilton Bair et al.

Viewed

Total article views: 264 (including HTML, PDF, and XML)
HTML PDF XML Total BibTeX EndNote
195 65 4 264 2 4
  • HTML: 195
  • PDF: 65
  • XML: 4
  • Total: 264
  • BibTeX: 2
  • EndNote: 4
Views and downloads (calculated since 30 Nov 2021)
Cumulative views and downloads (calculated since 30 Nov 2021)

Viewed (geographical distribution)

Total article views: 264 (including HTML, PDF, and XML) Thereof 264 with geography defined and 0 with unknown origin.
Country # Views %
  • 1
1
 
 
 
 
Latest update: 16 Jan 2022
Download
Short summary
Understanding how snow & ice reflect solar radiation (albedo) is important for global climate. Using high-resolution topography, darkening from surface roughness (apparent albedo) is separated from darkening by the composition of the snow (intrinsic albedo). Intrinsic albedo is usually greater than apparent albedo, especially during melt. Such high resolution topography is often not available, thus use of a shade component when modeling mixtures is advised.