Measurements of the bidirectional reflectance of snow at fine spectral and angular resolution (original) (raw)
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2004
We present 2 days' measurements of the hemispherical-directional reflectance factor (HDRF) of snow made at fine spectral and angular resolution with the Automated Spectro-Goniometer (ASG) for the range of solar zenith angles ($\ theta $0= 40°–50°) and snow textures (surface grain size= 80–280 μm). Measurements of the stratigraphy of snow texture and density accompanied each day's suite of measurements. These measurements represent the most detailed available in terms of angular and spectral resolution.
Seasonal Study of Directional Reflectance Properties of Snow
We present an analysis of the hemispherical-directional reflectance factor (HDRF) of snow, using 16 seasonal datasets of the spectral range from 400 to 2,500 nm. The data was measured under clear sky conditions in Davos Dorf (Grisons, Switzerland, 1,560 m a. s. l.). Fieldwork was carried out on seven days between February 5 and March 30 2004 with the Swiss Field Goniometer Sys- tem (FIGOS). In addition to the HDRF measurements, snow stratigraphy, temperature and density were measured, and chemical and photomicroscopical analyses of snow samples were performed. Concentration of organic and elemental carbon was determined by chemical analysis. The grain size analyses through image processing of micrographs revealed relatively small differences of 0.21 to 0.33 mm mean radius in the top layers of snow cover. Seven datasets present HDRF of wet snow surfaces with similar anisotropy at smaller sun zenith angles (θI =3.3 to 64.5°) compared to the nine surfaces measured at larger sun zenith ...
Atmospheric Chemistry and Physics, 2010
High-accuracy measurements of snow Bidirectional Reflectance Distribution Function (BRDF) were performed for four natural snow samples with a spectrogonioradiometer in the 500-2600 nm wavelength range. These measurements are one of the first sets of direct snow BRDF values over a wide range of lighting and viewing geometry. They were compared to BRDF calculated with two optical models. Variations of the snow anisotropy factor with lighting geometry, wavelength and snow physical properties were investigated. Results show that at wavelengths with small penetration depth, scattering mainly occurs in the very top layers and the anisotropy factor is controlled by the phase function. In this condition, forward scattering peak or double scattering peak is observed. In contrast at shorter wavelengths, the penetration of the radiation is much deeper and the number of scattering events increases. The anisotropy factor is thus nearly constant and decreases at grazing observation angles. The whole dataset is available on demand from the corresponding author.
Analysis of snow bidirectional reflectance from ARCTAS Spring-2008 Campaign
Atmospheric Chemistry and Physics, 2010
The spring 2008 Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARC-TAS) experiment was one of major intensive field campaigns of the International Polar Year aimed at detailed characterization of atmospheric physical and chemical processes in the Arctic region. A part of this campaign was a unique snow bidirectional reflectance experiment on the NASA P-3B aircraft conducted on 7 and 15 April by the Cloud Absorption Radiometer (CAR) jointly with airborne Ames Airborne Tracking Sunphotometer (AATS) and ground-based Aerosol Robotic Network (AERONET) sunphotometers. The CAR data were atmospherically corrected to derive snow bidirectional reflectance at high 1 • angular resolution in view zenith and azimuthal angles along with surface albedo. The derived albedo was generally in good agreement with ground albedo measurements collected on 15 April. The CAR snow bidirectional reflectance factor (BRF) was used to study the accuracy of analytical Ross-Thick Li-Sparse (RTLS), Modified Rahman-Pinty-Verstraete (MRPV) and Asymptotic Analytical Radiative Transfer (AART) BRF models. Except
The Cryosphere, 2021
Snow stands out from materials at the Earth's surface owing to its unique optical properties. Snow optical properties are sensitive to the snow microstructure, triggering potent climate feedbacks. The impacts of snow microstructure on its optical properties such as reflectance are, to date, only partially understood. However, precise modelling of snow reflectance, particularly bidirectional reflectance, are required in many problems, e.g. to correctly process satellite data over snow-covered areas. This study presents a dataset that combines bidirectional reflectance measurements over 500-2500 nm and the X-ray tomography of the snow microstructure for three snow samples of two different morphological types. The dataset is used to evaluate the stereological approach from Malinka (2014) that relates snow optical properties to the chord length distribution in the snow microstructure. The mean chord length and specific surface area (SSA) retrieved with this approach from the albedo spectrum and those measured by the X-ray tomography are in excellent agreement. The analysis of the 3D images has shown that the random chords of the ice phase obey the gamma distribution with the shape parameter m taking the value approximately equal to or a little greater than 2. For weak and intermediate absorption (high and medium albedo), the simulated bidirectional reflectances reproduce the measured ones accurately but tend to slightly overestimate the anisotropy of the radiation. For such absorptions the use of the exponential law for the ice chord length distribution instead of the one measured with the X-ray tomography does not affect the simulated re-flectance. In contrast, under high absorption (albedo of a few percent), snow microstructure and especially facet orientation at the surface play a significant role in the reflectance, particularly at oblique viewing and incidence.
Measurement of directional and spectral signatures of light reflectance by snow
IEEE Transactions on Geoscience and Remote Sensing, 2000
... Firn has the lowest albedo due to large grain sizes (in the order of several millimeters) and accumulated moraine dust contamination. ... [32] AW Nolin and J. Dozier, A hyperspectral method for remotely sensing the grain size of snow, Remote Sens. Environ., vol. 74, pp. ...
Hydrological Processes, 2004
Broadband albedo is a very important geophysical parameter in the Earth surface-atmosphere interaction in either global climate change or hydrological cycle and snowmelt runoff studies. To derive the broadband albedo accurately from satellite optical sensor observation at limited bands and at a single observation angle, the bidirectional reflectance factor (BRF) has to be specified quantitatively. In the present albedo derivation algorithms from the satellite radiance data, the BRF is either modelled or observed. Questions may arise as to how well a BRF model can be in the broadband albedo derivation. To help answer such questions, we studied the performance of a snow-surface BRF model for two specific cases under large solar zenith angles (65°and 85°). We measured snow-surface spectral directional reflectance under clear skies. The snow physical properties, such as snow grain size and snow density, at the same sites were also measured. In situ snow physical data are used to simulate the snow-surface BRF and hemispherical directional reflectance factor (HDRF) through a multilayered azimuth-and zenith-dependent plane-parallel radiative transfer model. The field measurements and BRF and HDRF simulations all reveal the forward-scattering nature of snow surface under large solar incidence angles, but the BRF model results depict the strongest forward-scattering patterns under such solar zenith angles. Because the HDRF is simulated through coupling of the surface BRF with radiative transfer in the atmosphere, the resulting HDRF patterns agree with the field measurements better than the simulated BRF does. The deviation of the simulated HDRF from field-based clear-sky directional reflectance (FCDR) is within š10% for the central (viewing zenith angle <45°) and lateral sides of the viewing hemisphere. This level of agreement between the simulated HDRF and FCDR also implies that the simulated BRF model can provide remote-sensing estimates of spectral albedo with an uncertainty of š10% for the same part of the viewing hemisphere. Further improvement in BRF model performance requires better handling of single scattering properties of snow grains, surface roughness, and atmospheric correction. Also, better procedures and techniques in field measurement are necessary for more accurate assessment of the performance of BRF models.
2011
We describe a two-parameter model for the reflectance of snow, and test it against multi-spectral and multi-angular observations. The first parameter of the model is proportional to the effective snow grain size. The second parameter accounts for the impact of soot and other pollutants on snow absorption. The model is analytical and is easily inverted against a set of multi-spectral observations. To test the ability of the model to reproduce snow reflectance, we use a multispectral and multi-directional set of measurements acquired by the POLDER-3 instrument onboard the Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with Observations from a Lidar (PARASOL) satellite launched by the French Space Agency CNES. We selected pure snow targets over Greenland and Antarctica. The model reproduces the main features of the snow angular reflectance: i) the snow reflectance generally decreases towards longer wavelengths, ii) the reflectance has maximum in the forward scattering direction at large view zenith angles, and iii) the reflectance variations in the perpendicular plane are small compared to those observed in the principal plane. The coefficient of correlation between the results of simulations and the measurements exceeds 85% in most of cases.
On the reflectance spectroscopy of snow 1
1. We acknowledge that the theory described must be extended to account for the possible snow vertical inhomogeneity and possible finite thickness of a snowpack. These topics are out of scope of this paper. The abstract, introduction, and conclusions have been modified to account for your comment. 2. We also agree that the retrieval approach will not work well in case of polluted snow with the spectral absorption coefficients of pollutants, which do not follow the Angström law. The abstract, introduction, and conclusions have been modified to account for your comment. Of course, general equations (Eqs. 1-3) to solve the direct problem of snow optics presented in the paper can be used anyway. Eq. 1 has a misprint (R0 is missed before the exponential term). We have corrected this misprint in the final version. 3. All equations and definitions are explained in the text. We have also prepared the Appendix A with all definitions and units. Also we have prepared a special section (appendi...
The effect of anisotropic reflectance on imaging spectroscopy of snow properties
2004
How does snow's anisotropic directional reflectance affect the mapping of snow properties from imaging spectrometer data? This sensitivity study applies two spectroscopy models to synthetic images of the spectral hemispherical–directional reflectance factor (HDRF) with prescribed snow-covered area and snow grain size. The MEMSCAG model determines both sub-pixel snow-covered area and the grain size of the fractional snow cover.