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Reflectance Modeling for Real Snow Structures Using a Beam Tracing Model

View Article: PubMed Central

ABSTRACT

It is important to understand reflective properties of snow, for example for remote sensing applications and for modeling of energy balances in snow packs. We present a method with which we can compare reflectance measurements and calculations for the same snow sample structures. Therefore, we first tomograph snow samples to acquire snow structure images (6 × 2 mm). Second, we calculated the sample reflectance by modeling the radiative transfer, using a beam tracing model. This model calculates the biconical reflectance (BR) derived from an arbitrary number of incident beams. The incident beams represent a diffuse light source. We applied our method to four different snow samples: Fresh snow, metamorphosed snow, depth hoar, and wet snow. The results show that (i) the calculated and measured reflectances agree well and (ii) the model produces different biconical reflectances for different snow types. The ratio of the structure to the wavelength is large. We estimated that the size parameter is larger than 50 in all cases we analyzed. Specific surface area of the snow samples explains most of the difference in radiance, but not the different biconical reflectance distributions. The presented method overcomes the limitations of common radiative transfer models which use idealized grain shapes such as spheres, plates, needles and hexagonal particles. With this method we could improve our understanding for changes in biconical reflectance distribution associated with changes in specific surface area.

No MeSH data available.


Experimental setup for measuring the reflectance of a snow sample. The description above the snow cube describes the reflectance measurement. The cylinder within the snow cube illustrates that we extracted after the reflectance measurement a snow sample of 6cm height. From this cylinder we tomographed a snow layer resulting in 600 tomographed slices. These cross sections are then used as input for the beam tracing model.
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f4-sensors-08-03482: Experimental setup for measuring the reflectance of a snow sample. The description above the snow cube describes the reflectance measurement. The cylinder within the snow cube illustrates that we extracted after the reflectance measurement a snow sample of 6cm height. From this cylinder we tomographed a snow layer resulting in 600 tomographed slices. These cross sections are then used as input for the beam tracing model.

Mentions: For the reflectance measurement in the laboratory we took the snow cube out of the Styrofoam box and prepared the sample surface with a sharp metal plate to be completely flat. The reflectance of the samples was measured with a standard field spectrometer “Field Spec Pro Dual VNIR” from “Analytical Spectral Devices”. The spectrometer measures the spectrum from 350 to 1050 nm using a 512-channel silicon photo-diode array. Since reflectance of snow varies strongly in the near infrared (NIR) spectrum [6] we made our analysis for one single wavelength, λ=870 nm. For our radiance measurements we attached a 3° field-of-view fore-optic to the glass fiber. Each sample was scanned 15 times (five repetitions at three different positions at the snow surface) to minimize measuring errors. The three positions were always close to the center of the surface of the snow sample. The geometric arrangement of the light source and the detector, and the distance of them to the snow surface were kept constant with respect to measured position at the snow surface (Figure 4).


Reflectance Modeling for Real Snow Structures Using a Beam Tracing Model
Experimental setup for measuring the reflectance of a snow sample. The description above the snow cube describes the reflectance measurement. The cylinder within the snow cube illustrates that we extracted after the reflectance measurement a snow sample of 6cm height. From this cylinder we tomographed a snow layer resulting in 600 tomographed slices. These cross sections are then used as input for the beam tracing model.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3675556&req=5

f4-sensors-08-03482: Experimental setup for measuring the reflectance of a snow sample. The description above the snow cube describes the reflectance measurement. The cylinder within the snow cube illustrates that we extracted after the reflectance measurement a snow sample of 6cm height. From this cylinder we tomographed a snow layer resulting in 600 tomographed slices. These cross sections are then used as input for the beam tracing model.
Mentions: For the reflectance measurement in the laboratory we took the snow cube out of the Styrofoam box and prepared the sample surface with a sharp metal plate to be completely flat. The reflectance of the samples was measured with a standard field spectrometer “Field Spec Pro Dual VNIR” from “Analytical Spectral Devices”. The spectrometer measures the spectrum from 350 to 1050 nm using a 512-channel silicon photo-diode array. Since reflectance of snow varies strongly in the near infrared (NIR) spectrum [6] we made our analysis for one single wavelength, λ=870 nm. For our radiance measurements we attached a 3° field-of-view fore-optic to the glass fiber. Each sample was scanned 15 times (five repetitions at three different positions at the snow surface) to minimize measuring errors. The three positions were always close to the center of the surface of the snow sample. The geometric arrangement of the light source and the detector, and the distance of them to the snow surface were kept constant with respect to measured position at the snow surface (Figure 4).

View Article: PubMed Central

ABSTRACT

It is important to understand reflective properties of snow, for example for remote sensing applications and for modeling of energy balances in snow packs. We present a method with which we can compare reflectance measurements and calculations for the same snow sample structures. Therefore, we first tomograph snow samples to acquire snow structure images (6 × 2 mm). Second, we calculated the sample reflectance by modeling the radiative transfer, using a beam tracing model. This model calculates the biconical reflectance (BR) derived from an arbitrary number of incident beams. The incident beams represent a diffuse light source. We applied our method to four different snow samples: Fresh snow, metamorphosed snow, depth hoar, and wet snow. The results show that (i) the calculated and measured reflectances agree well and (ii) the model produces different biconical reflectances for different snow types. The ratio of the structure to the wavelength is large. We estimated that the size parameter is larger than 50 in all cases we analyzed. Specific surface area of the snow samples explains most of the difference in radiance, but not the different biconical reflectance distributions. The presented method overcomes the limitations of common radiative transfer models which use idealized grain shapes such as spheres, plates, needles and hexagonal particles. With this method we could improve our understanding for changes in biconical reflectance distribution associated with changes in specific surface area.

No MeSH data available.