scholarly journals Vapor flux and recrystallization during dry snow metamorphism under a steady temperature gradient as observed by time-lapse micro-tomography

2012 ◽  
Vol 6 (3) ◽  
pp. 1673-1714 ◽  
Author(s):  
B. R. Pinzer ◽  
M. Schneebeli ◽  
T. U. Kaempfer
Keyword(s):  
Temperature Gradient ◽  
Physical Properties ◽  
Time Lapse ◽  
Steady Temperature ◽  
Data Set ◽  
External Temperature ◽  
Vapor Flux ◽  
Snow Metamorphism ◽  
Micro Tomography

Abstract. Dry snow metamorphism under an external temperature gradient is the most common type of recrystallization of snow on the ground. The changes in snow microstructure modify the physical properties of snow, and therefore an understanding of this process is essential for many disciplines, from modeling the effects of snow on climate to assessing avalanche risk. We directly imaged the microstructural changes in snow during metamorphism under a steady temperature gradient (STGM) of 50 K m−1, using in situ time-lapse X-ray micro-tomography. This novel and non-destructive technique directly reveals the amount of ice that sublimates and is deposited during metamorphism, and in addition the exact locations of these phase changes. From the four-dimensional data set, we calculated the average time that an ice volume stayed in place before it sublimated, and found a characteristic residence time of 2–3 days. This means that most of the ice changes its phase from solid to vapor and back many times in a seasonal snow pack, where similar temperature conditions can be found. Consistent with such a short timescale, we observed a mass turnover of up to 60 % of the total ice mass per day. The concept of hand-to-hand transport for the water vapor flux describes the observed changes very well. However, we did not find evidence for a macroscopic vapor diffusion enhancement. The picture of STGM that is produced by directly observing the microstructure of snow in situ sheds light on the micro-physical processes and could help to improve models that predict the physical properties of snow.

scholarly journals Vapor flux and recrystallization during dry snow metamorphism under a steady temperature gradient as observed by time-lapse micro-tomography

2012 ◽  
Vol 6 (5) ◽  
pp. 1141-1155 ◽  
Author(s):  
B. R. Pinzer ◽  
M. Schneebeli ◽  
T. U. Kaempfer
Keyword(s):  
Temperature Gradient ◽  
Physical Properties ◽  
Time Lapse ◽  
Phase Changes ◽  
External Temperature ◽  
Vapor Flux ◽  
Snow Metamorphism ◽  
Diffusion Enhancement ◽  
Micro Tomography ◽  
Snow Microstructure

Abstract. Dry snow metamorphism under an external temperature gradient is the most common type of recrystallization of snow on the ground. The changes in snow microstructure modify the physical properties of snow, and therefore an understanding of this process is essential for many disciplines, from modeling the effects of snow on climate to assessing avalanche risk. We directly imaged the microstructural changes in snow during temperature gradient metamorphism (TGM) under a constant gradient of 50 K m−1, using in situ time-lapse X-ray micro-tomography. This novel and non-destructive technique directly reveals the amount of ice that sublimates and is deposited during metamorphism, in addition to the exact locations of these phase changes. We calculated the average time that an ice volume stayed in place before it sublimated and found a characteristic residence time of 2–3 days. This means that most of the ice changes its phase from solid to vapor and back many times in a seasonal snowpack where similar temperature conditions can be found. Consistent with such a short timescale, we observed a mass turnover of up to 60% of the total ice mass per day. The concept of hand-to-hand transport for the water vapor flux describes the observed changes very well. However, we did not find evidence for a macroscopic vapor diffusion enhancement. The picture of {temperature gradient metamorphism} produced by directly observing the changing microstructure sheds light on the micro-physical processes and could help to improve models that predict the physical properties of snow.

scholarly journals Tomography-based observation of sublimation and snow metamorphism under temperature gradient and advective flow

2015 ◽  
Vol 9 (5) ◽  
pp. 4845-4864
Author(s):  
P. P. Ebner ◽  
M. Schneebeli ◽  
A. Steinfeld
Keyword(s):  
Water Vapor ◽  
Temperature Gradient ◽  
Phase Change ◽  
Atmospheric Chemistry ◽  
Solid Phase ◽  
Time Lapse ◽  
Advective Flow ◽  
Snow Metamorphism ◽  
Micro Tomography ◽  
Ice Sublimation

Abstract. Snow at or close to the surface commonly undergoes temperature gradient metamorphism under advective flow, which alters its microstructure and physical properties. Time-lapse X-ray micro-tomography is applied to investigate the structural dynamics of temperature gradient snow metamorphism exposed to an advective airflow in controlled laboratory conditions. The sublimation of water vapor for saturated air flowing across the snow sample was experimentally determined via variations of the porous ice structure. The results showed that the exothermic gas-to-solid phase change is favorable vis-a-vis the endothermic solid-to-gas phase change, thus leading to more ice deposition than ice sublimation. Sublimation has a marked effect on the structural change of the ice matrix but diffusion of water vapor in the direction of the temperature gradient counteracted the mass transport of advection. Therefore, the total net ice change was negligible leading to a constant porosity profile. However, the strong reposition process of water molecules on the ice grains is relevant for atmospheric chemistry.

scholarly journals Orientation selective grain sublimation-deposition in snow under temperature gradient metamorphism observed with Diffraction Contrast Tomography

2021 ◽  
Author(s):  
Rémi Granger ◽  
Frédéric Flin ◽  
Wolfgang Ludwig ◽  
Ismail Hammad ◽  
Christian Geindreau
Keyword(s):  
Temperature Gradient ◽  
Horizontal Plane ◽  
Time Lapse ◽  
Snow Sample ◽  
Crystalline Orientation ◽  
Diffraction Contrast ◽  
Crystallographic Facets ◽  
Over Time ◽  
Material Fluxes

Abstract. In this study on temperature gradient metamorphism in snow, we investigate the hypothesis that there exists a favorable crystalline orientation relative to the temperature gradient, giving rise to a faster formation of crystallographic facets. We applied in-situ time-lapse Diffraction Contrast Tomography on a snow sample with a density of 476 kg m−3 subject to a temperature gradient of 52 °C m−1 at mean temperatures in the range between −4.1 °C and −2.1 °C for three days. The orientations of about 900 grains along with their microstructural evolution are followed over time. Faceted crystals appear during the evolution and from the analysis of the material fluxes, we indeed observe higher sublimation-deposition rate for grains with their c-axis in the horizontal plane at the beginning of the metamorphism. This remains the case up to the end of the experiment for what concerns sublimation while the differences vanish for deposition. That latter observation is explained in terms of geometrical interactions between grains.

Time-series within automatically generated ROIs from wildlife cameras are well able to explain variability in forest phenology on a temperature gradient

2020 ◽  
Author(s):  
Lars Uphus ◽  
Annette Menzel
Keyword(s):  
Time Series ◽  
Temperature Gradient ◽  
Time Lapse ◽  
Relevant Information ◽  
High Temporal Resolution ◽  
Deciduous Species ◽  
Multiple Thresholds ◽  
In Situ Observations ◽  
Wildlife Cameras

<p>Using RGB camera data (e.g. webcams, wildlife cameras) has great potential to measure forest phenology over climate gradients, because of its very high temporal resolution, while at the same time being more objective and less time consuming than in situ observations. To make images useful for the purpose of measuring phenological events, such as Start of Season (SOS) and End of Season (EOS), there is need to derive Regions of Interest (ROI) objectively and (semi-)automatically. In order to answer this need, Bothmann et al. (2017) proposed a method which randomly sets a number of pinpricks in the image and calculates how greenness over time from all other pixels correlates to these different pinpricks. Subsequently, ROIs are created by discarding the pixels with low correlation, using multiple thresholds. Despite its advantage of being automated and more objective compared to prevailing expert-based ROIs, and therefore its potential applicability for phenological research using a large amount of cameras, the method has not been reproduced for this purpose so far. Therefore, we assess here how well this method is able to separate foliage of different deciduous species from evergreens and phenologically irrelevant components in time-lapse wildlife camera data and in that way how suitable it is in explaining variation in phenology over a temperature gradient. We used 73 Cuddleback wildlife cameras troughout Bavaria which were installed within nine quadrants of 6*6 kilometers spanning a temperature gradient of 2.5°C. Hourly taken images of deciduous forests in spring, summer and autumn 2019 were analysed. Half of them were facing canopy, and half of them were facing understory. We applied the principles of the method from Bothmann et al. (2017) and assigned the best matching ROI to foliage of <em>Fagus sylvatica</em> or other deciduous species. Within this ROI, mean Green Chromatic Coordinate (GCC), a greenness index, over all pixels within the ROI, was derived per time-stamp. Afterwards, a time-series was calculated on these GCC values and with a suitable combination of curve-fitting techniques, SOS and EOS were derived, expressed in Day of Year (DOY). We compared these SOS and EOS dates with weekly in situ observations of spring and autumn phenology, which were taken in the same quadrants. Despite that Bothmann's method was developed on a single tower-mounted scientific webcam which viewed on canopy from above, while we made use of wildlife cameras at 73 different locations facing either understory perpendicular or canopy from below, it was able to distinguish <em>F. sylvatica</em> and other deciduous foliage from phenologically less relevant information. Time-series derived from these ROIs were able to explain variability in phenology between understory and canopy and over the temperature gradient similarly and supplementary to in situ observations. </p>

scholarly journals Tomography-based monitoring of isothermal snow metamorphism under advective conditions

2015 ◽  
Vol 9 (1) ◽  
pp. 1021-1045
Author(s):  
P. P. Ebner ◽  
M. Schneebeli ◽  
A. Steinfeld
Keyword(s):  
Structural Dynamics ◽  
Transport Process ◽  
Laboratory Experiments ◽  
Time Lapse ◽  
Digital Geometry ◽  
X Ray ◽  
Effective Transport ◽  
Snow Metamorphism ◽  
Kelvin Effect ◽  
Micro Tomography

Abstract. Time-lapse X-ray micro-tomography was used to investigate the structural dynamics of isothermal snow metamorphism exposed to an advective airflow. Diffusion and advection across the snow pores were analysed in controlled laboratory experiments. The 3-D digital geometry obtained by tomographic scans was used in direct pore-level numerical simulations to determine the effective transport properties. The results showed that isothermal advection with saturated air have no influence on the coarsening rate that is typical for isothermal snow metamorphism. Diffusion originating in the Kelvin effect between snow structures dominates and is the main transport process in isothermal snow packs.

scholarly journals Amplification and Attenuation of Acoustic Phonons in Graphenelike Silicene

2021 ◽  
Vol 9 (12) ◽  
pp. 1594-1597
Author(s):  
Kasala Suresha
Keyword(s):  
Temperature Gradient ◽  
Physical Properties ◽  
Dirac Fermions ◽  
Acoustic Phonons ◽  
External Temperature ◽  
Electronic Dispersion

Abstract: Because of unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. We calculate theoretically in this article, the amplification and attenuation of acoustic phonons due to an external temperature gradient in Silicene at temperature ࢀ= 77K in the hypersound regime. The dependence of normalized amplification or attenuation on the frequency wasnumerically evaluated. It is observed from our calculations that when the temperature gradient is zero, absorption of acoustic phonons occurs and when temperature gradient is greater than zero, absorption switches to amplification of acoustic phonons. Keywords: Silicene, Amplification, Attenuation, Acoustic phonons, Temperature gradient.

scholarly journals Early-stage interaction between settlement and temperature-gradient metamorphism

2017 ◽  
Vol 63 (240) ◽  
pp. 652-662 ◽  
Author(s):  
MAREIKE WIESE ◽  
MARTIN SCHNEEBELI
Keyword(s):  
Mechanical Properties ◽  
Temperature Gradient ◽  
Early Stage ◽  
Large Change ◽  
Time Lapse ◽  
Micro Computed Tomography ◽  
Isothermal Conditions ◽  
Small Change ◽  
Avalanche Formation

ABSTRACTSnow metamorphism and settlement change the microstructure of a snowpack simultaneously. Past experiments investigated snow deformation under isothermal conditions. In nature, temperature gradient metamorphism and settlement often occur together. We investigated snow settlement in the first days after the onset of temperature-gradient metamorphism in laboratory experiments by means of in-situ time-lapse micro-computed tomography. We imposed temperature gradients of up to 95 K m−1 on samples of rounded snow with a density of ~230 kg m−3 and induced settlement by applying 1.7 kPa stress with a passive load on the samples simultaneously. We found that snow settled about half as fast when a temperature gradient was present, compared with isothermal conditions. The change in specific surface area after 4 days caused by temperature-gradient metamorphism was only a few percent. The viscosity evolution correlated with the amount of the temperature gradient. Finite element simulations of the snow samples revealed that stress-bearing chains had developed in the snow structure, causing the large increase in viscosity. We could show that a small change in microstructure caused a large change in the mechanical properties. This explains the difficulty of predicting snow mechanical properties in applications such as firn compaction or snow avalanche formation.

scholarly journals An instrumented sample holder for time-lapse micro-tomography measurements of snow under advective airflow

2014 ◽  
Vol 4 (1) ◽  
pp. 353-373
Author(s):  
P. P. Ebner ◽  
S. A. Grimm ◽  
M. Schneebeli ◽  
A. Steinfeld
Keyword(s):  
Structural Changes ◽  
Time Lapse ◽  
Sample Holder ◽  
Experimental Characterization ◽  
Computational Fluid Dynamics Simulations ◽  
Micro Tomography ◽  
Dynamics Simulations ◽  
Snow Samples

Abstract. An instrumented sample holder was developed for time-lapse micro-tomography of snow samples to enable in-situ nondestructive spatial and temporal measurements under controlled advective airflows, temperature gradients, and air humidities. The design was aided by computational fluid dynamics simulations to evaluate the airflow uniformity across the snow sample. Morphological and mass transport properties were evaluated during a 4 day test run. This instrument allows the experimental characterization of metamorphism of snow undergoing structural changes with time.

scholarly journals Orientation selective grain sublimation–deposition in snow under temperature gradient metamorphism observed with diffraction contrast tomography

2021 ◽  
Vol 15 (9) ◽  
pp. 4381-4398
Author(s):  
Rémi Granger ◽  
Frédéric Flin ◽  
Wolfgang Ludwig ◽  
Ismail Hammad ◽  
Christian Geindreau
Keyword(s):  
Temperature Gradient ◽  
Horizontal Plane ◽  
Time Lapse ◽  
Deposition Rates ◽  
Crystalline Orientation ◽  
Diffraction Contrast ◽  
Crystallographic Facets ◽  
Over Time ◽  
Material Fluxes

Abstract. In this study on temperature gradient metamorphism in snow, we investigate the hypothesis that there exists a favourable crystalline orientation relative to the temperature gradient, giving rise to a faster formation of crystallographic facets. We applied in situ time-lapse diffraction contrast tomography on a snow sample with a density of 476 kg m−3 subject to a temperature gradient of 52 ∘Cm-1 at mean temperatures in the range between −4.1 and −2.1 ∘C for 3 d. The orientations of about 900 grains along with their microstructural evolution are followed over time. Faceted crystals appear during the evolution, and from the analysis of the material fluxes, we observe higher sublimation–deposition rates for grains with their c axis in the horizontal plane at the beginning of the metamorphism. This remains the case up to the end of the experiment for what concerns sublimation while the differences vanish for deposition. The latter observation is explained in terms of geometrical interactions between grains.

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