|Foster, James - NASA GODDARD SPC FLT CTR|
|Kelly, Richard - NASA GODDARD SPC FLT CTR|
|Armstrong, Richard - NAT SNOW & ICE DATA CTR|
Submitted to: Scanning
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 8, 2006
Publication Date: December 7, 2006
Citation: Foster, J., Kelly, R., Rango, A., Armstrong, R., Erbe, E.F., Pooley, C.D., Wergin, W.P. 2006. Use of low-temperature scanning electron microscopy to compare and characterize three classes of snow cover. Scanning. 28:191-203. Interpretive Summary: It is difficult to characterize crystal forms in a snowpack, even if a snowpit is dug. Visual or light microscope observations of crystals are problematic because of exposure of the snowpit walls in long periods of time. Crystal structure and crystal metamorphosis are also difficult to characterize. We used methodology developed over several years to do the sampling and then analyzed the crystal features using Low Temperature Scanning Electron Microscopy (LTSEM). Falling snow crystals, partially metamorphosed grains and large depth hoar crystal could be identified easily from top to bottom of the snowpack. LTSEM techniques can be used to enhance visual observations and improve understanding of the physics of the snowpack. Scientists working on the estimation of snow water equivalent with microwave radiometry need this LTSEM data because large snow crystals deep in the snowpack more effectively scatter passive microwave radiation than do smaller snow grains near the surface of the snowpack.
Technical Abstract: This study, which uses low temperature scanning electron microscopy (LTSEM), systematically sampled and characterized snow crystals that were collected from three unique classes of snow cover: prairie, taiga and alpine. These classes, which were defined in previous field studies, result from exposure to unique climatic variables relating to wind, precipitation and air temperature. Snow samples were taken at 10 cm depth intervals from the walls of freshly excavated snow pits. The depth of the snow pits for the prairie, taiga and alpine covers were 28 cm, 81 cm and 110 cm, respectively. Visual examination revealed that the prairie snow cover consisted of two distinct layers whereas the taiga and alpine covers had four distinct layers. Visual measurements were able to establish the range of crystal sizes that occurred in each layer, the temperature within the pit and the snow density. The LTSEM observations revealed the detailed structures of the types of crystals that occurred in the snow covers and documented the metamorphosis that transpired in the descending layers. Briefly, the top layers from two of the snow covers, consisted of freshly fallen snow crystals that could be readily distinguished as plates and columns (prairie) or graupel (taiga). Alternatively, the top layer in the alpine cover consisted of older dendritic crystal fragments that had undergone early metamorphosis, i.e. they had lost their sharp edges and had begun to show signs of joining or bonding with neighboring crystals. A unique layer, known as sun crust, was found in the prairie snow cover; however, successive samplings from all three snow covers showed similar stages of metamorphism that led to the formation of depth hoar crystals. These changes included the gradual development of large, three-dimensional crystals having clearly defined flat faces, sharp edges, internal depressions and facets. The study, which indicates that LTSEM can be used to enhance visual data by systematically characterizing snow crystals that are collected at remote locations, is important for understanding the physics of snowpacks and the metamorphosis that leads to potential avalanche situations. In addition, the metamorphosis of snow crystals must be considered when microwave radiometry is used to estimate the snow water equivalent in the winter snowpack, because large snow crystals more effectively scatter passive microwave radiation than small crystals.