Snow Metamorphism, Near Surface Faceting

雪变质作用、近地表刻面

• 批准号: 0635977
• 负责人: Edward Adams
• 金额: $ 15.86万
• 依托单位: Montana State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-01 至 2011-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0635977&HistoricalAwards=false
• 关键词: Snow; Metamorphism; Near; Surface; Faceting

Near-surface snow metamorphism is to be examined, with particular emphasis on recrystallized, faceted snow. This meteorologically driven process is highly influenced by insolation and IR radiation. The hypothesis driving the investigation is that microstructural changes due to near-surface faceting of snow will alter the thermomechanical and optical properties. Typically, as snow ages, metamorphism leads to rounded grains accompanied by increased strength and reduced albedo. However, faceting of rounded grain snow yields a weaker structure but it may also become more reflective. In addition to the role that recrystallized layers play while at the surface, changes in thermodynamic properties will also influence the thermal state of the snowpack when subsequently buried. Clearly, snow is an essential water source so processes pertinent to the energy balance and the onset of melt are significant. Another important aspect is that the development of weak faceted layers on the surface may become a major factor in the avalanche potential when buried by additional snowfall.The project will combine laboratory, field and mathematical modeling studies. Fundamental to the study is an environmental chamber with programmable temperature, solar (metal-halide lamp), sky temperature (independently controlled ceiling temperature) and adjustable air circulation-velocity. Humidity will be monitored, but is not precisely controlled at sub-zero temperatures. A non contact sensor will monitor snow surface temperature and a thermocouple array will measure temperature gradients in the snow. The influence of weather on observed metamorphism at two field sites along with a calculated energy balance will guide the appropriate laboratory conditions. In the laboratory, microstructure will be inspected with a computed tomography (CT) scanner and an optical microscope. The potential to measure specific surface area employing part of the IR band from a hyperspectral imaging camera will be examined. Mechanical changes in strength as the metamorphism proceeds will be made using a snow micro-penetrometer. On selected samples an attempt will be made to incorporate the near-surface metamorphosed snow as a buried layer and to test its strength in cross shear, with a prescribed normal load. Thermal conductivity will be measured using a needle conductivity probe and albedo determined using hyper-spectral (0.4 to 0.912 m) imaging. Particulate contamination, which may influence energy absorption, will be ascertained with melted samples in a flow cytometer. A first principles energy balance model that accounts for complex natural topography, via lidar digital elevation map (DEM)'s, will be utilized and enhanced to calculate the thermal state of snow using measured meteorological conditions in a monitored mountain environment. Two instrumented field sites along with spatial surface temperature variations, which will be measured using a thermal imaging camera at discrete times, will be used to validate the model.

近地表的雪变质是要检查，特别强调重结晶，多面雪。这一气象驱动的过程受到太阳辐射和红外辐射的高度影响。驱动调查的假设是，由于雪的近表面刻面的微观结构的变化将改变热机械和光学性能。通常，随着雪的老化，变质作用导致圆形颗粒伴随着强度增加和硬度降低。然而，圆形颗粒雪的小面产生较弱的结构，但它也可能变得更具反射性。除了重结晶层在表面发挥的作用外，热力学性质的变化也会影响积雪随后被掩埋时的热状态。显然，雪是一种重要的水源，因此与能量平衡和融化开始有关的过程是重要的。另一个重要的方面是，当被额外的降雪掩埋时，地表上弱刻面层的发展可能成为雪崩潜力的主要因素。该项目将结合联合收割机实验室、现场和数学建模研究。研究的基础是一个环境室，具有可编程温度，太阳能（金属卤化物灯），天空温度（独立控制的天花板温度）和可调节的空气循环速度。湿度将受到监测，但在零度以下的温度下不会精确控制。非接触式传感器将监测雪表面温度，热电偶阵列将测量雪中的温度梯度。天气对两个现场观察到的变质作用的影响沿着计算出的能量平衡将指导适当的实验室条件。在实验室中，将使用计算机断层扫描（CT）扫描仪和光学显微镜检查微观结构。将研究利用高光谱成像相机的部分红外波段测量比表面积的可能性。随着变质作用的进行，强度的机械变化将使用雪微压计进行。在选定的样品上，将尝试将近地表的变质雪作为埋层，并在规定的法向载荷下测试其横向剪切强度。热导率将使用针式电导率探头测量，热导率将使用高光谱（0.4至0.912米）成像确定。可能影响能量吸收的微粒污染将在流式细胞仪中用熔化的样本进行确认。将利用并增强第一原理能量平衡模型，该模型通过激光雷达数字高程图（DEM）来解释复杂的自然地形，以使用监测的山区环境中测量的气象条件来计算雪的热状态。两个装有仪器的现场沿着与空间表面温度变化，这将使用热成像相机在离散时间测量，将用于验证模型。