Snow Metamorphism, Near Surface Faceting

要研究近地表雪的变质作用，特别是重结晶、多面化的雪。这一气象驱动的过程受到太阳辐射和红外辐射的强烈影响。推动这项研究的假设是，由于雪的近表面刻面而导致的微观结构变化将改变雪的热机械和光学性质。通常，随着积雪的老化，变质作用会导致圆形颗粒，伴随着强度的增加和反照率的降低。然而，圆形颗粒雪的刻面会产生较弱的结构，但它也可能变得更具反射性。除了再结晶层在表面起作用外，热力学性质的变化也会影响随后被埋藏的积雪的热状态。显然，雪是一种基本的水源，因此与能量平衡和融化开始相关的过程是重要的。另一个重要的方面是，当被更多的降雪掩埋时，地表弱分面层的发展可能成为雪崩可能性的主要因素。该项目将结合实验室、现场和数学模型研究。这项研究的基础是一个具有可编程温度、太阳能(金属卤化物灯)、天空温度(独立控制天花板温度)和可调节空气循环速度的环境室。湿度将被监测，但不能精确控制在零下的温度。非接触式传感器将监测雪面温度，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以及计算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仪(CT)和光学显微镜来检查微观结构。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将被研究。随着变质作用的进行，将使用雪微渗透仪进行强度的机械变化。在选定的样品上，将尝试将近地表变质的雪作为埋层，并在规定的法向载荷下测试其横向剪切强度。将使用针状导电性探头测量热导率，并使用高光谱(0.4m至0.912 m)成像来确定反照率。可能影响能量吸收的颗粒污染将在流式细胞仪中用熔融的样品来确定。通过激光雷达数字高程图(DEM)S，考虑复杂自然地形的第一性原理能量平衡模型将被利用和改进，以利用在监测的山地环境中测量的气象条件来计算雪的热状态。将使用两个仪表站以及空间表面温度变化来验证该模型，这两个现场将使用热成像相机在离散的时间测量。

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.