A mean field model of the decrease of the specific surface area of dry snow during isothermal metamorphism
A mean field model of the decrease of the specific surface area of dry snow during isothermal metamorphism
复制标题
等温变质作用过程中干雪比表面积减少的平均场模型
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
F. Dominé
中科院分区：
文献类型：
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作者：
L. Legagneux;F. Dominé
气体可接近的积雪表面面积是量化积雪与大气之间微量气体交换的重要参数，称为比表面积（SSA）。在变质作用期间，雪的SSA减少，但由于雪的物理和几何的复杂性，目前的雪模型没有描述这一点。在本文中，我们测试了是否有可能在不考虑真实雪的三维（3-D）结构的所有复杂性的情况下模拟等温变质过程中的雪SSA变化。我们建立了一个等温条件下雪变质的平均场模型，该模型基于瞬态奥斯特瓦尔德成熟的理论框架，并将雪表示为球形颗粒的分布。得到了这些球体生长速率的解析表达式，并模拟了表征雪几何特征的两个可测量参数SSA和曲率半径分布（DRC）的演变，并与x射线断层扫描得到的实验数据进行了比较。研究了温度、雪密度和凝结系数对SSA减少率的定性影响。该模型很好地预测了粒度分布的演化速率，验证了我们对等温变质作用的物理描述。特别地，我们发现气相扩散是限速的。然而，从DRC计算的SSA似乎很微妙，并且在我们对雪的三维几何形状的描述中证据过于粗糙。最后，强调初始DRC可以极大地影响SSA减少的速率，而SSA减少速率的实验测量表明，所有雪类型都以类似的方式演变。因此，大多数天然新鲜雪具有相似的drc。
[1] The surface area of snow that is accessible to gases is an essential parameter for quantifying the exchange of trace gases between the snowpack and the atmosphere and is called the specific surface area (SSA). Snow SSA decreases during metamorphism, but this is not described in current snow models owing to the complexity of the physics and geometry of snow. In this paper, we test whether it is possible to model snow SSA changes during isothermal metamorphism without accounting for all the complexity of the three-dimensional (3-D) structure of real snow. We have developed a mean field model of snow metamorphism under isothermal conditions, grounded in the theoretical framework of transient Ostwald ripening and representing snow as a distribution of spherical particles. Analytical expressions of the growth rates of these spheres are obtained, and the evolution of two measurable parameters that characterize snow geometry, the SSA and the distribution of radii of curvature (DRC), are simulated and compared to experimental data obtained by X-ray tomography. The qualitative effects of temperature, snow density, and the condensation coefficient on the rate of SSA decrease are examined. The model predicts very well the rate of evolution of the particle size distribution, which validates our physical description of isothermal metamorphism. In particular, we find that vapor phase diffusion is rate limiting. However, the calculation of the SSA from the DRC appears delicate and evidences too crude approximations in our description of the 3-D geometry of snow. Finally, it is stressed that the initial DRC can greatly influence the rate of SSA decrease, while experimental measurements of the rate of SSA decrease suggest that all snow types evolve in a similar way. It is thus proposed that most natural fresh snows have similar DRCs.