原始冰晶和聚集雪花的电容
The capacitance of pristine ice crystals and aggregate snowflakes
摘要：
描述了一种精确计算真实冰粒电容的新方法：这些值是数值预报模式中精确估计沉积和蒸发速率的关键。扩散的水分子的轨迹直接采样，使用随机的“步行者”。通过计算这些轨迹中有多少与冰粒（可以是任何形状）的表面相交，以及有多少逃离远离冰粒的球形边界，已经估计了一些模型冰粒习惯的电容，包括六边形柱和板，“不等边”柱和板，子弹，子弹状突起，树枝状突起和现实的聚集雪花。对于具有尖锐棱角的冰粒，该方法是求解拉普拉斯方程的电容的有效和直接的方法。如果使用足够多的随机游走器对颗粒几何形状进行采样，作者预计计算出的电容准确度在~ 1%以内。我们的建模总雪花（C/Dmax=0.25，归一化的最大尺寸Dmax）的电容示出与最近的飞机测量的雪花升华率密切一致。该结果表明，数值模型中常用的球体电容（C/Dmax=0.5）将蒸发速率高估了2倍。还研究了相邻生长的晶体对蒸气“屏蔽”的影响。结果清楚地表明，在云室实验中，在灯丝上生长的相邻晶体可以强烈地限制彼此的蒸汽供应，并且由此产生的生长速率测量可能严重低估孤立的单晶的速率（在我们的模型设置中为3倍）。
英文摘要：
A new method of accurately calculating the capacitance of realistic ice particles is described: such values are key to accurate estimates of deposition and evaporation rates in NWP models. The trajectories of diffusing water molecules are directly sampled, using random `walkers'. By counting how many of these trajectories intersect the surface of the ice particle (which may be any shape) and how many escape outside a spherical boundary far from the particle, the capacitance of a number of model ice particle habits have been estimated, including hexagonal columns and plates, `scalene' columns and plates, bullets, bullet-rosettes, dendrites, and realistic aggregate snowflakes. For ice particles with sharp edges and corners this method is an efficient and straightforward way of solving Laplace's equation for the capacitance. Provided that a large enough number of random walkers are used to sample the particle geometry the authors expect the calculated capacitances to be accurate to within ~1%. The capacitance for our modelled aggregate snowflakes (C/Dmax=0.25, normalised by the maximum dimension Dmax) is shown to be in close agreement with recent aircraft measurements of snowflake sublimation rates. This result shows that the capacitance of a sphere (C/Dmax=0.5) which is commonly used in numerical models, overestimates the evaporation rate by a factor of 2. The effect of vapor `screening' by crystals growing in the vicinity of one another has also been investigated. The results clearly show that neighbouring crystals growing on a filament in cloud chamber experiments can strongly constrict the vapor supply to each other, and the resulting growth rate measurements may severely underestimate the rate for a single crystal in isolation (by a factor of 3 in our model setup).
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