The Capacitance of Pristine Ice Crystals and Aggregate Snowflakes
The Capacitance of Pristine Ice Crystals and Aggregate Snowflakes
复制标题
原始冰晶和聚集雪花的电容
DOI：
10.1175/2007jas2315.1
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发表时间：
2006
影响因子：
3.1
通讯作者：
A. Illingworth
中科院分区：
文献类型：
--
作者：
C. Westbrook;R. Hogan;A. Illingworth
描述了一种精确计算真实冰粒电容的新方法：这些值是精确估计数值天气模型中沉积和蒸发（升华）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的表面相交（可以是任何形状），以及有多少逃逸到远离冰粒的球形边界之外，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，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一种有效而直接的方法。如果使用足够多的随机游走器对颗粒几何形状进行采样（1010 - 4），则作者预计计算的电容准确度在± 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）高估了雪花的蒸发速率两倍。还研究了在彼此附近生长的晶体对蒸气“屏蔽”的影响。结果清楚地表明，在云室实验中生长在灯丝上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了孤立的单晶的速率（在我们的模型设置中为3倍）。
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 (sublimation) rates in numerical weather 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 ofsolving Laplace’s equation for the capacitance. Provided that a large enough number of random walkers are used to sample the particle geometry (∼ 10 4 ) 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 of snowflakes by a factor of two. 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).