Rango Albert, Foster James, Josberger Edward G, Erbe Eric F, Pooley Christopher, Wergin William P
Jornada Experimental Range, Agricultural Research Service, U.S. Department of Agriculture, New Mexico State University, Las Cruces, New Mexico, USA.
Scanning. 2003 May-Jun;25(3):121-31. doi: 10.1002/sca.4950250304.
Snow crystals, which form by vapor deposition, occasionally come in contact with supercooled cloud droplets during their formation and descent. When this occurs, the droplets adhere and freeze to the snow crystals in a process known as accretion. During the early stages of accretion, discrete snow crystals exhibiting frozen cloud droplets are referred to as rime. If this process continues, the snow crystal may become completely engulfed in frozen cloud droplets. The resulting particle is known as graupel. Light microscopic investigations have studied rime and graupel for nearly 100 years. However, the limiting resolution and depth of field associated with the light microscope have prevented detailed descriptions of the microscopic cloud droplets and the three-dimensional topography of the rime and graupel particles. This study uses low-temperature scanning electron microscopy to characterize the frozen precipitates that are commonly known as rime and graupel. Rime, consisting of frozen cloud droplets, is observed on all types of snow crystals including needles, columns, plates, and dendrites. The droplets, which vary in size from 10 to 100 microm, frequently accumulate along one face of a single snow crystal, but are found more randomly distributed on aggregations consisting of two or more snow crystals (snowflakes). The early stages of riming are characterized by the presence of frozen cloud droplets that appear as a layer of flattened hemispheres on the surface of the snow crystal. As this process continues, the cloud droplets appear more sinuous and elongate as they contact and freeze to the rimed crystals. The advanced stages of this process result in graupel, a particle 1 to 3 mm across, composed of hundreds of frozen cloud droplets interspersed with considerable air spaces; the original snow crystal is no longer discernible. This study increases our knowledge about the process and characteristics of riming and suggests that the initial appearance of the flattened hemispheres may result from impact of the leading face of the snow crystal with cloud droplets. The elongated and sinuous configurations of frozen cloud droplets that are encountered on the more advanced stages suggest that aerodynamic forces propel cloud droplets to the trailing face of the descending crystal where they make contact and freeze.
通过气相沉积形成的雪晶在其形成和下降过程中偶尔会与过冷云滴接触。当这种情况发生时，云滴会在一个称为吸积的过程中附着并冻结在雪晶上。在吸积的早期阶段，呈现冻结云滴的离散雪晶被称为霜。如果这个过程持续下去，雪晶可能会完全被冻结云滴吞没。由此产生的粒子被称为霰。光学显微镜研究霜和霰已经有近100年的历史了。然而，与光学显微镜相关的有限分辨率和景深阻碍了对微观云滴以及霜和霰粒子三维形貌的详细描述。本研究使用低温扫描电子显微镜来表征通常被称为霜和霰的冻结沉淀物。在包括针状、柱状、片状和枝状在内的所有类型的雪晶上都观察到了由冻结云滴组成的霜。这些云滴大小从10到100微米不等，经常沿着单个雪晶的一个面聚集，但在由两个或更多雪晶（雪花）组成的聚集体上分布更随机。霜形成的早期阶段的特征是存在冻结云滴，这些云滴在雪晶表面呈现为一层扁平的半球体。随着这个过程的持续，云滴在与结霜的晶体接触并冻结时显得更加蜿蜒和细长。这个过程的后期阶段会产生霰，一种直径为1到3毫米的粒子，由数百个冻结云滴散布着相当大的空隙组成；原来的雪晶已不再可辨。这项研究增加了我们对霜形成过程和特征的了解，并表明扁平半球体的最初出现可能是由于雪晶的前沿面与云滴碰撞造成的。在后期阶段遇到的冻结云滴的细长和蜿蜒形态表明，空气动力将云滴推向下降晶体的后沿面，在那里它们接触并冻结。
