从过冷和冰川层云落下的定向平面冰晶的多普勒激光雷达测量
Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds
摘要：
利用垂直指向的多普勒激光雷达研究了水平沉降的平面冰晶的特性。强烈的镜面反射，观察到从他们的定向基面，通过比较与第二个激光雷达指向4度从天顶。对17个月连续高分辨率观测的分析表明，这些原始晶体经常在从中层混合相层云中落下的冰中观察到（-15 ℃层的时间为85%）。对一个实例的详细分析表明，晶体在过冷层内成核并迅速生长，然后脱落，形成定义明确的镜面反射层。从激光雷达单独的部分定向晶体不能量化，但偏振雷达测量证实，相当大一部分的晶体人口是良好的取向。当晶体落入欠饱和空气中时，观察到镜面反射随着晶体面变圆并失去其多面结构而关闭。从低于-22 ℃的过冷层落下的冰中也观察到镜面反射，但比在更温暖的温度下明显得多：我们认为，在冷云中，是分布中的小液滴冻结成板并产生镜面反射，而较大的液滴冻结成复杂的多晶体。激光雷达多普勒测量表明，典型的下降速度为0.3米/秒的定向晶体，与弱的温度相关性;相应的雷诺数为Re~10，与光柱测量一致。同时多普勒雷达观测表明镜面增强和涡流耗散率之间没有相关性，表明湍流不控制这些云的晶体取向。
英文摘要：
The properties of planar ice crystals settling horizontally have been investigated using a vertically-pointing Doppler lidar. Strong specular reflections were observed from their oriented basal facets, identified by comparison with a second lidar pointing 4deg from zenith. Analysis of 17 months of continuous high-resolution observations reveal that these pristine crystals are frequently observed in ice falling from mid-level mixed-phase layer clouds (85% of the time for layers at -15C). Detailed analysis of a case study indicates that the crystals are nucleated and grow rapidly within the supercooled layer, then fall out, forming well-defined layers of specular reflection. From the lidar alone the fraction of oriented crystals cannot be quantified, but polarimetric radar measurements confirmed that a substantial fraction of the crystal population was well oriented. As the crystals fall into subsaturated air, specular reflection is observed to switch off as the crystal faces become rounded and lose their faceted structure. Specular reflection in ice falling from supercooled layers colder than -22C was also observed, but was much less pronounced than at warmer temperatures: we suggest that in cold clouds it is the small droplets in the distribution that freeze into plates and produce specular reflection, whilst larger droplets freeze into complex polycrystals. The lidar Doppler measurements show that typical fall speeds for the oriented crystals are 0.3m/s, with a weak temperature correlation; the corresponding Reynolds number is Re~10, in agreement with light-pillar measurements. Coincident Doppler radar observations show no correlation between the specular enhancement and eddy dissipation rate, indicating that turbulence does not control crystal orientation in these clouds.
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