Mountain-top in-situ observations of mixed-phase clouds with a digital holographic instrument
Mountain-top in-situ observations of mixed-phase clouds with a digital holographic instrument
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利用数字全息仪对混合相云进行山顶原位观测
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发表时间：
2013
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通讯作者：
J. Henneberger
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作者：
J. Henneberger
云在降水的形成、辐射平衡、水文循环和大气化学中起着重要作用。混合相云（MPC）由过冷液滴和冰晶的混合物组成，热力学不稳定。没有任何明显的垂直速度和湍流的MPC将在相对较短的时间尺度上完全冰川化。然而，观察表明，MPC是一种常见现象，在全球所有季节、各种条件和所有纬度都观察到了。为了了解导致这种寿命的过程，以高空间分辨率测量MPC的液体和冰相的微物理特性非常重要。这项工作介绍了新开发的现场仪器HOLIMO II（全息成像显微物体II）的设计和表征。HOLIMO II使用数字同轴全息术在定义明确的样品体积内对云粒子的集合进行原位成像。用算法提取了直径在6 ~ 250μm之间的单粒子云的二维图像。通过分析粒子形状，将大于20μm的云粒子分类为水滴（圆形）或冰晶（非圆形）。因此，相位分辨的尺寸分布，浓度和云水含量的获得。山顶的实地测量，从高海拔研究站少女峰，瑞士。现场数据表明，HOLIMO II能够区分水和冰粒，并以高空间分辨率测量数量尺寸分布和含水量。HOLIMO II提供了一种在25米尺度上量化微物理特性变化的方法，并减少了冰晶数浓度测量的不确定性。一个案例研究在一段时间内的8小时进行了分析，探讨了液体的MPC的过渡。在测量期间，在JFJ观察到MPC至少4小时。分析了14天60多小时的数据，代表了大气科学中全息仪器的最长时间序列。两个拓扑影响的风制度被确定，每个具有不同的云属性。对于南风，与一个平稳的上升，MPC的属性是类似的层状北极MPC的调查结果。在北风的情况下，与一个尖锐的上升，更多的中间冰水含量，总水含量比（IWC/TWC），和较高的冰晶浓度，观察。在测得的冰晶浓度的云将有效地在相对较短的时间内冰川，这导致了假设，即高的上升气流速度稳定的MPC在少女峰更中间IWC/TWC分数。
Clouds play an important role in the formation of precipitation, in the radiative balance, in the hydrological cycle, and chemistry of the atmosphere. A mixed-phase cloud (MPC), which consist of a mixture of supercooled liquid droplets and ice crystals, is thermodynamically unstable. MPCs without any noticeable vertical velocity and turbulence will thoroughly glaciate on relative short time scales. However, observations have shown that MPCs are a common phenomenon and have been observed in all seasons, under a variety of conditions and at all latitudes worldwide. To understand the processes leading to this longevity measurements of the microphysical properties of the liquid and the ice phase of MPCs with high spatial resolution are important. This work describes the design and characterization of the newly developed field instrument HOLIMO II (HOLographic Imager for Microscopic Objects II). HOLIMO II uses digital in-line holography to in-situ image ensembles of cloud particles within a well defined sample volume. Two-dimensional images of single cloud particles between 6 and 250μm in diameter are algorithmically extracted. Particle shape is analyzed to classify cloud particles larger than 20μm as water droplets (circular) or ice crystals (non-circular). Thus phase-resolved size distributions, concentrations, and cloud water contents are obtained. Mountain-top field measurements from the high altitude research station Jungfraujoch, Switzerland, are presented. The field data shows that HOLIMO II is capable of discriminating between water and ice particles, and of measuring number size distributions and water content with a high spatial resolution. HOLIMO II provides a method to quantify variations in microphysical properties on a 25 m-scale and to reduce uncertainties in ice crystal number concentration measurements. A case study over a period of 8 h has been analyzed, exploring the transition of a liquid to a MPC. During the measurement period, a MPC was observed for at least four hours at JFJ. Data of more than 60 h on 14 different days was analyzed, representing the longest time series from a holographic instrument in atmospheric science. Two topologically-influenced wind regimes were identified, each with distinct cloud properties. For southerly winds, associated with a smoother ascent, the properties of the MPCs were similar to findings of stratiform Arctic MPCs. During north wind cases, associated with a sharper ascent, more intermediate ice-water-content to total-water-content ratios (IWC/TWC), and higher ice crystal concentrations, were observed. At the measured ice crystal concentrations the cloud would efficiently glaciate in relatively short time, which leads to the hypothesis that the high updraft velocities stabilize the MPCs at Jungfraujoch to more intermediate IWC/TWC fractions.