Abstract. The combination of downward-looking airborne lidar, radar, microwave, and imaging spectrometer measurements was exploited to characterize the vertical and small-scale (down to 10 m) horizontal distribution of the thermodynamic phase of low-level Arctic mixed-layer clouds. Two cloud cases observed in a cold air outbreak and a warm air advection event observed during the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign were investigated. Both cloud cases exhibited the typical vertical mixed-phase structure with mostly liquid water droplets at cloud top and ice crystals in lower layers. The horizontal, small-scale distribution of the thermodynamic phase as observed during the cold air outbreak is dominated by the liquid water close to the cloud top and shows no indication of ice in lower cloud layers. Contrastingly, the cloud top variability in the case observed during a warm air advection showed some ice in areas of low reflectivity or cloud holes. Radiative transfer simulations considering homogeneous mixtures of liquid water droplets and ice crystals were able to reproduce the horizontal variability in this warm air advection. Large eddy simulations (LESs) were performed to reconstruct the observed cloud properties, which were used subsequently as input for radiative transfer simulations. The LESs of the cloud case observed during the cold air outbreak, with mostly liquid water at cloud top, realistically reproduced the observations. For the warm air advection case, the simulated ice water content (IWC) was systematically lower than the measured IWC. Nevertheless, the LESs revealed the presence of ice particles close to the cloud top and confirmed the observed horizontal variability in the cloud field. It is concluded that the cloud top small-scale horizontal variability is directly linked to changes in the vertical distribution of the cloud thermodynamic phase. Passive satellite-borne imaging spectrometer observations with pixel sizes larger than 100 m miss the small-scale cloud top structures.

中文翻译：

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在冷空气爆发和热空气对流事件中通过机载遥感观测到的北极混合相云中热力学相的小规模结构

摘要。利用向下看的机载激光雷达，雷达，微波和成像光谱仪的测量结果来表征低层北极混合层云热力学相的垂直和小尺度（低至10 m）水平分布。研究了在北极云观测期间在冷空气暴发中观察到的两个云事件和在北极云观测期间观察到的热空气对流事件。两种云层均表现出典型的垂直混合相结构，云层顶部大部分为液态水滴，而下层则为冰晶。在冷空气爆发期间观察到的热力学阶段的水平，小规模分布主要由靠近云层顶部的液态水决定，在低云层中没有冰的迹象。相反，在热空气平流过程中观察到的云顶变化表明，在低反射率或云洞的区域有一些冰。考虑到液态水滴和冰晶的均匀混合物的辐射传递模拟能够重现这种暖空气对流中的水平变化。进行了大涡模拟（LESs）以重建观测到的云的性质，随后将其用作辐射传输模拟的输入。在冷空气暴发期间观察到的云事件的LESs，实际上是在云层顶部的液态水，真实地再现了这些观察结果。对于热空气对流情况，模拟的冰水含量（IWC）有系统地低于实测的IWC。尽管如此，LES揭示了靠近云顶的冰粒的存在，并确认了在云场中观察到的水平变化。结论是，云顶小尺度水平变化与云热力相垂直分布的变化直接相关。像素大小大于100 m的无源卫星成像光谱仪观测结果错过了小规模的云顶结构。