Observations of ice multiplication in a weakly convective cell embedded in supercooled mid-level stratus
Observations of ice multiplication in a weakly convective cell embedded in supercooled mid-level stratus
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DOI：
10.5194/acp-11-257-2011
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发表时间：
2011-01-01
影响因子：
6.3
通讯作者：
Blyth, A. M.
中科院分区：
文献类型：
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作者：
Crosier, J.;Bower, K. N.;Blyth, A. M.
云的微物理特性的同时观测获得了现场飞机测量和地面雷达/激光雷达。在3.6公里高度的逆温（类似于5摄氏度）以下存在广泛的中层层云。在雷达站以西20公里处观测到局部对流（最高上升气流1.5毫秒（-1））。这与2.5公里高度的辐合有关。对流无法穿透覆盖中层层云的逆温层，中层层云垂直厚度薄（约400 m），水平厚度大（约100 km），持续时间超过24 h。云由过冷水滴和小浓度的大（类似于1毫米）恒星/板状冰组成，这些冰慢慢沉淀出来。这种冰在温度高于-12.2摄氏度和低于-10.0摄氏度（分别为云顶和云底温度）时成核。没有观测到云层上方的冰种。这种冰是由初级成核形成的，无论是通过夹带有效的冰核从上面/下面的云，或通过缓慢的随机激活的过冷液滴内包含的浸没冷冻冰核。在云顶上方，观测到大量的亚微米气溶胶，由硫酸盐和碳质物质的混合物组成，是冰核的潜在来源。在云的上方和下方测量浓度接近25 cm（-3）的颗粒数浓度（尺寸范围为0.1 < D < 3.0 μ m）。云中的冰晶浓度恒定在0.2 L-1左右。据估计，气溶胶粒子夹带入云不能补充降水造成的云层冰核损失，中层层云的降水在到达地面前就蒸发了，而在对流特征层以下观测到的速率可达1 mm h（-1）。有强有力的证据表明，哈雷特-莫索普（HM）过程的二次冰粒的生产，导致形成的降水观察。这包括：（1）对流特征中的冰浓度比上覆层云中的原生冰浓度大一个数量级以上;（2）在类似-5 ℃的水平上观察到大量的小原始柱以及液态水滴和一些有边的冰粒;（3）柱在较低的温度下更大，越来越多地有边。如果放宽只有大于24 μ m的液滴才能产生二次冰碎片的条件，计算的冰碎片产生率与观测的浓度是一致的。这种情况表明理解在稍微过冷的温度下冰的形成的重要性，因为它可以导致二次冰的产生和在云中形成降水，否则可能不被认为是重要的降水来源
Simultaneous observations of cloud microphysical properties were obtained by in-situ aircraft measurements and ground based Radar/Lidar. Widespread mid-level stratus cloud was present below a temperature inversion(similar to 5 degrees C magnitude) at 3.6 km altitude. Localised convection (peak updraft 1.5ms(-1)) was observed 20 km west of the Radar station. This was associated with convergence at 2.5 km altitude. The convection was unable to penetrate the inversion capping the mid-level stratus.The mid-level stratus cloud was vertically thin (similar to 400 m), horizontally extensive (covering 100s of km) and persisted for more than 24 h. The cloud consisted of supercooled water droplets and small concentrations of large (similar to 1 mm) stellar/plate like ice which slowly precipitated out. This ice was nucleated at temperatures greater than -12.2 degrees C and less than -10.0 degrees C, (cloud top and cloud base temperatures, respectively). No ice seeding from above the cloud layer was observed. This ice was formed by primary nucleation, either through the entrainment of efficient ice nuclei from above/below cloud, or by the slow stochastic activation of immersion freezing ice nuclei contained within the supercooled drops. Above cloud top significant concentrations of sub-micron aerosol were observed and consisted of a mixture of sulphate and carbonaceous material, a potential source of ice nuclei. Particle number concentrations (in the size range 0.1 < D < 3.0 mu m) were measured above and below cloud in concentrations of similar to 25 cm(-3). Ice crystal concentrations in the cloud were constant at around 0.2 L-1. It is estimated that entrainment of aerosol particles into cloud cannot replenish the loss of ice nuclei from the cloud layer via precipitation.Precipitation from the mid-level stratus evaporated before reaching the surface, whereas rates of up to 1 mm h(-1) were observed below the convective feature. There is strong evidence for the Hallett-Mossop (HM) process of secondary ice particle production leading to the formation of the precipitation observed. This includes (1) Ice concentrations in the convective feature were more than an order of magnitude greater than the concentration of primary ice in the overlaying stratus, (2) Large concentrations of small pristine columns were observed at the similar to-5 degrees C level together with liquid water droplets and a few rimed ice particles, (3) Columns were larger and increasingly rimed at colder temperatures. Calculated ice splinter production rates are consistent with observed concentrations if the condition that only droplets greater than 24 mu m are capable of generating secondary ice splinters is relaxed.This case demonstrates the importance of understanding the formation of ice at slightly supercooled temperatures, as it can lead to secondary ice production and the formation of precipitation in clouds which may not otherwise be considered as significant precipitation sources.