Multi-thermals and high concentrations of secondary ice: A modelling study of convective clouds during the ICE-D campaign
Multi-thermals and high concentrations of secondary ice: A modelling study of convective clouds during the ICE-D campaign
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DOI：
10.5194/acp-2021-323
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发表时间：
2018-07
期刊：
影响因子：
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通讯作者：
Z. Cui;A. Blyth;G. Lloyd;T. Choularton;K. Bower;P. Field;R. Hawker;L. Bennett
中科院分区：
文献类型：
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作者：
Z. Cui;A. Blyth;G. Lloyd;T. Choularton;K. Bower;P. Field;R. Hawker;L. Bennett
抽象的。本文研究了矿物尘影响下对流云结冰的机制。2015年8月在佛得角附近进行的云中冰实验-尘埃（ICE-D）实地活动进行了观测。8月21日，FAAM飞机上的仪器透过云层进行的测量表明，在温度高于-8 °C时，观察到了高浓度的冰粒。利用现有的参数化方案在云模型中进行敏感性研究，以探讨冻结起始温度，冻结效率，矿物粉尘作为有效的冰核，和多热气流对二次冰生产的雾凇分裂过程的影响。使用默认的莫里森微观物理方案（莫里森等人，2005年），涉及一个单一的热产生的浓度的次生冰，这是远远低于观测值的总冰数浓度。将起始温度放宽到更高的值，提高冷冻效率，或这些的组合，增加了二次冰粒浓度，但没有足够的量。由于冻结开始温度较低，仅涉及尘埃颗粒作为冰成核颗粒的模拟产生了较低浓度的二次冰粒。模拟结果表明，较高浓度的冰成核颗粒具有较高的冻结起始温度可以解释一些观察到的高浓度的二次冰。然而，使用原始的莫里森方案，没有增强或松弛的两个热气流的模拟产生了最大浓度的二次冰粒。这是因为在Hallett-Mossop温度区，<$子颗粒暴露在显着的云液态水含量中的时间增加了。前向摄像机和对同一云反复通过的垂直风的测量表明，这些热带云包含多种热气流。因此，与世界其他地方观察到的其他对流云类似，多热气流可能在某些热带云中产生非常高浓度的次生冰粒方面发挥重要作用。
Abstract. This paper examines the mechanisms responsible for the production of ice in convective clouds influenced by mineral dust. Observations were made in the Ice in Clouds Experiment – Dust (ICE-D) field campaign which took place in the vicinity of Cape Verde during August 2015. Measurements made with instruments on the FAAM aircraft through the clouds on 21 August showed that ice particles were observed in high concentrations at temperatures greater than about −8 °C. Sensitivity studies were performed using existing parametrisation schemes in a cloud model to explore the impact of the freezing onset temperature, the efficiency of freezing, mineral dust as efficient ice nuclei, and multi-thermals on secondary ice production by the rime-splintering process. The simulation with the default Morrison microphysics scheme (Morrison et al., 2005) that involved a single thermal produced a concentration of secondary ice that was much lower than the observed value of total ice number concentration. Relaxing the onset temperature to a higher value, enhancing the freezing efficiency, or combinations of these, increased the secondary ice particle concentration, but not by a sufficient amount. Simulations that involved only dust particles as ice nucleating particles produced a lower concentration of secondary ice particles, since the freezing onset temperature is low. The simulations implicate that a higher concentration of ice nucleating particles with a higher freezing onset temperature may explain some of the observed high concentrations of secondary ice. However, a simulation with two thermals that used the original Morrison scheme without enhancement or relaxation produced the greatest concentration of secondary ice particles. It did so because of the increased time that graupel particles were exposed to significant cloud liquid water content in the Hallett-Mossop temperature zone. The forward-facing camera and measurements of the vertical wind in repeated passes of the same cloud suggested that these tropical clouds contained multiple thermals. Hence, in a similar way to other convective clouds observed elsewhere in the world, it is likely that multi-thermals play an important role in producing very high concentrations of secondary ice particles in some tropical clouds.