Abstract. The number of wildfire smoke layers in the upper troposphere per fire season increased at mid and high northern latitudes during the last years. To consider smoke in weather and climate models appropriately, the influence of smoke on a variety of atmospheric processes needs to be explored in detail. In this study, we focus on the potential impact of wildfire smoke on cirrus formation. For the first time, state-of-the-art aerosol and cirrus observations with lidar and radar, presented in part 1 of a series of two articles, are closely linked to comprehensive modeling of gravity-wave-induced ice nucleation in cirrus evolution processes, presented in part 2. The complex study is based on aerosol and ice cloud observations in the central Arctic during the one-year MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition. For almost a year (from the summer of 2019 to the spring of 2020), aged Siberian wildfire smoke polluted the tropopause region over the central Arctic and many cirrus systems developed in the polluted upper troposphere. Goal of the data analysis (part 1) is to provide observational evidence for a dominating impact of aged wildfire smoke (organic aerosol particles) on cirrus formation in the central Arctic (over the MOSAiC research icebreaker Polarstern) during the winter half year of 2019–2020. Aim of the simulations in part 2 is to gain a deeper and more detailed insight into the potential smoke impact on ice nucleation as a function of observed aerosol and meteorological conditions (temperature, relative humidity) and by considering realistic gravity wave characteristics (updraft speed, wave amplitude). Vertical movements of air parcels are essential to initiate cloud formation. The measurements presented in part 1 were conducted during the winter half year (October to March), aboard the ice breaker Polarstern. The research vessel Polarstern drifted with the pack ice in the central Arctic mainly at latitudes >85 °N during the winter half year. The cirrus statistics show typical properties of ice clouds of the synoptic cirrus category (top-down generation of cirrus structures). The ice clouds mostly started to evolve at heights close to the tropopause. Cirrus top temperatures accumulated between −60 and −75 °C. The cirrus optical thickness (COT at 532 nm) of the ice clouds covered a wide range of values from < 0.03 (subvisible cirrus fraction, 25 % out of all cases) over 0.03–0.3 (visible thin cirrus, 40 %) to > 0.3 (opaque cirrus fraction, 35 %). In about 30 % out of all high altitude lidar observations, cirrus signatures were detected, much more than expected (10 %). This fact may be taken as a first hint that wildfire smoke was significantly involved in Arctic cirrus formation. The smoke particle surface area concentration around the tropopause was of the order of 5–15 µm² cm⁻³ and indicated considerably enhanced levels of aerosol pollution in the upper troposphere. Based on the combined lidar-radar measurements, we analyzed 20 cirrus cases in terms of profiles of the ice water content (IWC), ice crystal number concentration (ICNC), and visible extinction coefficient. IWC mostly ranged from 0.001–0.01 g/m⁻³ and ICNC (in the ice virga) accumulated in the range from 0.01 to 10 L⁻¹. Three facts (in combination) corroborate our hypothesis that aged wildfire smoke triggered ice formation in many of the observed cirrus systems: (1) The low ICNC values point to heterogeneous ice nucleation, (2) the elevated smoke pollution levels (in terms of particle surface area concentrations) were high enough to significantly influence ice nucleation in the upper troposphere, and (3) the high ice saturation ratios accumulating around 1.3–1.4 in the upper part of the observed and analyzed cirrus decks indicate quite inefficient INPs as expected in the case of organic aerosol particles (wild fire smoke particles).

中文翻译：

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野火烟雾对北极卷云形成的影响，第 1 部分：MOSAiC 2019-2020 年观测结果分析


摘要。过去几年，北纬中高纬度地区每个火灾季节对流层上层野火烟雾层的数量有所增加。为了在天气和气候模型中适当考虑烟雾，需要详细探讨烟雾对各种大气过程的影响。在这项研究中，我们重点关注野火烟雾对卷云形成的潜在影响。两篇文章系列的第 1 部分中首次介绍了利用激光雷达和雷达进行的最先进的气溶胶和卷云观测，与卷云演化过程中重力波引起的冰核的综合建模密切相关，第 2 部分介绍。这项复杂的研究基于为期一年的 MOSAiC（北极气候研究多学科漂流观测站）考察期间对北极中部的气溶胶和冰云的观测。在近一年的时间里（从2019年夏天到2020年春天），古老的西伯利亚野火烟雾污染了北极中部上空的对流层顶区域，许多卷云系统在受污染的对流层上层发展起来。数据分析（第 1 部分）的目标是提供观测证据，证明 2019 年冬半年，老化的野火烟雾（有机气溶胶颗粒）对北极中部（MOSAiC 研究破冰船 Polarstern 上空）卷云形成的主要影响 - 2020.第 2 部分中模拟的目的是通过考虑实际的重力波特征（上升气流速度、波幅）。气团的垂直运动对于云的形成至关重要。 第 1 部分中介绍的测量是在冬半年（十月至三月）在 Polarstern 破冰船上进行的。冬半年，Polarstern 号科考船随浮冰一起漂流到北极中部，主要分布在纬度 >85 °N 的地区。卷云统计数据显示了天气卷云类别（自上而下生成卷云结构）的冰云的典型属性。冰云大多在接近对流层顶的高度开始演化。卷云顶部温度累积在-60 至-75 °C 之间。冰云的卷云光学厚度（COT，532 nm）涵盖了从 < 0.03（亚可见卷云分数，占所有情况的 25%）到 0.03–0.3（可见薄卷云，40%）到 > 0.3 的广泛值。 （不透明卷云部分，35%）。在所有高空激光雷达观测中，约 30% 的观测结果检测到了卷云特征，远高于预期 (10%)。这一事实可以被视为野火烟雾与北极卷云形成密切相关的第一个暗示。对流层顶周围的烟雾颗粒表面积浓度约为 5–15 µm ² cm ⁻³ ，表明对流层上层气溶胶污染水平显着增强。基于激光雷达-雷达联合测量，我们分析了 20 个卷云实例的冰水含量 (IWC)、冰晶数浓度 (ICNC) 和可见光消光系数的剖面。 IWC 的范围大多为 0.001–0.01 g/m ⁻³ ，ICNC（在冰 virga 中）累积范围为 0.01 至 10 L ⁻¹ 。 三个事实（组合在一起）证实了我们的假设，即老化的野火烟雾在许多观察到的卷云系统中引发了冰的形成：（1）低 ICNC 值表明异质冰核，（2）烟雾污染水平升高（就颗粒而言）表面积浓度）足够高，足以显着影响对流层上部的冰成核，并且（3）在观测和分析的卷云层上部，冰饱和比累积在 1.3-1.4 左右，这表明 INP 的效率相当低，正如在遇有机气溶胶颗粒（野火烟雾颗粒）。