Understanding the evolution of the ice phase within mixed-phase clouds (MPCs) is necessary to reduce uncertainties related to the cloud radiative feedback in climate projections and precipitation initiation. Both primary ice formation via ice-nucleating particles (INPs) and secondary ice production (SIP) within MPCs are unconstrained, not least because of the lack of atmospheric observations. In the past decades, advanced remote sensing methods have emerged which provide high-resolution data of aerosol and cloud properties and could be key in understanding microphysical processes on a global scale. In this study, we retrieved INP concentrations and ice multiplication factors (IMFs) in wintertime orographic clouds using active remote sensing and in situ observations obtained during the RACLETS campaign in the Swiss Alps. INP concentrations in air masses dominated by Saharan dust and continental aerosol were retrieved from a polarization Raman lidar and validated with aerosol and INP in situ observations on a mountaintop. A calibration factor of 0.0204 for the global INP parameterization by DeMott et al. (2010) is derived by comparing in situ aerosol and INP measurements, improving the INP concentration retrieval for continental aerosols. Based on combined lidar and radar measurements, the ice crystal number concentration and ice water content were retrieved and validated with balloon-borne in situ observations, which agreed with the balloon-borne in situ observations within an order of magnitude. For seven cloud cases the ice multiplication factors (IMFs), defined as the quotient of the ice crystal number concentration to the INP concentration, were calculated. The median IMF was around 80, and SIP was active (defined as IMFs > 1) nearly 85 % of the time. SIP was found to be active at all observed temperatures (−30 to −5 ^∘C), with the highest IMFs between −20 and −5 ^∘C. The introduced methodology could be extended to larger datasets to better understand the impact of SIP not only over the Alps but also at other locations and for other cloud types.

中文翻译：

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利用现场观测验证的主动遥感反演冰核粒子浓度和冰倍增因子

了解混合相云 (MPC) 中冰相的演变对于减少与气候预测和降水起始中的云辐射反馈相关的不确定性是必要的。通过冰核粒子 (INP) 形成的初级冰和 MPC 内的次级冰生成 (SIP) 都不受限制，尤其是因为缺乏大气观测。在过去的几十年里，出现了先进的遥感方法，这些方法可以提供气溶胶和云特性的高分辨率数据，并且可能是理解全球范围内的微物理过程的关键。在这项研究中，我们使用主动遥感和在瑞士阿尔卑斯山的 RACLETS 活动期间获得的现场观测，检索了冬季地形云中的 INP 浓度和冰倍增因子 (IMF)。从偏振拉曼激光雷达中检索到以撒哈拉尘埃和大陆气溶胶为主的气团中的 INP 浓度，并通过山顶上的气溶胶和 INP 原位观测进行验证。校准系数DeMott 等人的全局 INP 参数化为 0.0204 。(2010)是通过比较原位气溶胶和 INP 测量得出的，改进了大陆气溶胶的 INP 浓度反演。基于激光雷达和雷达联合测量，利用气球原位观测对冰晶数浓度和冰水含量进行了反演和验证，与气球原位观测在一个数量级内吻合。对于七种云情况，计算了定义为冰晶数量浓度与 INP 浓度的商的冰倍增因子 (IMF)。IMF 的中位数约为 80，并且 SIP 在  近 85% 的时间内处于活跃状态（定义为 IMF > 1）。发现 SIP 在所有观察到的温度下都处于活动状态（-30到-5  ^∘ C），最高 IMF 在-20到-5  ^∘ C 之间。引入的方法可以扩展到更大的数据集，以更好地了解 SIP 不仅对阿尔卑斯山而且在其他位置和其他云类型。