Abstract. Ice formation in cold temperature regimes is most probably dominated by homogeneous freezing of aqueous solution droplets. The nucleation rate as derived from laboratory experiments can be represented as a function of water activity. For idealized nucleation events as modelled with a state-of-the-art ice microphysics, the impact of different approximations of the nucleation rate on the resulting ice crystal number concentrations and maximum supersaturation ratios is investigated. The nucleation events are sensitive to the slope of the nucleation rate but only weakly affected by changes in its absolute value. This leads to the conclusion that details of the nucleation rate are less important for simulating ice nucleation in bulk models, if the main feature of the nucleation rate (i.e. its slope) is represented sufficiently. The weak sensitivity on the absolute values of the nucleation rate suggests that the amount of available solution droplets also does not crucially affect nucleation events. The use of just one distinct nucleation threshold for analysis and model parameterisation should be reinvestigated. The frequently used thresholds corresponding to a very high nucleation rate value is not reached in many nucleation events with low vertical updrafts. In contrast, the maximum supersaturation and thus the nucleation thresholds reached during an ice nucleation event depend on the vertical updraft velocity or cooling rate. This feature might explain some high supersaturation values during nucleation events in cloud chambers and suggests a reformulation of ice nucleation schemes used in coarse models based on a fixed nucleation threshold.

中文翻译：

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成核速率公式对冰成核事件的影响

摘要。低温条件下的冰形成很可能主要是水溶液液滴的均匀冻结。从实验室实验得出的成核率可以表示为水分活度的函数。对于用最先进的冰微物理学建模的理想化成核事件，研究了成核率的不同近似值对所得冰晶数浓度和最大过饱和比的影响。成核事件对成核速率的斜率敏感，但仅受其绝对值变化的微弱影响。这得出这样的结论，如果成核率的主要特征（即斜率）被充分表示，则成核率的细节对于在体模型中模拟冰成核不太重要。对成核速率绝对值的敏感性较弱表明可用溶液液滴的数量也不会对成核事件产生关键影响。应重新研究仅使用一个不同的成核阈值进行分析和模型参数化。在许多具有低垂直上升气流的成核事件中，没有达到与非常高的成核率值相对应的常用阈值。相反，在冰成核事件期间达到的最大过饱和度以及因此达到的成核阈值取决于垂直上升气流速度或冷却速率。这一特征可能解释了云室中成核事件期间的一些高过饱和值，并建议重新制定基于固定成核阈值的粗模型中使用的冰成核方案。