Climate indices are used to reduce the complex climate system and its changes to simple measures. The data basis – whether observational data or climate model data – to which the climate indices are applied, is usually subject to uncertainties. For threshold-based climate indices, the data uncertainty influences the threshold value, and, hence, the uncertainty can influence the values for the climate index. What the actual impacts of these uncertainties are on threshold-based climate indices is examined in this paper. The focus is not only on the climate model uncertainty, but also on the observational data uncertainty. The general sensitivity of each of the chosen climate indices to arbitrary changes in the threshold is studied. This shows a higher sensitivity of indices assessing extremes (ice days, heavy precipitation days) to changes in the threshold than indices that integrate a quantity over a given time interval (coldsum, consecutive days). For assessing an ensemble of climate model data with respect to their ability to reproduce the index values for current climate, the reference data uncertainty is applied to the chosen threshold-based climate indices by changing their threshold value by its corresponding uncertainty. It is shown that the climate model uncertainty can be within the range of the reference data uncertainty. When using threshold-based climate indices to assess changes in future climate periods, uncertainties should always be taken into account and ideally corrected in an appropriate way. This is especially important for indices that assess extremes.

中文翻译：

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数据不确定性对基于冷季阈值的气候指数的影响

气候指数用于将复杂的气候系统及其变化简化为简单的衡量标准。气候指数所应用的数据基础——无论是观测数据还是气候模型数据——通常存在不确定性。对于基于阈值的气候指数，数据不确定性会影响阈值，因此不确定性会影响气候指数的值。本文探讨了这些不确定性对基于阈值的气候指数的实际影响。重点不仅在于气候模型的不确定性，还在于观测数据的不确定性。研究了每个选定的气候指数对阈值任意变化的一般敏感性。这表明评估极端情况（冰天、强降水天数）与阈值变化的关系，而不是对给定时间间隔内数量进行积分的指数（coldsum、连续天数）。为了评估气候模型数据集合重现当前气候指数值的能力，通过通过相应的不确定性改变其阈值，将参考数据不确定性应用于所选的基于阈值的气候指数。结果表明，气候模型的不确定性可以在参考数据不确定性的范围内。当使用基于阈值的气候指数来评估未来气候时期的变化时，应始终考虑不确定性，并最好以适当的方式进行纠正。这对于评估极端情况的指数尤其重要。