Abstract. Aviation is currently estimated to contribute ~3.5 % of the net anthropogenic effective radiative forcing (ERF) of Earth's atmosphere. The largest component of this forcing comes from contrail cirrus (also with a large associated uncertainty of ~70 %), estimated to be two times larger than the contribution from aviation CO₂ emissions. Here we implement the contrail parameterisation previously developed for the USA NCAR (National Center for Atmospheric Research) Community Atmosphere Model (CAM) in the UK Met Office Unified Model (UM). By using for the first time the same contrail parameterisation in two different host climate models, this work investigates the impact of key features of the host climate model on quantifying contrail cirrus radiative impacts. We find that differences in the background humidity (in particular ice supersaturation) in the two climate models lead to substantial differences in simulated contrail fractions, with UM values being two to three times as large as those from CAM. We also find contrasting responses in overall global cloud fraction due to air traffic, with contrails causing increases and decreases in total cloud fraction in the UM and in CAM, respectively. The different complexity of the two models’ cloud microphysics schemes (i.e. single and double-moment cloud schemes in the UM and CAM, respectively) results in significant differences in the simulated changes in cloud ice water content due to aviation. When accounting for the difference in cloud microphysics complexity, we estimate the contrail cirrus ERF of the year 2018 to be 40.8 mWm⁻² in the UM and 60.1 mWm⁻² in CAM. While these two estimates are not entirely independent, they indicate a substantial (i.e. factor of ~2) uncertainty in contrail cirrus ERF from differences in the microphysics and radiation schemes of the two host climate models. We also find a factor of 8 uncertainty in contrail cirrus ERF due to existing uncertainty in contrail cirrus optical depth. We suggest that future work on the contrail cirrus climate impact should focus on better representing the microphysical and radiative contrail characteristics in different climate models and on improved observational constraints.

中文翻译：

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宿主气候模型对轨迹卷云有效辐射强迫估计的影响


摘要。目前估计航空业贡献了地球大气净人为有效辐射强迫 (ERF) 的约 3.5%。这种强迫的最大组成部分来自轨迹卷云（也具有大约 70% 的相关不确定性），估计比航空 CO ₂ 排放的贡献大两倍。在这里，我们实现了之前为英国气象局统一模型 (UM) 中的美国 NCAR（国家大气研究中心）社区大气模型 (CAM) 开发的轨迹参数化。通过首次在两个不同的宿主气候模型中使用相同的轨迹参数化，这项工作研究了宿主气候模型的关键特征对量化轨迹卷云辐射影响的影响。我们发现，两个气候模型中背景湿度（特别是冰过饱和度）的差异导致模拟轨迹分数存在显着差异，UM 值是 CAM 值的两到三倍。我们还发现，空中交通对全球总体云量的影响存在对比，凝结尾迹分别导致 UM 和 CAM 中总云量的增加和减少。两个模型的云微物理方案（即UM和CAM中分别为单矩云方案和双矩云方案）的复杂程度不同，导致模拟的航空引起的云冰水含量变化存在显着差异。考虑到云微物理复杂性的差异，我们估计 2018 年 UM 中的轨迹卷云 ERF 为 40.8 mWm ⁻² ，CAM 中为 60.1 mWm ⁻² 。虽然这两个估计并不完全独立，但它们表明了很大的（即 ~2) 因两个宿主气候模型的微物理和辐射方案的差异而导致轨迹卷云 ERF 的不确定性。由于轨迹卷云光学深度存在不确定性，我们还发现轨迹卷云 ERF 存在 8 个不确定性。我们建议未来关于尾迹卷云气候影响的工作应侧重于更好地表示不同气候模型中的微物理和辐射尾迹特征以及改进观测约束。