We present a new cloud scheme for the ECHAM‐HAM global climate model (GCM) that includes prognostic cloud fraction and allows for subsaturation and supersaturation with respect to ice separately in the cloud‐free and cloudy air. Stratiform clouds form by convective detrainment, turbulent vertical diffusion, and large‐scale ascent. For each process, the corresponding cloud fraction is calculated, and the individual updraft velocities are used to determine cloud droplet/ice crystal number concentrations. Further, convective condensate is always detrained as supercooled cloud droplets at mixed‐phase temperatures (between 235 and 273 K), and convectively detrained ice crystal number concentrations are calculated based on the updraft velocity. Finally, the new scheme explicitly calculates condensation/evaporation and deposition/sublimation rates for phase‐change calculations. The new cloud scheme simulates a reasonable present‐day climate, reduces the previously overestimated cirrus cloud fraction, and in general improves the simulation of ice clouds. The model simulates the observed in‐cloud supersaturation for cirrus clouds, and it allows for a better representation of the tropical to extra‐tropical ratio of the longwave cloud radiative effect. Further, the ice water path, the ice crystal number concentrations, and the supercooled liquid fractions in mixed‐phase clouds agree better with observations in the new model than in the reference model. Ice crystal formation is dominated by the liquid‐origin processes of convective detrainment and homogeneous freezing of cloud droplets. The simulated ice clouds strongly depend on model tuning choices, in particular, the enhancement of the aggregation rate of ice crystals.

中文翻译：

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为ECHAM‐HAM开发具有预测云分数和两个矩微物理学的云方案

我们为ECHAM-HAM全球气候模型（GCM）提出了一种新的云方案，该方案包括可预测的云分数，并允许在无云和多云的空气中分别对冰进行亚饱和和过饱和。层状云是由对流减缓，湍流垂直扩散和大规模上升形成的。对于每个过程，都将计算相应的浊度分数，并使用各个上升气流的速度来确定浊滴/冰晶数浓度。此外，对流凝结水总是在混合相温度（235和273 K之间）下以过冷云滴的形式被消除，并且基于上升气流速度来计算对流被消除的冰晶数浓度。最后，新方案显式地计算凝结/蒸发和沉积/升华速率，以进行相变计算。新的云方案模拟了当今的合理气候，减少了先前被高估的卷云比例，并总体上改善了对冰云的模拟。该模型模拟了观测到的卷云的云内过饱和度，并且可以更好地表示长波云辐射效应的热带与温带比例。此外，与参考模型相比，新模型中的观测结果与混合相云中的冰水路径，冰晶数浓度和过冷液体分数更吻合。冰晶的形成主要由对流减缓和云滴均匀冻结的液体起源过程决定。