In the Met Office Unified Model prognostic cloud scheme (PC2), thermodynamic variability is represented by considering two distributions, based on the properties of the clear and cloudy portions of the grid‐box. To calculate a cloud fraction increment, the equation set needs to be closed. The closure assumption determines how these two distributions are blended together at the saturation boundary. The current closure is reviewed. In the limit of vanishing liquid condensate, this can lead to very tall but narrow probability distribution functions (PDFs). At present, the two parts of the PDF are weighted based on the area under their curves. As a result, these very tall PDFs are allowed to contribute significantly, since their area is finite. This closure is shown to sometimes lead to unphysically large cloud fraction changes. However, they are prevented from affecting the model evolution by being overwritten if their magnitude is unrealistically large. A new blending closure is proposed. This weighs the two PDFs based on their widths. The contribution of very tall, but narrow, PDFs is hence drastically reduced. While this removes the undesirable need for overwriting unrealistic increments, it is shown that overall model performance did not improve or worsen.

中文翻译：

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在Met Office Unified Model预后云方案中修改热力学可变性封闭

在Met Office统一模型预后云方案（PC2）中，基于网格箱的阴天和阴天部分的属性，通过考虑两个分布来表示热力学变异性。要计算云分数增量，需要关闭方程组。闭合假设确定了这两个分布如何在饱和边界处混合在一起。当前关闭已被审查。在消失的液体冷凝物的极限内，这可能会导致非常高但狭窄的概率分布函数（PDF）。目前，PDF的两个部分是根据其曲线下的面积加权的。结果，由于它们的面积有限，因此允许这些非常高的PDF做出显着贡献。事实表明，这种关闭有时会导致物理上较大的云量变化。然而，如果它们的大小过大，则可以通过覆盖它们来防止影响模型演化。提出了一种新的混合瓶盖。这将根据两个PDF的宽度对其进行加权。因此，非常高但很窄的PDF的贡献被大大减少了。虽然这消除了覆盖不切实际的增量的不希望的需求，但事实表明，总​​体模型性能没有改善或恶化。