Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review
Low-Cloud Feedbacks from Cloud-Controlling Factors: A Review
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DOI：
10.1007/s10712-017-9433-3
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发表时间：
2017-11-01
影响因子：
4.6
通讯作者：
Pincus, Robert
中科院分区：
文献类型：
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作者：
Klein, Stephen A.;Hall, Alex;Pincus, Robert
热带低层云（包括海洋层积云和信风积云）对变暖的响应是未来气候预测中不确定性的主要来源。气候模型对响应的模拟差异很大，反映了模型在模拟这些云时的困难。这些不足导致了替代方法来预测低云反馈。在这里，我们回顾了一种观测方法，它依赖于这样一个假设，即观测到的低云和大尺度环境的“云控制因素”之间的关系在时间尺度上是不变的。有了这个假设，并给出云控制因素如何随着气候变暖而变化的预测，人们可以在不使用任何低云模型模拟的情况下预测低云反馈。我们讨论了这种方法的基本问题和实施问题，并提出了可以减少预测的低云反馈的不确定性的步骤。最近使用这种方法的研究预测，热带低云的反馈是积极的，主要是由于观测到低云对太阳辐射的反射随着温度的升高而减少，而所有其他控制云的因素都是固定的。温度的正反馈部分地被逆温强度在变暖的世界中增加的趋势的负反馈所抵消，其他控制云的因素发挥的作用较小。这些研究一致估计热带低云对全球平均云反馈的贡献为0.25 +/- 0.18 W m（-2）K-1（90%置信区间），表明热带低云不太可能减少全球总的云反馈。由于用这种方法预测的正热带低云反馈与使用高分辨率云模型进行的低云反馈研究的独立证据一致，因此在减少这一关键气候不确定性方面正在取得进展。
The response to warming of tropical low-level clouds including both marine stratocumulus and trade cumulus is a major source of uncertainty in projections of future climate. Climate model simulations of the response vary widely, reflecting the difficulty the models have in simulating these clouds. These inadequacies have led to alternative approaches to predict low-cloud feedbacks. Here, we review an observational approach that relies on the assumption that observed relationships between low clouds and the "cloud-controlling factors" of the large-scale environment are invariant across time-scales. With this assumption, and given predictions of how the cloud-controlling factors change with climate warming, one can predict low-cloud feedbacks without using any model simulation of low clouds. We discuss both fundamental and implementation issues with this approach and suggest steps that could reduce uncertainty in the predicted low-cloud feedback. Recent studies using this approach predict that the tropical low-cloud feedback is positive mainly due to the observation that reflection of solar radiation by low clouds decreases as temperature increases, holding all other cloud-controlling factors fixed. The positive feedback from temperature is partially offset by a negative feedback from the tendency for the inversion strength to increase in a warming world, with other cloud-controlling factors playing a smaller role. A consensus estimate from these studies for the contribution of tropical low clouds to the global mean cloud feedback is 0.25 +/- 0.18 W m(-2) K-1 (90% confidence interval), suggesting it is very unlikely that tropical low clouds reduce total global cloud feedback. Because the prediction of positive tropical low-cloud feedback with this approach is consistent with independent evidence from low-cloud feedback studies using high-resolution cloud models, progress is being made in reducing this key climate uncertainty.