What is it about?

Our study shows that a recently proposed index derived from the inequality in the cloud-top entrainment criterion, which relates temperature and water vapor profiles in the lower atmosphere, predicts well the reduction in low cloud cover to sea surface temperature warming.

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Why is it important?

Low clouds are mainly stratiform and prevalent over eastern subtropical oceans. They have a strong cooling effect on the Earth’s radiation budget because of their high albedo relative to the surface. An increase (a decrease) in these clouds in response to global warming thus leads to dampening (amplifying) the warming. Although most of the state-of-the-art climate models indicate reductions in low cloud cover with warming, the spread of the changes simulated by individual models widely ranges from small positive to large negative values. This is the primary source of uncertainty in climate projections. While low clouds are maintained by a delicate balance of complex physical processes, a predictive index for low cloud cover can be obtained from large-scale temperature and water vapor profiles, of which the responses to warming are more reliable. Our result supports the expectation from climate models that low clouds will amplify global warming and leads to reduced uncertainty in climate projections.


In our previous paper in 2017, which proposed the estimated cloud-top entrainment index (ECTEI), we mathematically predicted that ECTEI could explain the reduction in low cloud cover in a warming climate from its definition. We are excited and pleased to clearly demonstrate this prediction by analyzing the simulation results of the state-of-the-art climate models.

Tsuyoshi Koshiro
Meteorological Research Institute

Read the Original

This page is a summary of: Estimated cloud-top entrainment index explains positive low-cloud-cover feedback, Proceedings of the National Academy of Sciences, July 2022, Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2200635119.
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