The Earth’s changing climate is the product of complex exchanges between the ocean and atmosphere. Scientists explore these exchanges using general circulation models (GCMs), which calculate the circulation of the ocean or atmosphere. The models account for the Earth’s rotation and thermodynamic interactions, like radiation and latent heat, but scientists still struggle to pin down how clouds influence climate feedback. Here Qu et al. develop a methodology to identify how low-lying tropical marine cloud cover responds to temperature changes in GCMs and assess the role of clouds in a changing climate.
The scientists first identified two major variables that control cloud coverage: latent heat flux and moisture gradient. In this case, latent heat flux describes the energy released as water evaporates from the Earth’s surface and collects in the troposphere. The moisture gradient represents how moisture collects in the air with respect to altitude: the boundary layer closest to the Earth’s surface gets drier with increasing altitude. In this study, the team used sea surface temperature as a proxy for these two variables.
They found that the cloud feedbacks created in GCMs could be attributed to three key changes. The first is stronger tropical inversions—meteorological phenomena in which the normal stratification of the atmosphere is reversed, so that cool, moist air is trapped under warm, dry air. The stronger the inversion is, the less mixing occurs between the boundary layer and the dry upper air of the troposphere. Less mixing produces a shallower, wetter, cloudier boundary layer.
The other changes are an increase in surface latent heat flux and an increase in the difference in moisture content at different heights (due to a greater increase in moisture near the surface compared with higher altitudes). In contrast to stronger tropical inversions, these changes hinder the formation of low cloud cover.
Overall, the team found that when models accurately simulated the response of low cloud coverage to inversion strength and sea surface temperature variations, low cloud cover decreased. Without the reflective layer of clouds, the Earth’s surface absorbs more solar radiation. Therefore, a decrease in low cloud coverage can drive an overall increase in warming. The authors hope further study of cloud sensitivity will help to improve cloud feedback modeling. (Geophysical Research Letters, doi:10.1002/2015GL065627, 2015)
—Lily Strelich, Freelance Writer
Citation: Strelich, L. (2015), Low-altitude clouds play an important role in a changing climate, Eos, 96, doi:10.1029/2015EO039715. Published on 18 November 2015.