UW Study Reveals Why Similar Cloud Changes Can Have Different Climate Effects

Marine low clouds over the world’s eastern subtropical oceans strongly influence Earth’s temperature by reflecting sunlight back to space. How these clouds change as the planet warms is one of the biggest uncertainties in predicting the severity of future warming.

However, Earth system models have had difficulty simulating these clouds because the small-scale turbulence and rain processes that shape them must be approximated.

A study headed by University of Wyoming researchers used 250 experimental configurations of an existing climate model to investigate how clouds respond to changes in their environment. The researchers found that similar changes in cloud coverage can have very different climate effects depending on how reflective the clouds are to begin with. They also identified a persistent problem in how the simulated clouds respond to winds moving air across ocean regions with different temperatures. The findings identify opportunities to improve cloud simulations, although the study did not implement or test those improvements.

“The findings highlight why evaluating climate models requires examining both how clouds change and how reflective they are before changing, with implications for assessing cloud feedback and future warming,” says Timothy Myers, an associate research scientist in the UW Department of Atmospheric Science. “To understand how clouds affect warming, we need to know both how their coverage changes and how strongly they reflect sunlight.”

Myers was the lead author of a paper, titled “Cloud Controlling Factor Analysis Reveals E3SMv3 Structural Bias and Mean State Controls on Stratocumulus Radiative Sensitivity,” that was published Oct. 18 in Geophysical Research Letters, a gold open-access journal that publishes high-impact, innovative and timely communications-length articles on major advances spanning all of the major geoscience disciplines.

Associate research scientists Travis Aerenson and Jacqueline Nugent; Hunter Brown, a postdoctoral research associate; and Daniel McCoy, an associate professor, all in the UW Department of Atmospheric Science, contributed to the paper, along with Mark Zelinka, an atmospheric scientist at Lawrence Livermore National Laboratory.

The study, conducted from February through August, builds on Nephele, a UW-led effort to explore uncertainty in cloud processes through hundreds of climate model simulations.

Nephele involves 250 Earth system model simulations with systematically varied cloud, convection and aerosol parameters, Myers says. The main simulations and satellite observations analyzed cover the period from 2010 to 2016.

Researchers used the Energy Exascale Earth System Model version 3 (E3SMv3) for testing cloud simulations. E3SMv3 is a computer representation of Earth’s climate system developed by the U.S. Department of Energy (DOE). It simulates interactions among the atmosphere, oceans, land and ice to investigate how climate works and how it may change.

The study used the Derecho and Casper computing systems at the National Science Foundation’s National Center for Atmospheric Research-Wyoming Supercomputing Center. E3SMv3 divides Earth into a grid and uses physical equations of fluid motion and thermodynamics to calculate how air and water move, and how heat and moisture are exchanged, Myers says.

“Some processes, such as individual cloud droplets growing into raindrops, are too small to simulate directly, so the model uses simplified approximations of them,” Myers says. “Our study examines how these approximations affect clouds and their influence on climate.”

Researchers compared satellite observations with 250 simulations of E3SMv3, using different settings for uncertain cloud and aerosol processes, Myers says.

“We found that how readily the model converts cloud water into rain strongly controls cloud reflectivity,” he says. “When that conversion is less efficient, more water remains in the clouds, making them more reflective and increasing the climate impact of a given change in cloud cover.”

The model captures the direction of most observed cloud responses to meteorological and aerosol changes but often exaggerates their strength. It also consistently misses the cloud response to winds moving cool ocean air over warmer water, Myers adds.

“Our study shows why predicting changes in cloud cover is just one part of the challenge. The same loss of cloud cover can have very different effects on climate depending on how reflective the clouds were to begin with,” Myers explains. “Losing highly reflective clouds allows more solar energy to remain in the climate system than losing the same area of less reflective clouds.”

One challenge the model has yet to solve is its response to surface temperature advection, which means transport by moving air. For this study, it refers to winds moving air across ocean regions with different surface temperatures, Myers says.

When air moves from cooler to warmer ocean regions, the ocean supplies additional heat and moisture to the air and strengthens mixing near the surface. These conditions tend to support more low cloud cover and greater reflection of sunlight.

“We see that relationship in observations, but the model does not reproduce it,” he says. “None of the 250 simulations recovered the observed response across the settings we tested. Correcting the problem will require closer examination of how the model fundamentally represents the underlying processes.”

Myers says immediate beneficiaries of the study are climate researchers and model developers.

“Our findings identify specific processes to investigate and improve. More reliable climate simulations support climate assessments and help governments, water managers, energy planners and communities prepare for changes in temperature, rainfall and other climate risks,” he says. “Understanding clouds is essential because they strongly influence how much warming results from increasing greenhouse gases.”

The work of the UW researchers was supported by the DOE, Office of Science, Office of Biological and Environmental Research, through the Established Program to Stimulate Competitive Research under Award DE-SC0024161. Zelinka’s work was supported by the U.S. DOE Office of Science Biological and Environmental Research program -- Regional and Global Model Analysis program area.