UW Faculty Member Contributes to Research that Suggests Rainier Future

A University of Wyoming researcher played a key role in a new study that suggests that most global climate models may underestimate the amount of rain that will fall in Earth’s tropical regions as the Earth continues to warm. That’s because existing models underestimate decreases in high clouds over the tropics seen in recent NASA observations.

“Global climate models need to represent many complex processes in order to predict future climate,” says Zhien Wang, a UW professor in the Department of Atmospheric Science and the Templeton Faculty Fellow. “Part of the processes control cloud and precipitation formation, which are not fully understood and represented in climate models; and introduce uncertainties in predicted future climate as the Earth continues to warm.” 

The new study, titled “Tightening of Tropical Ascent and High Clouds Key to Precipitation Change in a Warmer Climate,” was published earlier this month (June 7) in Nature Communications, an open access journal that publishes high-quality research in biology, physics, chemistry, Earth science and all related areas.

Hui Su, a scientist with NASA’s Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, Calif., was the paper’s lead author. Wang was the paper’s co-author. Other contributing authors were from UCLA, Ewha Women’s University in Seoul, South Korea, and the Division of Geological and Planetary Sciences at the California Institute of Technology.

Regional precipitation changes accompanying anticipated global warming could exert profound impacts on ecosystems and human society, making the need for more accurate global climate models crucial and necessary, according to the paper.

“The study points an important research direction to improve climate model reliability by better representing microphysical and dynamical processes controlling tropical cirrus (clouds),” Wang says. “Satellite and aircraft observations will continue to play a critical role in studying these processes. We are actively working in this research direction.”

Wang says his contribution to the research is to prove one of NASA’s data products used in the study. The data set is generated by combined cloud radar and LIDAR measurements from different satellites to provide high-resolved, high-cloud structure. Wang says this work has been funded continuously during the last 15 years by NASA and the Jet Propulsion Laboratory.

Fewer Clouds, More Rainfall

Logically, it could be asked, “How do fewer clouds lead to more rainfall?”

Globally, rainfall isn’t related only to the clouds that are available to make rain but also to Earth’s “energy budget,” defined as incoming energy from the sun compared to outgoing heat energy. High-altitude tropical clouds trap heat in the atmosphere. If there are fewer of these clouds in the future, the tropical atmosphere will cool. Judging from observed changes in clouds over recent decades, it appears that the atmosphere would create fewer high clouds in response to surface warming. It also would increase tropical rainfall, which would warm the air to balance the cooling from the high cloud shrinkage.

Rainfall warming the air also sounds counterintuitive. Typically, people are used to rain cooling the air around them, not warming it. Several miles up in the atmosphere, however, a different process is occurring.

When water evaporates into water vapor on Earth’s surface and rises into the atmosphere, it carries with it the heat energy that made the water evaporate. In the cold upper atmosphere, when the water vapor condenses into liquid droplets or ice particles, it releases its heat and warms the atmosphere.

It puts the decrease in high tropical cloud cover in context as one result of a planet-wide shift in large-scale air flows that occurs as Earth’s surface temperature warms. These large-scale flows are called the atmospheric general circulation, and they include a wide zone of rising air centered on the equator. Observations over the last 30 to 40 years have shown that this zone is narrowing as the climate warms, causing the decrease in high clouds.

Su, her colleagues at the Jet Propulsion Laboratory, and at three other universities, compared climate data from the past few decades with 23 climate model simulations of the same period. Climate modelers use retrospective simulations like these to gauge how well their numerical models are able to reproduce observations. For data, the research group used observations of outgoing thermal radiation from NASA’s space-borne Clouds and the Earth’s Radiant Energy System (CERES) and other satellite instruments, as well as ground-level observations.

Su’s team found that most of the climate models underestimated the rate of increase in precipitation for each degree of surface warming that has occurred in recent decades. The models that came closest to matching observations of clouds in the present-day climate showed a greater precipitation increase for the future than the other models.

By tracing the underestimation problem back to the models’ deficiencies in representing tropical high clouds and the atmospheric general circulation, Su says, “This study provides a pathway for improving predictions of future precipitation change.”

Wang plans to use the Cheyenne supercomputer to run high-resolution models to better understand related physical processes, together with observations. 

“The improved understanding can be used to improve climate model representation in the future,” Wang says.