Antarctic Cloud Cover Caused Glitch In Tropical Rainfall Model

April Flowers for – Your Universe Online

One would not assume that cloud cover over Antactica’s Southern Ocean could cause rainfall in Zambia or the tropical island of Java. New research from the University of Washington, however, finds that a phantom band of rainfall just south of the equator that does not occur in reality is caused by poor simulation of the cloud cover thousands of miles farther to the south. This illusionary band of rainfall is one of the most persistent biases in global climate models.

Atmospheric scientists at Washington hope that their results will help explain why global climate models duplicate the inter-tropical convergence zone, a band of heavy rainfall in the northern tropics, on the other side of the equator by mistake.

The results of the study appear in a recent issue of Proceedings of the National Academy of Sciences (PNAS).

“There have been tons of efforts to get the tropical precipitation right, but they have looked in the tropics only,” said Yen-Ting Hwang, a UW doctoral student in atmospheric sciences who found the culprit in one of the most remote areas of the planet.

“What we found, and that was surprising to us, is the models tend to be not cloudy enough in the Southern Ocean so too much sunlight reaches the ocean surface and it gets too hot there,” Hwang said. “People think of clouds locally, but we found that these changes spread into the lower latitudes.”

Prior studies examined tropical sea-surface temperatures, or better ways to represent tropical winds and clouds. None managed to correctly stimulate rainfall in the tropics, however, which is an important region for global climate models since small shifts in rainfall patterns can have huge effects on climate and agriculture.

“The rain bands are very sharp in this area,” commented Dargan Frierson, a UW associate professor of atmospheric sciences. “You go from some of the rainiest places on Earth to some of the driest in [less than a few hundred miles].”

Recent theories have suggested that tropical rainfall might be linked to global processes. The new research looked for possible connections to ocean temperatures, air temperatures, winds and cloud cover.

“For the longest time we were expecting that it would be a combination of different factors,” Frierson said, “but this one just stood out.”

Cloud biases over the Southern Ocean are the primary contributor to the phantom double rain band problem existing in most modern climate models, the research showed.

“It almost correlates perfectly,” Hwang said in a statement. “The models that are doing better in tropical rainfall are the ones that have more cloud cover in the Southern Ocean.”

Hwang will present the findings at the World Climate Research Programme (WCRP) in April. The findings have also been submitted for inclusion in the fifth report of the Intergovernmental Panel on Climate Change (IPCC).

The team found that most modern climate models do not generate enough low-level clouds over the perpetually stormy Southern Ocean, allowing heat to accumulate in the Southern Hemisphere.

“Basically hot air rises, and it rains where air rises. So it´s kind of obvious that the rain is going to be over warmer ocean temperatures,” Frierson said. “Our new thinking is that the heat spreads — it´s the warmth of the entire hemisphere that affects tropical rainfall.”

Climatologists can look for ways to improve the models to increase cloud cover over the Southern Ocean in the short term. Eventually, however, more powerful computers may permit models that are able to create accurate simulations of global cloud coverage.

“We have confidence in climate predictions outside the tropics, but tropical rainfall forecasts are much less certain,” Frierson said. “We hope this work will lead to better rainfall forecasts in regions like equatorial Africa, where it´s so important to have accurate predictions of future patterns.”