Phantom Rain Clouds
Climate models are far from perfect. But then again the prediction of weather is a far from perfect science. It seems counterintuitive that clouds over the Southern Ocean, which circles Antarctica, would cause rain in Zambia or the tropical island of Java. But new research finds that one of the most persistent biases in global climate models — 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.
Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the climate system to projections of future climate. The most talked-about use of climate models in recent years has been to project temperature changes resulting from increases in atmospheric concentrations of greenhouse gases.
Models try to simulate as much as possible about the climate system: the incoming and outgoing radiation, the way the air moves, the way clouds form and precipitation falls, the way the ice sheets grow or shrink, etc. They are frequently coupled to a representation of the ocean. They may take into account how the vegetation on the Earth's surface changes. Critically, they try to calculate how all these different parts of the climate system interact, and how the feedback processes work.
University of Washington atmospheric scientists hope their results help explain why global climate models mistakenly duplicate the inter-tropical convergence zone, a band of heavy rainfall in the northern tropics, on the other side of the equator. The study appears this week in the Proceedings of the National Academy of Sciences.
"There have been tons of efforts to get the tropical precipitation right, but they have looked in the tropics only," said lead author Yen-Ting Hwang, a UW doctoral student in atmospheric sciences. She 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."
Previous studies looking at the problem investigated tropical sea-surface temperatures, or ways to better represent tropical winds and clouds. But none managed to correctly simulate rainfall in the tropics — an important region for global climate predictions, since small shifts in rainfall patterns can have huge effects on climate and agriculture.
Recent theories suggest tropical rainfall might be linked to global processes. Hwang’s research, funded by the National Science Foundation, 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."
The paper shows that cloud biases over the Southern Ocean are the primary contributor to the double-rain band problem that exists in most modern climate models.
"It almost correlates perfectly," Hwang said. "The models that are doing better in tropical rainfall are the ones that have more cloud cover in the Southern Ocean."
Most models don’t generate enough low-level clouds over the perpetually stormy Southern Ocean, the authors found, so heat accumulates 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."
In the short term, climate scientists can look for ways to improve the models to increase cloud cover over the Southern Ocean. Eventually, more powerful computers may permit models that are able to accurately simulate clouds over the entire planet.
For further information see Phantom Clouds.
Rain Cloud image via Wikipedia.