Remote clouds responsible for climate models' glitch in tropical rainfall

Mar 11, 2013
Earth

New research 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 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 models – a phantom band of rainfall just south of the equator that does not occur in reality – is caused by poor simulation of the thousands of miles farther to the south. University of Washington hope their results help explain why 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 .

"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 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 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 can have huge effects on climate and agriculture.

"The rain bands are very sharp in this area," commented co-author 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 just a few hundred kilometers."

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 .

"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."

Hwang will speak on her results in April to scientists at the World Climate Research Programme. The results have also been submitted for inclusion in the fifth report of the Intergovernmental Panel on Climate Change, which is expected to appear next year.

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 . Eventually, more powerful computers may permit models that are able to accurately simulate clouds over the entire planet.

"We have confidence in climate predictions outside the tropics, but 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."

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dav_daddy
1.3 / 5 (12) Mar 12, 2013
Ok what happened to the guy who said "we have precise models that can mimic past changes in climate and are accurate enough to predict details regarding current and future climate?"
runrig
4.6 / 5 (9) Mar 12, 2013
Ok what happened to the guy who said "we have precise models that can mimic past changes in climate and are accurate enough to predict details regarding current and future climate?"


That "guy" does not exist, as anyone with a wit of intelligence would know. Models are only an approximation of the real world - an approximation that climate scientists are constantly trying to improve upon. Whoever led you to believe that forecasting the future is 100% correct? - that's just as impossible as being 100% wrong. Try looking at a normal distribution curve. If you understand one that is.
Dug
1 / 5 (5) Mar 12, 2013
That guy may not exist, but the majority of people seem to think that climate models are a lot more accurate than they are. Those people include legislators and numerous planners and decision makers trying to figure out just how accurate climate change predictions might be and what actions need to be taken. In the last year their have been at least three major climate model glitches reported - not counting this one. We need to understand that models are not science in themselves, but are based on selected information (some times extrapolated and interpolated data) and employed potentially with bias. Much of the information used in climate models is outside of the accuracy of the instruments used. Examples being temps and sea level are predicting changes outside the range of accuracy of both satellite altimetry (fraction of mm.) and temp accuracy beyond the range of most recording temperature devices - i.e. 1.3 F is beyond the accuracy of the avg. field grade thermometer.
ryggesogn2
1.8 / 5 (10) Mar 12, 2013
Water vapor affects climate and models don't account for water vapor very well?
Howhot
5 / 5 (5) Mar 12, 2013
R2, you know very well that water vapor is taken into account for computer models of climate change! The funny little twist in the argument is that the climate models are actually UNDER predicting the global rise of temperatures from green house gasses. And wow, what an effect that will have on the denialist libertarian survivalist scourge. How do you deal with a film like "Beasts of the Southern Wild" that is so libertarian survivalist in its philosophy but so very concerned with the environment.

As always, I think your barking up the wrong tree.
Sherrin
1.4 / 5 (8) Mar 13, 2013
Shows the northern hemisphere bias in research. Nobody thought to do enough ground truth checking in the southern hemisphere (until now), because the southern weather patterns must surely be a mirror image of the northern hemisphere patterns. Mustn't they? Grrr... Not very happy with Yen-Ting Hwang's language "...so too much sunlight reaches the ocean surface and it gets too hot there," Hmmm. She better turn the southern hemisphere thermostat down then if it doesn't suit her.
runrig
5 / 5 (5) Mar 13, 2013
Shows the northern hemisphere bias in research. Nobody thought to do enough ground truth checking in the southern hemisphere (until now), because the southern weather patterns must surely be a mirror image of the northern hemisphere patterns. Mustn't they? Grrr... Not very happy with Yen-Ting Hwang's language "...so too much sunlight reaches the ocean surface and it gets too hot there," Hmmm. She better turn the southern hemisphere thermostat down then if it doesn't suit her.


No, actually the SH circulation behaves very differently than that of the NH. Greater land-mass in the north with large mountain ranges disturbs/deflects the Jet-stream. SH is dominated by Antarctica at it's centre, with a vast expanse of ocean surrounding, keeping a tight zonal circulation.