David L. Chandler, MIT News Offic

Heavier rainstorms lie in our future. That’s the clear conclusion of a new MIT and Caltech study on the impact that global climate change will have on precipitation patterns.

But the increase in extreme downpours is not uniformly spread around the world, the analysis shows. While the pattern is clear and consistent outside of the tropics, climate models give conflicting results within the tropics and more research will be needed to determine the likely outcomes in tropical regions.

Overall, previous studies have shown that average annual precipitation will increase in both the deep tropics and in temperate zones, but will decrease in the subtropics. However, it’s important to know how the magnitude of extreme precipitation events will be affected, as these heavy downpours can lead to increased flooding and soil erosion.

It is the magnitude of these extreme events that was the subject of this new research, which will appear online in the Proceedings of the National Academy of Sciences during the week of Aug. 17. The report was written by Paul O’Gorman, assistant professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT, and Tapio Schneider, professor of environmental science and engineering at Caltech.

Model simulations used in the study suggest that precipitation in extreme events will go up by about 5 to 6 percent for every one degree Celsius increase in temperature. Separate projections published earlier this year by MIT’s Joint Program on the Science and Policy of Global Change indicate that without rapid and massive policy changes, there is a median probability of global surface warming of 5.2 degrees Celsius by 2100, with a 90 percent probability range of 3.5 to 7.4 degrees.

Specialists in the field called the new report by O’Gorman and Schneider a significant advance. Richard Allan, a senior research fellow at the Environmental Systems Science Centre at Reading University in Britain, says, “O’Gorman’s analysis is an important step in understanding the physical basis for future increases in the most intense rainfall projected by climate models.” He adds, however, that “more work is required in reconciling these simulations with observed changes in extreme rainfall events.”

The basic underlying reason for the projected increase in precipitation is that warmer air can hold more water vapor. So as the climate heats up, “there will be more vapor in the atmosphere, which will lead to an increase in precipitation extremes,” O’Gorman says.

However, contrary to what might be expected, precipitation extremes do not increase at the same rate as the moisture capacity of the atmosphere. The extremes do go up, but not by as much as the total water vapor, he says. That is because water condenses out as rising air cools, but the rate of cooling for the rising air is less in a warmer climate, and this moderates the increase in precipitation, he says.

The reason the climate models are less consistent about what will happen to precipitation extremes in the tropics, O’Gorman explains, is that typical weather systems there fall below the size limitations of the models. While high and low pressure areas in temperate zones may span 1,000 kilometers, typical storm circulations in the tropics are too small for models to account for directly. To address that problem, O’Gorman and others are trying to run much smaller-scale, higher-resolution models for tropical areas.

David L. Chandler, MIT News Office

The Air Canada plane from Victoria to Toronto was cruising last year in clear skies; neither the pilots’ eyes nor the air traffic controllers’ radar screens saw any bad weather. Then, abruptly, the plane plummeted 2,000 feet in 15 seconds, and then another 2,000, before the pilot was able to regain control and level off.

The terrified passengers and crew — 10 of whom suffered injuries that would require hospital treatment — had experienced a phenomenon called an internal wave, something that is relatively unknown to the general public, and which is beginning to yield its secrets to scientists under new observational and modeling techniques.

Internal waves, which are prevalent in the oceans as well as in the atmosphere, are hidden from obvious view most of the time. But these colossal phenomena — a single wave can span 1,000 kilometers or more — can have profound effects on Earth’s climate as well as on drilling rigs, undersea cables and even vehicles such as submarines and, as those Air Canada passengers and crew can attest, to aircraft in flight.

Thomas Peacock, an associate professor of mechanical engineering at MIT, has been studying these waves for more than five years, and has uncovered important new details of how they form and propagate. His two latest papers on the subject are being published over the coming months in the Journal of Fluid Mechanics. One of these provides new insight into the forms of internal waves generated in the oceans, and the other helps explain the mystery of a narrow, focused beam of internal waves that recurs regularly twice a day – tied to the tides – in a channel in the Hawaiian Islands, but vanishes near the ocean surface.

Understanding these waves is important for models of climate change, because breaking internal waves in the ocean are believed to be a significant part of the mixing process by which warmer surface ocean water can be carried to the depths and colder water to the surface. This potentially makes them one of several important ocean mechanisms that impact the Earth’s climate.

Ron Prinn, director of MIT’s Center for the Science and Policy of Global Climate Change, says the mixing rate in the oceans — the rate at which warm surface waters get mixed with colder deep water and remove heat from the atmosphere — is one of the biggest remaining uncertainties in climate modeling, so understanding the mechanisms better could make a big difference in the accuracy of climate projections.

“It’s one of the outstanding problems” in climate modeling, says Raffaele Ferrari, professor of physical oceanography in the Department of Earth, Atmospheric and Planetary Sciences. He notes that although researchers have made great progress in understanding how internal waves are produced, when it comes to figuring out how they break — that is, how their energy is dissipated — “we can probably account for 20 percent, but we can’t account for the other 80 percent. It’s a missing link.”

Since such waves can disrupt both moving airplanes and underwater vehicles, and stationary equipment such as undersea drilling rigs and communications cables, this research could also have important practical consequences – at some point perhaps yielding improved ways of predicting the times and locations where they may occur. Because of that, Peacock’s work has drawn funding from the National Science Foundation, the MIT France Program and the Office of Naval Research.

The existence of internal waves has been known for more than a century, but exactly how they form and dissipate, and their effects on both natural and technological systems, are still being explored and are yielding new insights. Peacock and his students in the Experimental and Nonlinear Dynamics Lab (ENDLab) have made significant strides by coupling mathematical modeling of the behavior of these waves with laboratory experiments in wave tanks he has designed, and participation in field research at sea, in an effort to better understand not only how the waves form, but also how they then lose their energy. Bruce Sutherland, professor of physics at the University of Alberta, Canada, says that this combined approach is unique among climate scientists. “Such a holistic view has already benefited our understanding of climate,” he says, “through his studies of wave generation by tidal flow over ridges, and by the examination of the life-cycle of these waves emanating from sills and seamounts.”

The internal waves themselves are made up of moving regions of air or water that are more dense or less dense than their surroundings because of differences in temperature and, in the water, differences in salinity. In principle, they are similar to the familiar waves on the ocean’s surface, but because they occur within the water their visible manifestations are subtle, or sometimes nonexistent.

In the ocean, these waves form when tidal currents pass over an obstacle such as a submerged ocean ridge. “Cold, heavy water from the bottom gets pushed up over the ridge, and sets up a disturbance,” Peacock explains. They can also be generated by powerful storms, such as hurricanes, displacing the ocean surface. In the atmosphere, internal waves can be produced by thunderstorms and when air passes over a mountain range, in which case they are sometimes called “mountain waves.” It was just such a mountain wave, in the lee of the Rockies, that caused last year’s Air Canada plunge.

Besides his efforts to understand these waves, Peacock has been working to increase public understanding of these little-known yet widespread effects. To illustrate their power and immense scale, he plans to travel to Australia this fall to film a segment for a Discovery Channel program he is co-producing about internal waves; two segments in the South China Sea and off the West Coast of Australia have already been completed. He and the film crew hope to be able to catch an atmospheric internal wave that produces something called a Morning Glory cloud, in a location where they typically form at this time of year, and “surf” that wave in a glider. Because the long, narrow cylindrical cloud formation can span hundreds of miles, the popular Lonely Planet travel guide has described it as the most exciting natural phenomenon to observe in the sky next to a total eclipse.

“It’s a challenge, to try to be there when it happens,” he says. “But it will be a once-in-a-lifetime experience to surf the Morning Glory.”

David L. Chandler, MIT News Office
August 17, 2009

Heavier rainstorms lie in our future. That’s the clear conclusion of a new MIT and Caltech study on the impact that global climate change will have on precipitation patterns.

But the increase in extreme downpours is not uniformly spread around the world, the analysis shows. While the pattern is clear and consistent outside of the tropics, climate models give conflicting results within the tropics and more research will be needed to determine the likely outcomes in tropical regions.

Overall, previous studies have shown that average annual precipitation will increase in both the deep tropics and in temperate zones, but will decrease in the subtropics. However, it’s important to know how the magnitude of extreme precipitation events will be affected, as these heavy downpours can lead to increased flooding and soil erosion.

It is the magnitude of these extreme events that was the subject of this new research, which will appear online in the Proceedings of the National Academy of Sciences during the week of Aug. 17. The report was written by Paul O’Gorman, assistant professor in the Department of Earth, Atmospheric and Planetary Sciences at MIT, and Tapio Schneider, professor of environmental science and engineering at Caltech.

Model simulations used in the study suggest that precipitation in extreme events will go up by about 5 to 6 percent for every one degree Celsius increase in temperature. Separate projections published earlier this year by MIT’s Joint Program on the Science and Policy of Global Change indicate that without rapid and massive policy changes, there is a median probability of global surface warming of 5.2 degrees Celsius by 2100, with a 90 percent probability range of 3.5 to 7.4 degrees.

Specialists in the field called the new report by O’Gorman and Schneider a significant advance. Richard Allan, a senior research fellow at the Environmental Systems Science Centre at Reading University in Britain, says, “O’Gorman’s analysis is an important step in understanding the physical basis for future increases in the most intense rainfall projected by climate models.” He adds, however, that “more work is required in reconciling these simulations with observed changes in extreme rainfall events.”

The basic underlying reason for the projected increase in precipitation is that warmer air can hold more water vapor. So as the climate heats up, “there will be more vapor in the atmosphere, which will lead to an increase in precipitation extremes,” O’Gorman says.

However, contrary to what might be expected, precipitation extremes do not increase at the same rate as the moisture capacity of the atmosphere. The extremes do go up, but not by as much as the total water vapor, he says. That is because water condenses out as rising air cools, but the rate of cooling for the rising air is less in a warmer climate, and this moderates the increase in precipitation, he says.

The reason the climate models are less consistent about what will happen to precipitation extremes in the tropics, O’Gorman explains, is that typical weather systems there fall below the size limitations of the models. While high and low pressure areas in temperate zones may span 1,000 kilometers, typical storm circulations in the tropics are too small for models to account for directly. To address that problem, O’Gorman and others are trying to run much smaller-scale, higher-resolution models for tropical areas.