Atmospheric Motion: The Why and The How
Story posted February 25, 2003
If a bright blue plume of smoke were released over the mountains in Alaska, how long would it take before particles from that plume reached Brunswick? And how dense would they be when they got here? It's not so easy to release blue smoke in one place and then wait for it to arrive in another, so how do scientists learn about this type of atmospheric movement? Perhaps more importantly - why do they care?
Assistant professor of physics Mark Battle set about answering those questions in a recent faculty seminar: "Atmospheric motion: The why and the how."
"The title is both grandiose and enigmatic," Battle said, "and I can't possibly live up to it." The why is why we should care about atmospheric motion, and the how is how we know about it, short of releasing that blue plume of smoke.
Atmospheric motion is closely related to weather systems and how they move, and "weather prediction is directly relevant to your lives," Battle said. His interest in atmospheric motion, however, has not much to do with when to expect snow, but a lot to do with global climate change, and what we can expect from it.
That the climate is warming is virtually undisputed. "The temperature changes are well outside the range of natural variability over the past 1000 years," Battle said.
Likewise, scientists generally agree that the introduction of carbon dioxide (CO2 ) into the atmosphere is the greatest manmade influence on climate change.
Greenhouse gases, among them carbon dioxide, are released into the atmosphere and absorbed back into the water or land. This happens through natural and manmade processes, and scientists know it's happening - though they don't know exactly where. Battle's research will lead to a better understanding of these release and absorption sites, or "sources" and "sinks" as they are known.
Sources and sinks are not necessarily static, either. In the early years of the 20th century, much of New England was basically devoid of trees, and instead the landscape was made up of fields and farms. Now forests are being replenished, and because trees absorb carbon dioxide the re-growth of the forests is creating a sink area. By the same token, if a forest is cut down and an industry built, what was once a sink area could become a source of greenhouse gases.
Each year, human industry releases about six gigatons of carbon, in the form of carbon dioxide, into the atmosphere. In rough terms, about one third stays in the air; one third dissolves into the ocean (which is very gradually becoming more acidic); and one third is absorbed into the land.
Understanding the movement of the atmosphere and the sources and sinks for carbon dioxide could make it possible to predict future CO2 levels and better prepare for, or combat, changes in the climate.
To better pinpoint the sources and sinks requires measuring the concentrations CO2 in the atmosphere and then using knowledge of atmospheric motion to do inverse modeling and discover where the carbon dioxide was introduced.
Battle has been working with eight other scientists from four other institutions to model atmospheric motion. To assess their understanding, Battle and his colleagues run a "tracer" substance that is analogous to carbon dioxide through a computer model, then compare those results to what is happening in reality.
To accurately represent CO2 and test the computer model, the tracer needs to share many of carbon dioxide's characteristics. CO2 is basically inert in the atmosphere; it is known to have sources and sinks for distributed throughout the earth; it varies seasonally over both land and water; and it is measurable.
Battle and his colleagues recognized that Ar/N2 is the best available analog for CO2 . The atmospheric Ar/N2 ratio is also inert; the sources and sinks are globally distributed; there is a seasonal variance, though only over the ocean. It's not easy to measure, but it can be done. (Though before Battle and his colleagues began their work, no one had measured it with the requisite precision.)
When ocean water warms, the gases leave it in proportion to their solubility, which leads to changes in the Argon/Nitrogen ratio in the atmosphere. Regardless of the specific quantities of Argon and Nitrogen, the ratio is consistent, which is important in collecting measurements.
"The atmosphere is a big reservoir, so the changes that we see are very small," Battle said.
Battle and his colleagues are among the first to use the Argon-Nitrogen ratio (Ar/N2 ) as their tracer. Battle's colleagues at Princeton use an Isotope Ratio Mass Spectrometer to measure the ratio in air samples gathered at six spots around the globe, from the North Slope of Alaska to Antarctica.
So far, they've collected three years of data, and they've largely been pleased with the results. The scientists use two different models of atmospheric movement and two different models of heat fluxes, which allow calculation of gas emissions, so they can compare the results and start judging the reliability of the models.
As he displayed the graph of their results on the screen, Battle said, "This was just a delight the first time this plot popped out of the computer."
The graph shows the permutations of atmospheric movement and heat flux models closely agreeing with the real-world data.
So far, Battle said, the model developers seem to be getting the heat fluxes and the atmospheric transport models right, though there are phasing differences between the models and the data collected from the real world. This means that while the variation of the Ar/N2 ratios in the model and the real world appear to match, the variations occur earlier in the model than in the real world. There are also a few other discrepancies at different air collection sites; still, the models look good on the whole.
"These models are not made in a vacuum (I didn't mean that as a pun), an intellectual vacuum. They're constantly refined and adapted," Battle said, "so this agreement isn't a complete surprise."
The research is far from over. Battle and his colleagues are tweaking the models to try to improve the accuracy. They're also encouraging collaborators to develop a dedicated model of heat fluxes, since those used in their research were designed for other purposes, not to be used with Ar/N2 . Dedicated models and more data in the years to come gives Battle hope that refinements to these models will lead to much more robust predictions of both carbon dioxide levels in the atmosphere and associated temperature changes.
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