Students tend to be focused on, 'How do I solve this particular problem?' which is a good way to think in the middle of a design problem. But I want to give them tools they are going to be able to use for any design problem.

Some people see beauty in the soaring silhouette of a city skyline. Richard Crago sees roughness.

As a professor of civil and environmental engineering, Crago studies the intersection where the land meets the sky. More specifically, he studies how the movement of constituents in the air, such as water vapor, is affected by the interaction of the atmosphere and the land surface. City skylines present an interesting case because their size and irregularity create a significant drag force.

"A major urban area with skyscrapers and so forth is one of, or maybe the, aerodynamically roughest surfaces on Earth," Crago says. As air flows over this rough surface, the drag force creates turbulence, which affects the mixing of air pollutants and the movement of water vapor and heat. Understanding this drag force could help people better predict the weather, both in and downwind of cities, and better understand how pollutants such as ozone will disperse.

Crago has also developed and tested models of how evaporation and transpiration from land surfaces affect the atmosphere and vice versa. Wet areas tend to have high evaporation rates and therefore create high humidity in the atmosphere above, while drier areas tend to have lower rates of evaporation and lower atmospheric humidity. Counter-intuitively, this means that a dry atmosphere, for instance, can lead to a high potential for evaporation but the actual rate might be low, thanks to the lack of water on the ground.

Evaporation rates are also affected by the temperature of the Earth's surface, so accurate temperature measurements are vital to developing accurate models of evaporation. Satellites, however, typically generate a composite temperature measurement that essentially flattens the Earth and ignores the variation between the soil surface and the leaf-canopy of plants. In reality, temperature can vary by 8 to 10 degrees Celsius from the soil into the vegetation. Crago is teasing apart the effects of this variability on models of evaporation.

All of Crago's research focuses on understanding fundamental principles. That's an unusual approach in engineering, a field more often associated with solving specific, practical problems. Problem-solving, however, can require a thorough understanding of complex underlying systems. Crago endeavors to instill an appreciation for basic principles in his students.

"Students tend to be focused on, 'How do I solve this particular problem?' which is a good way to think in the middle of a design problem. But I want to give them tools they are going to be able to use for any design problem," Crago says. "The only way to do that is to help them to understand the basic fundamental principles behind the tools you use."

Posted Aug. 31, 2009

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