Brewster returns to roots in new research
A common misconception is that carbon dioxide (CO2) is the most important greenhouse gas within the Earth’s atmosphere, but it is actually water vapor. The emphasis on CO2 comes from our active creation of it, but overall the net effect of water vapor in the atmosphere does far more to trap heat. And while reducing CO2 emissions is important, so is understanding water’s effect in Earth’s energy balance better.
“I’m intrigued by water because there are still big uncertainties about its radiative properties and effects in the atmosphere,” Brewster said. “We’re working on these properties and effects with both the vapor form of water and also the condensed form.”
With his doctoral work focusing specifically on radiation in scattering media, Brewster brings that perspective to the problems that atmospheric scientists are also working to solve.
“I mean, I love radiation. I love heat and mass transfer whether the water is in liquid form, condensed into clouds, or the vapor form,” he said. “I just got into tons of different interesting problems.”
With an NSF grant for geophysical science research, Brewster is focusing on establishing a better understanding of how water in different forms effects heat transfer in the atmosphere. In his lab, they work with mists and cloud droplets to determine their behavior and how radiation influences them and vice versa.
Part of the motivation for the work is to help explain how clouds become rain. Although there is a fair amount of knowledge established about clouds and precipitation, there is still a major question on how cloud droplets make the transition into rain drops in “warm” (temperatures above freezing) clouds.
The volume of a rain drop is about a million times larger than the volume of a single cloud droplet. The atmospheric science community has not fully agreed on how the transition or growth in size between cloud drops and precipitation occurs in warm clouds. Brewster is testing to see if this phenomenon, the so-called “condensation-coagulation bottleneck,” can be at least partially explained by radiative cooling.
There is also an unsolved problem in atmospheric radiation called the “water-vapor continuum” that Brewster is trying to make progress on. And there is still uncertainty on the net effect of clouds on Earth’s radiation balance, whether they trap more infrared heat in or scatter more solar radiation back out to space.
“It’s fun to work on big-picture questions in atmospheric sciences as a mechanical engineer,” he said. “And even though there’s a lot of uncertainty about water’s energetic role in the atmosphere, one thing is certain: the amount of water in the atmosphere is increasing as temperatures warm. And when water condenses, it releases a lot of energy, the equivalent of tornadic wind-speed kinetic energies from a relatively small amount of water vapor condensing.”
In addition to his work in environmental sciences, Brewster has other concurrent projects in heat and mass transfer. Using his expertise in combustion, he is tackling the problem of creating safer, more environmentally friendly airbag inflator materials. Another project he is working on involves using radiation to create more efficient refrigerator-freezers for consumers that don’t ruin frozen food by thawing it during the defrost cycle.