As clouds change shape, mixing occurs, as drier air mingles with water-saturated air. New research led by Michigan Technological University analyzes this mixing with a holographic imaging instrument called HOLODEC and an airborne laboratory. The work was done in collaboration with the National Center for Atmospheric Research (NCAR), Max Planck Institute for Chemistry and Mainz University.
This new way of seeing clouds--and the way wet and dry air form sharp boundaries--is the focus of the team's study, published in Science this week. What the team found with these naturally created boundaries, formed by completely evaporating some water drops and leaving others unscathed, is called inhomogenous mixing. And it goes against base assumptions used in most computer models assume for cloud formations.
The findings will influence models that help predict weather and climate change.
Raymond Shaw, a professor of physics at Michigan Tech, looks at the smallest part of clouds: droplets. To understand groups of droplets, Shaw and the NCAR team flew airplanes through fluffy, cottonball cumulus clouds in Wyoming and Colorado. Aboard the plane, the team took detailed 3-D images with an instrument called the Holographic Detector for Clouds (HOLODEC--yes that's like Star Trek "holodeck"). These particular clouds were only made up of liquid water and the size of those drops is a key part of cloud formation and mixing.
"You can take a certain amount of water, and divide it up into many small drops or just a few big drops," Shaw says, explaining that it's like having a lot of sand or just a few boulders. "And just by dividing it up in different ways, you can change the optical properties of the clouds, making them brighter or darker, more or less reflective."
The differences affect how much sunlight makes it into the lower atmosphere and can reflect, buffer or trap in heat. The challenge, however, is that clouds don't blanket regions--let alone the world--in a uniform layer. Plus, on even smaller scales within clouds, mixing affects the spacing between drops, what size they are and how they are distributed throughout the clouds.