Scientists have just made a breakthrough in understanding how clouds interact with the surrounding air by studying clouds in unprecedented detail. 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 .These particular clouds were only made up of liquid water, but the size of those drops is a key part of cloud formation and mixing.
The HOLODEC instrument, which is a tube about six inches in diameter and several feet long, samples a cloud volume about the size of a marker and provides a unique insight into cloud mixing. Jeff Stith, who manages the Research Aviation Facility at NCAR, compares this mingling to marbled cake. The wet air, filled with droplets, is like red cake, the dry air is like white cake. Stith, Shaw and their team then want to know how fine the boundaries are between red and white cake, and how much pink cake there will be. Visible with the HOLODEC, immediately following cloud mixing the team observed clear boundaries, distinct lines between wet and dry air. Additionally, the size of the droplets remained unaffected. The ones that dissipate go away completely and the ones left behind stay the same size, making a marbled cake. Together the open space and large drops encourage clouds to grow more. With enough mixing, the droplets tend to spread out more evenly, blurring the distinct boundary.
“We can not only see how many droplets there are, and how big they are, we can also see how they are distributed in space,” Shaw told NPR. The droplets that are remaining are just as big as the droplets that are in the center, kind of the protected core of the cloud, where no evaporation has taken place,” he says. “That was a surprise to us.” “But the amazing thing is that it really does have consequences,” he adds. “Those little details in the cloud eventually form a link in a chain that leads to a weather forecast, or an understanding of how climate will change.” 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.
Shaw and his colleagues found that at the cloud edges, dry air mixes in with the water, causing some droplets to evaporate completely while others form wispier filaments resulting in those ultra-thin strands that hang off the edge of clouds you see in the air. It’s in contrast to the initial thought that all or most of the droplets would evaporate in the dry air and cause the cloud to shrink. Shaw says these little details are just one step in a domino effect that leads to bigger weather patterns and atmospheric processes. Meteorologists and climatologists use these details to better understand what’s going on with larger phenomena, and predict what future trends or natural events might look like. The study also provides good support for the usefulness of the HOLODEC. One can easily imagine that other scientists will start to employ it in more and more of their own research. Other kinds of clouds such as colder clouds with ice crystals might behave differently, Baker says. Shaw says he’s really interested in understanding those clouds, too.
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