A group of researchers from Michigan Technological University, the National Centre for Atmospheric Research (NCAR), Max Planck Institute for Chemistry and Mainz University, had used an airborne lab and a holographic imaging tool, called Holographic Detector for Clouds (or HOLODEC) to better understand the microstructure of clouds and the process whereby dry air mixes with water-saturated air to create them.
The HOLODEC instrument, a tube about 15 centimetres in diameter and about 60 centimetres long, samples the volume of a cloud about the size of a marker and provides unique insights into cloud mixing.
The key finding of the study, published this week in the journal Science, called inhomogeneous mixing, was achieved though studying the clear boundaries between wet and dry air, formed by completely evaporating some water drops and leaving others unscathed. The new finding goes against the base assumptions used in most computer models of cloud formation.
In the study, the team flew NSF/NCAR C-130 military airplanes through fluffy, cottonball cumulus clouds in Wyoming and Colorado, and took detailed, holographic 3D images with the HOLODEC device, attached to one of the wings of the aircraft.
This lab-in-the-sky took about a decade to build and refine. According to Raymond Shaw, a Physics Professor at Michigan Tech, it all started with a prototype HOLODEC, machined and cobbled together over several months.
“And it actually worked – then we had to figure out what to do with the data. Now, we’ve built a better instrument and refined the technique, and we are finally able to handle large amounts of data.” Processed with high-end graphics cards – the kind that hardcore gamers equip their rigs with – these models allow the team to not only visualize mixing in clouds, but also gain insight into ways for improving weather and climate models.
The particular clouds Shaw’s group tested were only made up of liquid water and the size of those drops is a key part in 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,” explained Shaw. “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.”
These tiny water droplets are immensely important to weather and climate, as their relative levels affect how much sunlight makes it into the lower atmosphere, and can reflect, buffer or trap in heat. The difficult part about studying them, however, is that clouds do not cover different parts of the world in the same, uniform layer, and even on smaller scales, mixing exerts an influence on the spacing, size and distribution of the droplets within clouds themselves.
According to the science and technology writer Allison Mills, watching clouds is “serious business, and these instruments and data processing have provided unique insight into the microscopic world of cloud mixing”.