The U.S. Department of Energy has awarded a Colorado State University atmospheric scientist a grant to understand more about a rare class of atmospheric aerosols that could improve weather models.
Here in the West, this winter’s snow is next year’s water. Weather models are critically important tools in agriculture and resource management. But gaps in our understanding of a rare class of atmospheric aerosols known as ice-nucleating particles have made it difficult to accurately predict rain and snowfall. This modeling weakness leaves stakeholders guessing how much water a winter will provide, and where it will end up.
DOE awarded Russell Perkins, a research scientist in CSU’s Department of Atmospheric Science, $668,000 to advance our understanding and modeling of the role these aerosols play in cloud formation.
A recipe for snow
If you could rewind and watch the formation of any individual snowflake plucked from a mountainside in the Rockies, chances are that it began with crystals forming around an ice-nucleating particle.
Without a nucleating particle present, atmospheric water droplets can supercool, remaining liquid at temperatures as low as minus 36 F before freezing. For this reason, atmospheric conditions often don’t favor spontaneous freezing.
These particles can prompt crystallization at much higher temperatures, depending on the chemistry of the individual particle, but they are extremely rare among atmospheric aerosols. At minus 4 F , ice nucleation occurs about one part per million among aerosol particles. The physics of condensing liquid water from vapor is similar, with vapor always condensing on aerosol particles known as cloud condensation nuclei, but these are much more common among aerosols.
A variety of substances can act as ice-nucleating particles, including biological compounds, naturally occurring organics such as wildfire byproducts, or man-made substances. The most widely known man-made ice-nucleating particle, silver iodide, is commonly used in cloud seeding operations. Another interesting particle is a dried cultured bacteria product marketed under the trade name Snomax, which is used in the ski industry to allow artificial snow-making at temperatures as high as 28 F.
Not enough is known about the sources, abundance, and transport of naturally occurring ice nucleating particles to accurately incorporate them into weather models, Perkins said.
“How likely it is for a particular cloud to precipitate, to have rain or snow fall out of it, can all be really dependent on the aerosols that get fed into it,” he said.
Samples, data, and modeling
Perkins and co-investigator Sonia Kreidenweis, a University Distinguished Professor of Atmospheric Science, will investigate how different aerosols affect cloud properties like ice/water ratio, average droplet size, and other factors. It’s part of a larger effort by the DOE’s Atmospheric Radiation Measurement program to improve the snowfall forecasting capability of the complicated scientific simulations known as earth system models or what scientists call ESMs.
“ESMs have persistently failed in predicting the timing of peak snow depth and snowmelt in winter, and in capturing decadal trends in the timing of summer precipitation,” Perkins said.
The researchers will work with data and samples collected by the Surface Atmosphere Integrated field Laboratory to identify and characterize ice-nucleating particles, cloud condensation nuclei, and other atmospheric aerosols. Along the way, they’ll integrate records of snowmaking and cloudseeding activity into their data, helping to quantify the impact these human inputs have. And, using the varied elevations of the mountain sites in the Surface Laboratory project’s distributed sensor network, they’ll work toward an understanding of vertical transport of these aerosols so that future surface measurements can provide useful information about cloud inputs.
