Where does snow come from? This may sound like a simple question to think about as half of the planet emerges from a season of watching whimsical flakes fall from the skies – and move them away from drones. But a new study of ice water in slightly supernatural Arctic clouds may reconsider the simplicity of the moving material. The study, published by scientists from the Brookhaven National Laboratory of the U.S. Department of Energy (DOE) in the Proceedings of the National Academy of Sciences, including new direct evidence that drizzle droplets disrupt “ice proliferation” explosive events. The decisions have an impact on weather forecasts, climate modeling, water supply – and even energy and transport infrastructure.
“Our results shed new light on pre-laboratory understanding based on how supercooled water droplets – water that still melts under its freezing point – turn to ice and eventually snow,” said the person. Brookhaven Lab atmosphere science, Edward Luke, lead author of the paper The new results, from long-range cloud radar in the world and weather balloon measurements in mixed-stage clouds (made up of melt water and ice) at temperatures between 0 and -10 degrees Celsius (32 and 14 ° Fahrenheit), providing evidence that freezing temperatures of permeable droplets are important for the formation of ice that can fall from these clouds. as snow.
“Now climate models and the weather forecasting models used to determine how much snow you need can jump forward using physics a lot more reasonable to simulate the shape of ‘high school’ ice, ”Luke said.
What is high school ice?
Snow rising from supernatural clouds usually comes from “primary” ice fragments, which form when water crystallizes on tiny particles of dust or aerosols in the atmosphere. called ice grains. However, at slightly supernatural temperatures (ie, 0 to -10 ° C), observation of planes has shown that clouds can contain many more ice crystals than can be explained by the small number of grains. nucleating ice present. This phenomenon has been a concern of the atmospheric research community for decades. Scientists believe the definition is a “secondary” ice product, in which the extra ice grains are generated from other ice grains. But it is difficult to capture the process in action in the natural environment.
Previous explanations of how secondary ice forms relied heavily on laboratory experiments and limited, short-lived, air-based sampling flights were available. A common understanding from a number of laboratory experiments was that relatively large, fast ice particles, known as rimers, can “collect” and freeze tiny droplets of clouds – which in turn lead to more tiny ice grains. called splinters. But it turned out that such “rime splintering” is almost not the whole story.
The latest results from the Arctic show that larger supercooled water droplets, known as drizzle, play a more important role in the production of secondary ice granules than was commonly thought.
“When an ice ball hits one of these drizzle droplets, it triggers a frost, which first forms a hard ice shell around the drop,” explained Fan Yang, co-author of the paper. “Then, as the frost moves in, the pressure starts to build up as water expands as it freezes. That pressure causes the drizzle to fall. , generating more ice grains. “
The data shows that this “freeze breaking” process can be explosive.
“If you had one ice cream that would stimulate the production of one more ice grain, it wouldn’t be that important,” said Luke. increase density of ice grains in clouds 10 to 100 times – and even 1,000 from time to time!
“Our findings may provide the link needed for the imbalance between the scarcity of the primary primary ice floes and snowfall from those slightly supernatural clouds.”
Millions of samples
The new results span six years of data collected by a millimeter-wave Doppler radar at the DOE Atmospheric Radiation Atmospheric Radiation (ARM) depot in Utqiagvik (formerly Barrow), Alaska. The radar data is amplified by measurements of temperature, humidity, and other atmospheric conditions collected by weather balloons launched from Utqiagvik during the study period.
Brookhaven Lab atmospheric scientist and study co-author Pavlos Kollias, who is also a professor in the department of atmospheric sciences at Stony Brook University, was instrumental in collecting this millimeter wave radar data in a which made it possible for the experts. find out how secondary ice was formed.
“ARM has begun using shortwave cloud radiators since the 1990s to better understand the microphysical processes of clouds and how these affect weather on Earth today. Our team ‘lead the development of their data sampling strategy to provide information on cloud and rainfall processes so that one presented in this study can be obtained,’ “Kollias said.
The radar’s millimeter-scale wavelength makes it particularly sensitive to ice masses and water droplets in clouds. Its double polarization provides particle-shaped information, allowing scientists to identify needle-shaped ice crystals – the preferred form of secondary ice grains in slightly overcast cloud conditions. Doppler spectra observations recorded every few seconds provide information on how many grains are present and how quickly they fall to the ground. This information is crucial for figuring out where secondary rimers, drizzle, and ice grains are.
Using intelligent automated inspection methods developed by Luke, Yang, and Kollias, the scientists studied through millions of these Doppler radar glasses to sort the grains into a data bucket by size and shape – and matched the data with contemporary weather observations of their presence. of supercooled cloud water, temperature, and other variables. The detailed data mining allowed them to compare the number of secondary ice needles generated under different conditions: the presence of straight rimers, rimers as well as drizzle droplets, or just drizzle.
“The sheer number of ideas allows us for the first time to take the secondary ice signal out of the ‘back sound’ of all other atmospheric processes taking place – and guess how and what are the conditions in which high school ice events occur, “Luke said.
The results were clear: Situations with supercooled drizzle droplets resulted in spectacular ice propagation events, much more than rimers.
Short-term and long-term effects
The real-world data allows scientists to quantify the “ice multiplication factor” for different cloud conditions, which improves the accuracy of climate models and weather forecasts.
“Weather forecasting models can’t handle the complexity of cloud microphysical processes. We have to take advantage of the calculations, otherwise you would never get a forecast,” said Andrew Vogelmann, co. another author in the study. “To do that, you have to figure out which parts of physics are most important, and then describe that physics as accurately and simply as possible. This study makes it clear that you know about pouring in those mixed stage clouds is necessary.
In addition to helping you budget what extra time you need to drive your way and get to work, a clearer understanding of what drives high school ice formation can help scientists get a better forecast of the amount of snow that accumulates in waters to provide drinking water throughout the year. The new data will also help improve our understanding of how long clouds stay around, which has a significant impact on climate.
“More ice grains generated by secondary ice production will have a significant effect on precipitation, solar radiation (than the expulsion of sunlight clouds into space), the water cycle, and the evolution of phase clouds. mixed, “Yang said.
Cloud life is particularly important for the Arctic climate, Luke and Vogelmann noted, and the Arctic climate is very important for the overall energy balance of the Earth.
“Mixed-grade clouds, which contain both supercooled melt water and ice particles, can last for weeks in the Arctic,” Vogelmann said. “But if you have a handful of ice grains, the cloud can clear out. after they grow and fall to the ground like snow. Sunlight is then able to pass through to start heating up the land or ocean surface. “
That could change the season of snow and ice on the ground, causing melting and then even less reflection of sunlight and more heating.
“If we can predict in a climate model that something is going to change the balance of ice formation, rain showers, and other factors, then we will be better able to anticipate expectations. the weather and climate in the future, and perhaps be better prepared for those effects, “Luke said.