Explosive source of ‘secondary’ ice and snow

Where does snow come from? That may be a simple question to ponder, as half the planet emerges from a season to watch whimsical flakes fall from the sky – and hoe them from ramps. But a new study on how water becomes ice in slightly sub-cooled Arctic clouds, you can rethink the simplicity of the soft things. The study, published by scientists from the US Department of Energy (DOE), Brookhaven National Laboratory in the Proceedings of the National Academy of Sciences, contains new direct evidence that crushing droplets cause explosive “ice multiplication” events. The findings have implications for weather forecasts, climate modeling, water supply – and even energy and transport infrastructure.

“Our results shed new light on the concept previously based on laboratory experiments on how sub-cooled water droplets – water that is still liquid below freezing – turn into ice and eventually snow,” said Brookhaven Lab atmospheric scientist Edward Luke , the lead author of the paper. The new results, of long-term real-world cloud radar and balloon measurements in mixed-phase clouds (composed of liquid water and ice) at temperatures between 0 and -10 degrees Celsius (32 and 14 ° Fahrenheit), provide evidence that the freezing fragmentation of drizzle drops are important for how much ice will form and possibly if snow can fall out of these clouds.

“Now climate models and the weather forecasting models can determine how much snow you need to kick, make a leap forward by using much more realistic physics to simulate ‘secondary’ ice formation,” Luke said.

What is secondary ice?

Precipitation of snow from sub-cooled clouds usually comes from ‘primary’ ice particles that form when water crystallizes on certain small dust spots or aerosols in the atmosphere, known as ice-core particles. However, at slightly sub-cooled temperatures (ie 0 to -10 ° C), aircraft observations have shown that clouds can contain much more ice crystals than the relatively few ice-core particles can explain. This phenomenon has amazed the atmospheric research community for decades. Scientists thought that the explanation was ‘secondary’ ice production, in which the additional ice particles are generated from other ice particles. But it was difficult to set up the process in the natural environment.

Previous explanations for how secondary ice forms are based primarily on laboratory experiments and limited, short-term aircraft-based sampling flights. A common notion that emerged from various laboratory experiments was that relatively large ice particles that fall rapidly, called rimers, can “collect” and freeze droplets of cloud and freeze – which then produce even smaller ice particles, called splinters. But it seems that such ‘rhyme splintering’ is not nearly the whole story.

The new results from the North Pole show that larger sub-cooled water droplets, which are classified as drizzle, play a much more important role in the production of secondary ice particles than is commonly thought.

“When an ice particle hits one of the droplets, it causes freezing, which first forms a solid ice shell around the droplet,” explained Fan Yang, a co-author of the page. “Then, as the freezing point moves inward, the pressure starts to build up because water expands as it freezes. That pressure drops the drizzle and then more ice particles are produced.”

The data show that this ‘freezing fragmentation’ process can be explosive.

“If you have one ice particle that causes the production of another ice particle, it’s not that important,” Luke said. ‘But we have provided evidence that, with this cascade process, freezing rain can increase the ice particle concentrations in clouds 10 to 100 times – and sometimes even 1,000!

“Our findings may provide the missing link for the mismatch between the scarcity of primary ice-core particles and snowfall from these slightly sub-cooled clouds.”

Millions of monsters

The new results depend on six years of data collected by a millimeter-wavelength upward Doppler radar at the DOE Atmospheric Radiation Measuring (ARM) user facility’s Northern Slope of Alaska Atmospheric Observatory in Utqiagvik (formerly Barrow), Alaska. The radar data is supplemented by measurements of temperature, humidity and other atmospheric conditions collected by weather balloons launched from Utqiagvik during the study period.

The atmospheric scientist from Brookhaven Lab and co-author of the study, Pavlos Kollias, who is also a professor in the Department of Atmospheric Sciences at Stony Brook University, was instrumental in collecting this millimeter-wavelength radar data in a way that made it possible for the scientists to deduce how secondary ice was formed.

“ARM has been a pioneer in the use of short wavelength cloud radars since the 1990s to better understand the clouds’ microphysical processes and how they affect the weather on earth today. Our team has led to the optimization of their strategy for data sampling, so that information on cloud and precipitation processes such as the one presented in this study can be obtained, ‘Kollias said.

The radar’s wavelength on a millimeter scale makes it uniquely sensitive to the size of ice particles and water droplets in clouds. Its dual polarization provides particle shape information, allowing scientists to identify needle-like ice crystals – the preferred shape of secondary ice particles in slightly sub-cooled cloud conditions. Doppler spectra observations taken every few seconds provide information on how many particles are there and how fast they fall to the ground. This information is critical to determine where there are ridges, drizzle and secondary ice particles.

Using sophisticated, automated analysis techniques developed by Luke, Yang and Kollias, the scientists scanned millions of these Doppler radar spectra to sort the particles in data buckets by size and shape – and linked the data to temporary observations of balloons about the presence of sub-cooled cloud water, temperature and other variables. The detailed extraction of data enabled them to compare the number of secondary ice needles generated under different conditions: in the presence of only rimers, rimers plus drizzle or just drizzle.

“The large number of observations allows us to lift the secondary ice signal for the first time from the ‘background noise’ of all the other atmospheric processes that take place – and to quantify how and under what conditions secondary ice events occur,” Luke said.

The results were clear: conditions with subcooled droplets produced dramatic ice multiplication events, far more than rhymes.

Short- and long-term impact

These actual data give scientists the ability to quantify the “ice multiplication factor” for different cloud conditions, which will improve the accuracy of climate models and weather forecasts.

“Weather forecasting models can not handle the full complexity of the cloud microphysical processes. We have to make savings on the calculations, otherwise you will never get a forecast,” said Andrew Vogelmann, another co-author of the study. “To do this, you need to find out which aspects of physics are most important, and then account for the physics as accurately and simply as possible in the model. This study makes it clear that the knowledge about drizzle in these mixed-phase clouds is necessary. ‘

In addition to helping you budget how much extra time you need to kick your driveway and get to work, a clearer understanding of what drives secondary ice formation can help scientists better predict how much snow accumulates in watersheds for drinking water during the year. to provide. The new data will also help improve our understanding of how long clouds will last, which has important implications for the climate.

“More ice particles generated by secondary ice production will have a major impact on precipitation, solar radiation (how many sunlight clouds reflect back into space), the water cycle and the evolution of mixed-phase clouds,” Yang said.

The lifespan of the cloud is especially important for the climate in the North Pole, Luke and Vogelmann noted, and the Arctic climate is very important for the overall energy balance on earth.

“Mixed-phase clouds, which contain both subcooled liquid water and ice particles, can last for weeks on end in the Arctic,” Vogelmann said. “But if you have a whole bunch of ice particles, the cloud can clear away after it grows and falls to the ground like snow. Then your sunlight will be able to go straight to heat the ground or the ocean surface.”

This can change the seasonality of snow and ice on the ground, melt and then cause even less reflection of sunlight and more heating.

‘If we can predict in a climate model that something will change the balance between ice formation, drizzle and other factors, then we will be better able to expect what we can expect in the future weather and climate, and possibly be better prepared for these consequences, said Luke.