Explosive origins of 'secondary' ice and snow

Date:
March 22, 2021
Source:
DOE/Brookhaven National Laboratory
Summary:
Scientists publish new direct evidence that shattering drizzle
droplets drive explosive 'ice multiplication' events. The findings
have implications for weather forecasts, climate modeling, water
supplies - - and even energy and transportation infrastructure.



FULL STORY ========================================================================== Where does snow come from? This may seem like a simple question to ponder
as half the planet emerges from a season of watching whimsical flakes
fall from the sky -- and shoveling them from driveways. But a new study
on how water becomes ice in slightly supercooled Arctic clouds may make
you rethink the simplicity of the fluffy stuff. The study, published by scientists from the U.S. Department of Energy's (DOE) Brookhaven National Laboratory in the Proceedings of the National Academy of Sciences,
includes new direct evidence that shattering drizzle droplets drive
explosive "ice multiplication" events.

The findings have implications for weather forecasts, climate modeling,
water supplies -- and even energy and transportation infrastructure.


==========================================================================
"Our results shed new light on prior lab-experiment-based understanding
about how supercooled water droplets -- water that's still liquid below
its freezing point -- turn into ice and eventually snow," said Brookhaven
Lab atmospheric scientist Edward Luke, the lead author on the paper. The
new results, from real-world long-term cloud radar and weather-balloon measurements in mixed- phase clouds (composed of liquid water and ice) at temperatures between 0 and - 10 degrees Celsius (32 and 14DEG Fahrenheit), provide evidence that freezing fragmentation of drizzle drops is important
to how much ice will form and potentially fall from these clouds as snow.

"Now climate models and the weather forecast models used to determine
how much snow you'll have to shovel can make a leap forward by using much
more realistic physics to simulate 'secondary' ice formation," Luke said.

What is secondary ice? Precipitating snow from supercooled clouds
usually originates from "primary" ice particles, which form when water crystallizes on select tiny specks of dust or aerosols in the atmosphere,
known as ice-nucleating particles. However, at slightly supercooled temperatures (i.e., 0 to -10DEGC), aircraft observations have shown that
clouds can contain far more ice crystals than can be explained by the relatively few ice-nucleating particles present. This phenomenon has
puzzled the atmospheric research community for decades. Scientists have
thought that the explanation is "secondary" ice production, in which the additional ice particles are generated from other ice particles. But
catching the process in action in the natural environment has been
difficult.

Previous explanations for how secondary ice forms relied mainly on
laboratory experiments and limited, short-term aircraft-based sampling
flights. A common understanding that came out of several lab experiments
was that relatively big, fast-falling ice particles, called rimers,
can "collect" and freeze tiny, supercooled cloud droplets -- which then
produce more tiny ice particles, called splinters. But it turns out that
such "rime splintering" isn't nearly the whole story.



==========================================================================
The new results from the Arctic show that larger supercooled water
droplets, classified as drizzle, play a much more important role in
producing secondary ice particles than commonly thought.

"When an ice particle hits one of those drizzle drops, it triggers
freezing, which first forms a solid ice shell around the drop,"
explained Fan Yang, a co- author on the paper. "Then, as the freezing
moves inward, the pressure starts to build because water expands as it
freezes. That pressure causes the drizzle drop to shatter, generating
more ice particles." The data show that this "freezing fragmentation"
process can be explosive.

"If you had one ice particle triggering the production of one other
ice particle, it would not be that significant," Luke said. "But we've
provided evidence that, with this cascading process, drizzle freezing fragmentation can enhance ice particle concentrations in clouds by 10 to
100 times -- and even 1,000 on occasion! "Our findings could provide
the missing link for the mismatch between the scarcity of primary ice-nucleating particles and snowfall from these slightly supercooled
clouds." Millions of samples


==========================================================================
The new results hinge upon six years of data gathered by
an upward-pointing millimeter-wavelength Doppler radar at the DOE
Atmospheric Radiation Measurement (ARM) user facility's North Slope of
Alaska atmospheric observatory in Utqiagvik (formerly Barrow), Alaska. The radar data are complemented by measurements of temperature, humidity,
and other atmospheric conditions collected by weather balloons launched
from Utqiagvik throughout the study period.

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

"ARM has pioneered the use of short-wavelength cloud radars since the
1990s to better understand clouds' microphysical processes and how those
affect weather on Earth today. Our team led the optimization of their
data sampling strategy so information on cloud and precipitation processes
like the one presented in this study can be obtained," Kollias said.

The radar's millimeter-scale wavelength makes it uniquely sensitive to the sizes of ice particles and water droplets in clouds. Its dual polarization provides information about particle shape, allowing scientists to
identify needlelike ice crystals -- the preferential shape of secondary
ice particles in slightly supercooled cloud conditions. Doppler spectra observations recorded every few seconds provide information on how many particles are present and how fast they fall toward the ground. This information is critical to figuring out where there are rimers, drizzle,
and secondary ice particles.

Using sophisticated automated analysis techniques developed by Luke,
Yang, and Kollias, the scientists scanned through millions of these
Doppler radar spectra to sort the particles into data buckets by size
and shape -- and matched the data with contemporaneous weather-balloon observations on the 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: in
the presence of just rimers, rimers plus drizzle drops, or just drizzle.

"The sheer volume of observations allows us for the first time to lift
the secondary ice signal out of the 'background noise' of all the other atmospheric processes taking place -- and quantify how and under what circumstances secondary ice events happen," Luke said.

The results were clear: Conditions with supercooled drizzle drops produced dramatic ice multiplication events, many more than rimers.

Short- and long-term impacts These real-world data give the scientists
the ability to quantify the "ice multiplication factor" for various
cloud conditions, which will improve the accuracy of climate models and
weather forecasts.

"Weather prediction models can't handle the full complexity of the
cloud microphysical processes. We need to economize on the computations, otherwise you'd never get a forecast out," said Andrew Vogelmann, another co-author on the study. "To do that, you have to figure out what aspects
of the physics are most important, and then account for that physics as accurately and simply as possible in the model. This study makes it clear
that knowing about drizzle in these mixed-phase clouds is essential."
Besides helping you budget how much extra time you'll need to shovel
your driveway and get to work, a clearer understanding of what drives
secondary ice formation can help scientists better predict how much snow
will accumulate in watersheds to provide drinking water throughout the
year. The new data will also help improve our understanding of how long
clouds will stick around, which has important consequences for climate.

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

Cloud lifetime is particularly important to the climate in the Arctic,
Luke and Vogelmann noted, and the Arctic climate is very important to
the overall energy balance on Earth.

"Mixed-phase clouds, which have both supercooled liquid water and ice
particles in them, can last for weeks on end in the Arctic," Vogelmann
said. "But if you have a whole bunch of ice particles, the cloud can get cleared out after they grow and fall to the ground as snow. Then you'll
have sunlight able to go straight through to start heating up the ground
or ocean surface." That could change the seasonality 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, drizzle, and other factors, then we'll
have a better ability to anticipate what to expect in future weather and climate, and possibly be better prepared for these impacts," Luke said.

========================================================================== Story Source: Materials provided by
DOE/Brookhaven_National_Laboratory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Edward P. Luke, Fan Yang, Pavlos Kollias, Andrew M. Vogelmann,
Maximilian
Maahn. New insights into ice multiplication using remote-sensing
observations of slightly supercooled mixed-phase clouds in the
Arctic.

Proceedings of the National Academy of Sciences, 2021; 118 (13):
e2021387118 DOI: 10.1073/pnas.2021387118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210322175036.htm

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