October
2007
Ice in clouds
Front Range field campaign begins next month
During ICE-L, researchers will study
ice formation processes in lenticlar clouds (such
as the one pictured here) along the Front Range. |
Much of Earth’s precipitation begins its journey from
the atmosphere to the ground in the form of ice. The process
by which ice crystals (which become rain, snow, and hail)
initially form in clouds is a complicated one that scientists
have been studying for more than 50 years without coming
to consensus.
A team of researchers hopes to shine more light on the issue
next month during a series of research flights along the
Front Range, using NCAR’s C-130 and
a privately-owned Lear Jet. The field campaign, called Ice
in Clouds Experiment–Layer (ICE-L), will improve scientists’ understanding
of ice formation, with applications for cloud models,
precipitation forecasts, and climate change research.
“We’re basically looking at how the ice nucleation
process occurs, and whether we can predict it based on measurements
of ice nuclei,” explains Andy Heymsfield (ESSL/MMM),
who’s leading the project along with Paul Field (ESSL/MMM;
UK Met Office), David Rogers (EOL), and Jeff Stith (EOL).
In total, about 35 co-investigators are participating,
drawn from NCAR, universities, government agencies,
and private companies.
The initial formation of ice is closely tied to the physical
and chemical properties of aerosol particles. (Aerosols
are tiny particles suspended in the air from sources such
as dust, volcanoes, wildfires, vegetation, sea spray, and
fossil fuel burning.) Among the wide variety of aerosol particles
in the atmosphere, there are a few that catalyze ice formation.
These are called ice nuclei. Depending on temperature and
humidity, ice nuclei may make up only one out of a million
particles. They are difficult to identify, but critical for
forming ice in clouds and, ultimately, precipitation.
At sufficiently cold temperatures
(–40ºF/C), pure liquid water freezes spontaneously
in a process called homogeneous nucleation. At warmer temperatures,
ice nuclei form ice from vapor or supercooled liquid water
(chilled below freezing point without having turned solid).
This process is known as
heterogenous nucleation.
Kip Eagan (EOL) prepares a cloud
radar for installation on the C-130. The radar
has the ability to simultaneously look both up
and down into clouds. |
The ICE-L project will make airborne observations of both
types of nucleation, with the C-130 focusing on the heterogenous
process. The
researchers hope to show that direct measurements
of ice nuclei and ice nucleation can be used to predict
the number of ice particles that form in a cloud. The research
is especially important because scientists’ lack
of understanding of the processes of ice initiation in
clouds leads to uncertainties in modeling precipitation
amounts and distribution, as well as in predicting climate
change.
“In any reasonably good-to-excellent cloud model, you
need to predict the concentration of ice crystals as a function
of cloud conditions,” Andy says.
A number of recent advances in instrumentation make this
an ideal time for ICE-L. For example, it’s been difficult
for scientists to measure the number of ice particles in
clouds using airborne instruments because large ice particles
can shatter on the edges of the probes, producing high concentrations
of smaller particles.
“Until now, we’ve been unable to accurately measure
ice concentrations in clouds to within probably about a factor
of ten,”
Andy says.
Faster electro-optics and innovative designs have improved
this situation. The researchers will use new instruments
that reduce the likelihood of shattering ice particles and
allow them to sample true concentrations of smaller ice crystals.
They’ll employ three different probes (the small ice
detector, cloud droplet probe, and 2D stereo) to sample ice
crystals. Using aerosol mass spectrometers, they’ll
measure particle chemical composition. A special cloud chamber
on the aircraft will allow the sampling of ice nuclei, and
a specialized inlet will evaporate cloud ice particles to
allow the residual aerosol particles they contain to be tested
for their ice-forming abilities.
The team will also use two instruments from the University
of Wyoming, a cloud radar and cloud lidar. The radar has
the ability to look both up and down into clouds simultaneously
while giving information on the growth of ice particles and
the vertical winds above and below a cloud, while the lidar
functions as a very sensitive indicator of the first ice
forming in wave clouds.
“The unprecedented part of this project is that we’re
going to have the Wyoming lidar and radar—that’s
just incredible,” Andy says.
A major goal of the project is to determine the effectiveness
of new instruments at identifying critical ice formation
regions that have previously been out of scientists’ reach. “This
will be a starting point for testing instruments,” Andy
says. “If it works, the instruments can be taken to
other places.”
The researchers plan to make about three five-hour flights
per week for four weeks, starting in November. They’ll
focus mainly on lenticular clouds forming over the Front
Range (which is conveniently one of the best places in the
world for the formation of these clouds). Lenticular clouds
provide an almost laboratory-like setting for studying ice
formation processes. Air in these nearly-stationary clouds
ascends in mountain-induced waves, making the growth time
of particles the transit time through the cloud. Researchers
aboard aircraft can repeat sampling the clouds at different
levels to study influences of time and temperature on ice
production processes.
“We’ll also be looking at ice formation in upslope
clouds, which are common during this time of year and contribute
to precipitation along the Front Range,” Jeff Stith
says.
In addition, the project’s proximity to Denver will
allow the team to study the effects of urban aerosols on
ice nucleation processes, as well as the impact of any Asian
dust outbreaks that happen to occur during ICE-L’s
time frame.
Although some initial discoveries are expected during the
field project, the analysis of data and publication of the
results will occur over a period of several years following
the study.
On
the Web
More about ICE-L
In this issue...
Ice
in clouds
Surprise
finding in the desert
Tackling
disasters in an energy-restricted Boulder
Jeffco
bears fruit
Short
Takes
Getting
their paws wet
Delphi
Question
Random
profile: Chris Golubieski
Just One Look
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