The University of Bonn ADMIRARI Radiometer deployed at the CARE site. This instrument measures microwaves that are naturally emitted from Earth’s surface to determine water vapor and cloud and liquid water in the air column. Credit: NASA / Walt Petersen
the future is always a tricky business—just watch a TV weather report.
Weather forecasts have come a long way, but almost every season there’s a
snowstorm that seems to come out of nowhere, or one that’s forecast as
‘the big one’ that turns out to be a total bust.
the last ten years, scientists have shown that it is possible to detect
falling snow and measure surface snowpack information from the vantage
point of space. But there remains much that is unknown about the fluffy
still figuring out how to measure snow from space,” says Gail
Skofronick-Jackson, a specialist in the remote sensing of snow at NASA’s
Goddard Space Fight Center, Greenbelt, Md. “We’re where we were with
measuring rain 40 years ago.”
is part of a team of scientists from NASA and Environment Canada who
are running a large experiment in Southern Ontario to improve snow
detection. Their GPM Cold-season Precipitation Experiment (GCPEx)
supports the new Global Precipitation Measurement (GPM) mission whose
Core satellite is scheduled to launch in 2014.
is an international satellite mission that will unify and set new
standards for precipitation measurements from space, providing the
next-generation observations of rain and snow worldwide every three
part of their snow detection efforts, the GCPEx science team is
collecting as much data as they can to improve understanding of snow
dynamics inside clouds, because they relate to how snow moves through
Earth’s water and climate cycles.
snowfall measurements are important for more than just weather
forecasts. Snow is one of the primary sources of water in mountainous
regions. For example, the snow pack on the Sierra Nevada Mountains
accounts for one-third of the water supply for all of California. The
snowmelt that enters the water cycle in spring and summer provides both
drinking water and irrigation for California’s $37.5 billion dollar
agricultural industry. Droughts and climate change are making the snow
pack a shrinking water resource, and managers have a greater need than
ever to know exactly how much water is locked in snow.
But to know that, scientists first have to know just how much water snow carries as it falls to the ground.
Rain vs. snow
out how much water comes down as snow first requires being able to tell
snow from rain. On the surface, it’s obvious, rain is liquid and wet,
snow is solid and frozen. But rain and snow often happen at the same
time or snow can melt into rain.
The NASA D3R radar deployed at the CARE site. This radar scans the air column for snow falling from the clouds to the ground. Credit: NASA / Walt Petersen
tell the difference between rain and snow from space, scientists use an
instrument called a microwave radiometer. It works by measuring the
microwaves that naturally radiate from Earth all the time. Different
natural phenomena radiate at different frequencies. For example rain
causes a response at lower frequency microwaves while falling snow
affects higher frequency microwave measurements. A radiometer, like a
car radio picking up different stations, picks up responses at different
frequencies and thus distinguishes between rain and snow. The signal
gets stronger for heavy rain or intense snow rates.
radiometers on some satellites can tell the difference between rain and
snow—but only to a point. Complications occur where the frequencies
respond to both liquid rain and falling snow or even to the Earth’s
surface, says Skofronick-Jackson. “There’s a mixed response in those
channels, so you kind of have to know what you’re looking at.”
those overlapping frequencies, what distinguishes rain from snow is
temperature, size and shape, a current unknown in most precipitation
we know about rain is that raindrops are spherical or slightly
flattened spheres. So they all basically have one shape,” says
Skofronick-Jackson. “With snow we have so many different shapes.”
Needles in a stack of snowflakes
come in a wide variety of shapes and sizes. Individual flakes can be
long thin needles, hollow columns, or flat plates with millions of
different patterns. Their fluffy shapes and sheer variety are what make
measuring snow rates tricky.
are going to pretty much fall straight down as fairly dense liquid
particles. Snowflakes wobble; they’re blown by the wind. They’re going
to have all these different characteristics as to how they fall. And
that makes a difference in what the satellite sees,” says
variety of snowflake shapes also complicate estimates of how much water
snow holds. A “wet” snow of fluffy flakes has more water per unit
volume than “dry” snow. On the ground, the same physical volume of those
types of snow contain very different amounts of water, and this water,
called snowmelt, is what ultimately ends up in reservoirs, rivers and
other sources of freshwater.
GPM satellite will measure global precipitation, be it heavy tropical
rain, moderate rain, light rain or snow. GPM’s radar instrument, built
by mission partner, the Japanese Aerospace Exploration Agency, provides
essential measurements of the size of the flakes and how much water they
hold. The radar works by actively sending out microwaves on two
different frequencies. When the microwave pulses encounter a raindrop or
snowflake, it reflects part of both pulses back to the radar’s sensors.
By timing the interval between when the pulse was sent and then
received the radar knows how far away the particles are in the cloud.
up all the particles and you get a full picture of all the rain and
snow in one weather event. “It’s like a CAT-scan,” says
Skofronick-Jackson. “You can actually see layer by layer what’s in the
Atmospheric layer cake with marble swirl frosting
atmosphere does not lend itself to easy understanding. Temperature,
humidity and winds change with altitude, creating many layers of air
with different properties. The topography of land surfaces and oceans
affect global weather patterns. Those conditions then combine to create
both short-term weather and long-term climate. Part of resolving that
picture at high latitudes is to understand snow’s dynamics inside
This image of falling snowflakes was taken by the Snow Video Imager (SVI) at one of the auxiliary ground sites, the Steamshow Fairgrounds, 5 miles (8 km) south of the main CARE site, during a light snowfall on Saturday, Jan. 21, 2012. The SVI is set up about two feet off the ground and the snowflakes are falling from top to bottom through the frame. They can be seen here in different three-dimensional orientations at 5x magnification. Credit: NASA
know most clouds don’t have just one classic snowflake shape, but what
we don’t know is what are the mixtures of snowflake types,” says
combining the broader radiometer measurements that distinguish liquid
from ice and tell how much water the clouds hold with the vertical
details provided by the radar, the GPM science team may be able to see
what mixture of particles are falling to the ground, or if a snowflake
makes it all the way to the ground at all.
they’re falling, snowflakes will sometimes go through warm layers and
start to melt and start to look like raindrops,” says
Skofronick-Jackson. “So do you call that rain or do you call that snow?”
Finding that line is one of the goals of the cold-season experiment.
How many ways can you measure a snowflake?
GPM GCPEx field mission is currently underway just north of Toronto,
Canada in Egbert, Ontario. Located near Lake Huron, the region is prone
to both lake effect snow squalls and widespread snowstorms. NASA is
working with Environment Canada to measure snow as many ways as possible
to match snow on the ground with snow in the clouds and with simulated
satellite passes measured from aircraft flying overhead.
on the ground at the Center for Atmospheric Research Experiments
measure the quantity of snow, how fast it falls and how much water it
holds. Radar and radiometers on the ground also get an up-close look at
the snow as it falls from clouds to the surface. Meanwhile, two research
planes, the University of North Dakota’s Citation and the Canadian
National Research Council Convair 580, fly though the clouds measuring
snowflake sizes and water content, temperature and cloud water.
“They’ll do spirals so you can see all the way from the top of the cloud to the bottom of the cloud,” says Skofronick-Jackson.
the clouds at 33,000 feet, a third plane, NASA Dryden’s airborne
laboratory DC-8, carries NASA Goddard-developed Conical Scanning
Millimeter-wave Imaging Radiometer (CoSMIR) radiometer and NASA’s Jet
Propulsion Laboratory-developed Airborne Precipitation Radar-2 (APR-2).
Together these two instruments simulate the instruments that the GPM
satellite will carry into orbit.
datasets will complement current measurements made by radiometers on
Earth-observing satellites Aqua and Soumi NPP and the Cloud Profiling
Radar on CloudSat.
we can do with all these measurements is learn these relationships
between what the radar and the radiometer sees, what’s in the cloud, and
what’s falling out,” says Skofronick-Jackson.
The GCPEx campaign, running from Jan. 17 through Feb. 29, is on its way to filling in that picture.
Conversations With Goddard – Gail Skofronick-Jackson