|
+ Play
Audio
|
+ Download Audio | +
Email to a friend | + Join mailing list
March
2, 2007: People have lived with rain and snow for
millennia, and scientists have studied weather for more than
a century. You might think that, after all that time, we would
have precipitation pretty much figured out. And you'd be wrong.
 "It's
amazing how much we don't know about global patterns of rain
and snow," says Walt Petersen, an atmospheric scientist
with the National Space Science and Technology Center (NSSTC)
and the University of Alabama (UAH) in Huntsville.
For
instance, how much snow falls worldwide each day--and where?
How much water falls to Earth in the form of light, drizzly
rain?
"These
are just a few of the outstanding questions," he says.
Answering them would fill significant gaps in our understanding
of the Earth's climate system. What to do? "The best
way to study global precipitation is from space."
Right:
Artist's concept of a space-based precipitation radar. Credit:
Walter A. Petersen, NSSTC/UAH.
That's
why NASA recently funded a suite of 59 research proposals
under the agency's ongoing Precipitation Measurement Mission.
The studies will look at ways to improve measurements of rain
and snow from Earth orbit. Petersen is among the winners,
and one of the things he'll be studying is snow:
"Snow
is a huge problem," says Petersen. It turns out that
estimating snowfall is very hard to do with radar. Rain is
easier because it always consists of simple liquid-filled
droplets. Radar echoes from rain clouds can be converted into
rates of rainfall with fairly good precision. A radar onboard
NASA's Tropical Rainfall Measurement Mission (TRMM) satellite,
for instance, measures monthly rainfall within an accuracy
of about 10%.
But
frozen precipitation such as snow is much more variable. Famously,
no two snowflakes are alike. The differing sizes, shapes,
and densities of individual flakes mean they won't all fall
at the same speed, complicating efforts to estimate rates
of snowfall. Also, snowflakes have lots of crazy angles and
planar "surfaces," which can make tricky radar echoes.
Above:
Particles of snow on the ground in Canada. Photo credit: Walter
A. Petersen, NSSTC/UAH. [Larger
image]
The
problems don't end there. "Ice and snow have variable
dielectric behavior depending on how much ice and how much
air is contained in the particle," he adds. (Note: The
dielectric constant of a substance tells how strongly the
substance will interact with a radar wave.) "With raindrops,
you are dealing primarily with water, which has a known and
fixed dielectric constant. With snow, we know the dielectric
constant for pure ice and we know the dielectric constant
for air, but, the amounts of air and ice can vary quite a
bit from snowflake to snowflake. Further, snowflakes also
rime and melt. This means you can also have water on the surface--another
complication!"
For
these reasons, "our estimates of global snowfall are
very uncertain," Petersen says. This applies to both
ground- and space-based radars. Only in areas where snow depth
is routinely measured via "stick-in-the-ground"
methods do scientists have good estimates for the amount of
water that falls as snow. The problem is, "there are
relatively few of these measurement sites compared to the
large area that needs to be measured."
 Right:
Ground-based snow gauges. Photo credit: Gail Skofronick-Jackson,
NASA/GSFC. [Larger
image]
Snow
plays a big role in climate. When water evaporates, it carries
away a lot of heat (which is why sweat cools down your skin
as it evaporates). Later, when that moisture condenses inside
clouds to form snowflakes, it releases this stored heat, warming
the air. As more snow crystallizes, more heat is released, which
in turn makes wind. When the snow falls, it takes water out
of the atmosphere, leaving it drier. Snow on the ground also
reflects sunlight back into space, which helps cool the planet.
So learning to portray global snowfall correctly in computer
climate simulations is critical for accurately predicting how
the real climate will behave in the future.
Many
of the newly funded studies will develop ways to estimate
snowfall rates from radar data.
This
is timely because in 2013 NASA plans to launch a new radar
onboard the Global Precipitation Mission satellite (GPM).
GPM will extend TRMM's observations by looking at precipitation
beyond the tropics for the first time, orbiting at an angle
that will bring it almost to the Arctic Circle (65 degrees
latitude). At these higher latitudes, GPM will encounter lots
of snow.
Along with snow,
GPM will be able to detect lighter rainfall than TRMM can.
If less than about 1 millimeter of rain falls per hour, it
is invisible to TRMM. That's rarely an issue in the tropics,
but at higher latitudes, light, drizzly rain is common. Although
it's light, this rain can last for days, moving large amounts
of water and releasing lots of heat into the atmosphere.
In industrialized
nations with extensive rain-gauge networks, this light rainfall
is well documented. But in much of the world, light rain goes
unrecorded, leaving a large gap in our knowledge of global
water cycles. GPM will be able to sense rain down to about
2/10 mm per hour.
Heavy
rain, drizzle, snow—"it's all water," says Petersen.
"We've got to keep track of it in every form to truly
understand the climate of Earth."
SEND
THIS STORY TO A FRIEND
Authors: Patrick Barry,
Dr. Tony Phillips
| Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
|
