Combined radars may boost weather prediction

System with short-, long-wavelength radars looks to improve accuracy

By Todd Neff, Camera Staff Writer
March 26, 2004

Like the wren on the back of Aesop's eagle, a 30-inch disc coupled to
a 30-foot radar dish may help weather prediction reach new heights.

The boost in accuracy provided by combining the two radars could be
the key. And of interest to the Federal Aviation Administration and
others is how the radar could help small aircraft avoid weather that
leads to ice-related crashes.

A team of eight Boulder scientists and engineers with the National
Center for Atmospheric Research have since February tested a system
combining short- and long-wavelength radars to see inside
clouds. Together, the two radars of the "S-Polka" system can detect
everything from baseball-sized hail to ice or water particles
one-hundredth of a millimeter in diameter.

On Thursday, the bright white radars waited under a cloudless sky at
NCAR's Marshall Field Test Site atop Marshall Mesa near Superior. The
big dish -- the same type as the Nexrad weather radars used by the
National Weather Service and local TV stations -- beams and receives
polarized S-band radar, or S-pol. It emits 10-centimeter radio waves
used to discern airborne liquid down to about the size of a fine
drizzle. With smaller particles, S-pol radar knows something's there,
but can't tell exactly what it is.

The smaller radar operates in Ka-band, the same frequency used by
satellite television providers. With wavelengths of about a
centimeter, it can sense the finest mist or ice crystals. The Federal
Aviation Administration and the National Science Foundation have each
contributed $300,000 to developing the radar, NCAR scientist and
project-team leader Marcia Politovich said.

Putting them together has been the main challenge, said Jothiram
Vivekanandan, the lead scientist on the project. NCAR scientists tried
it with limited success in the early 1990s, Vivekanandan said. Because
of their differing bandwidths, the radars need to be painstakingly
calibrated to work together.

"It's an extremely complex problem," and one that could take years to
master, Vivekanandan said.

The first round of testing, which began in February, is winding up
next week. Sunny spring weather hasn't helped the team, which is
looking for storms, Politovich said.

On May 1, the project's array of white shipping containers will be
loaded onto trucks and taken to Mexico's Baja California coast, where
scientists will test the radar on the North Atlantic Monsoon.

Charles Ryerson, a scientist with the Army Corps of Engineers' Cold
Regions Research and Engineering Lab in New Hampshire, said the NCAR
team's use of existing radar technologies was a clever approach.

"If you add Ka-band capability, you can detect information about icing
you can't do now, but with essentially existing systems with just a
small add-on," he said.

Politovich said ground-based dual-band radar could be in use in
aviation within the decade. But it could take longer to convince
weather forecasters to use it.

Vivekanandan said it took 30 years for weather services to upgrade to
the current S-pol radar systems. Unlike previous systems S-pol can
tell hail from snow and, with the addition of Doppler, can determine a
storm system's velocity.

Local National Weather Service Meteorologist Kyle Fredin said current
radar works well, but dual-band radar could help improve local
forecasts. As an example, Fredin said, forecasters knew the upslope
flow of storms last month would make conditions ripe for snowfall. But
they couldn't discern the differing water content in clouds to
understand how much snow might fall where.

With dual-band radar, he said, meteorologists could combine an exact
understanding of cloud moisture with temperature and wind information
to provide more detailed precipitation forecasts.


Contact Staff Writer Todd Neff at (303) 473-1327 or nefft@dailycamera.com.