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| Dusan Zrnic. Image Credit: NSSL. |
By Bob Henson
The path to polarization
After years of development, the concept of polarizing radar signals
for meteorology took root in the fertile soil of Canada’s Prairie
provinces. NSSL’s Richard Doviak and Dusan Zrnić traveled to Alberta in
1979 to check out a circularly polarized radar pioneered by McGill
University. “Dick had been interested in doing polarization research as
early as 1971, but NSSL was deeply immersed in Doppler work at the
time,” recalls Zrnić.
Once the lab decided to build its own polarized radar, it went for a
dual-pol approach, with signals oriented in the horizontal or vertical
rather than circularly. “We made this choice for good reason,” says
Zrnić. Research by Thomas Seliga and Viswanathan Bringi, then both at
Ohio State University, had shown how signals from the two orientations
might yield critical data on the character of precipitation.
This hypothesis was confirmed through measurements in a 1977 Oklahoma field project using the CHILL
dual-pol radar under the leadership of Gene Mueller. Now based at CSU,
CHILL—the first university-based dual-pol radar—was named after Chicago,
Illinois, where it was launched by the University of Chicago and the
Illinois State Water Survey. NCAR was another dual-pol pioneer,
converting its CP-2 radar and creating the first real-time displays of
differential reflectivity (relating horizontal to vertical returns).
Starting in the mid-1980s, Zrnić teamed with postdoctoral fellows
Mangalore Sachidananda (now at the Indian Institute of Technology) and
Narasimha Balakrishnan (now at the Indian Institute of Science) to work
out the details of distinguishing rain from hail and other hydrometeors
using dual-pol data, with contributions from Jerry Straka (University of
Oklahoma). By 1996, Zrnić had written a paper for the Bulletin of the American Meteorological Society pondering the eventual role of dual-pol in operational settings.
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| Jothiram Vivekanandan. Image Credit: NCAR. |
While it would take years to iron out the wrinkles of dual-pol for
everyday use, the technology quickly made its way into field studies. In
the mid-1980s, NCAR converted one of its existing cloud-physics radars
to dual-pol, but scientists found that its usefulness for collaborative
work was limited.
“Unless you have a good engineering background, you can’t interpret
the raw data very easily. It was not that popular,” says NCAR’s Jothiram
Vivekanandan. A breakthrough came in the early 1990s, when NSSL devised
a technique for obtaining the major dual-pol products in real time. CSU
and NCAR followed suit, which paved the way for more widespread use of
dual-pol among researchers. Further progress came in 1998, when Anthony
Holt (University of Essex) convinced CSU to run both of the CHILL
transmitters—horizontal and vertical—at the same time, rather than
alternating between the two modes. This simultaneous technique has been
incorporated into the NWS dual-pol upgrade, where it meshes well with
hardware requirements and existing software.
Across the globe with dual-pol
Dual-pol’s appeal to university scientists got a major boost with NCAR’s S-Pol,
an NSF-funded community resource that debuted in 1996. This radar’s
ingenious design allowed the entire mechanism to be packed into four
shipping containers (seatainers); these can be arranged in a matter of
days to form a pedestal for the radar with minimal impact on the
landscape. “No one wants to have a big concrete pad in their backyard,”
says Vivekanandan.
Managed by NCAR’s Tammy Weckwerth, S-Pol has played a lead role in 18
field projects around the world over the past 15 years (see photos). A
high-frequency, dual-pol Ka-band radar appended to the unit transforms
S-Pol into S-PolKa. This hybrid setup can detect fine-scale cloud droplets and gauge humidity and liquid water amounts.
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| NCAR's S-Pol. Image Credit: Scott Ellis, NCAR. |
CSU’s Steven Rutledge has used S-Pol on four different projects,
including the radar’s first campaign in 1997. “S-Pol has provided
high-quality data from all of these projects, allowing my students,
staff, and me to do some very interesting research,” he says. “Its high
degree of portability and high data quality, and the excellent staff
that come along with the radar, are real pluses.”
Two S-Pol deployments near mountain ranges—the Alps in 1999 and the
Oregon Cascades in 2001—helped Robert Houze (University of Washington)
examine how mountain barriers influence fronts. S-Pol tracked the
evolution of ice particles created as air is forced up a mountain range,
with the particles eventually melting out as rain. At both sites, the
polarimetric and wind data revealed that this process boosted
precipitation in both stable and unstable air, but through different
mechanisms for the two kinds of air masses.
“S-Pol has been groundbreaking in this work,” says Houze. Later this
year, he and Rutledge will be taking S-Pol to the Maldives to study how
tropical clouds impact large-scale climate variations emanating from the
tropics. The project is called DYNAMO (Dynamics of the Madden-Julian Oscillation).
While putting S-Pol on the road, NCAR has also teamed with NSSL on
various improvements to the software that processes dual-pol signals.
These innovations include a phase-coding algorithm that allows radars to
capture usable signals from as far as 260 kilometers (160 miles),
nearly twice as far as before.
Also at NCAR, John Hubbert led a team effort to better identify
ground clutter—unwanted radar signals from buildings and other fixed
objects—and distinguish it from weather data. The software was added to
current NWS radars in 2009, and a dual-pol version will accompany the
NWS upgrade.
On the drawing board is a plan to cooperatively operate S-PolKa and
CHILL with another CSU radar (CSU-Pawnee) and the Denver and Cheyenne
NWS radars. The result, dubbed FRONT, would be a five-radar powerhouse
for field programs and instrument testing. “We envision the addition of
multiple and diverse instruments within FRONT to help sample
more components of the atmosphere, so that it won’t be only a radar
network,” says Weckwerth.
Stocking the forecaster’s toolbox
Researchers at NSSL and elsewhere are now racing to develop
algorithms that will help forecasters get the most out of dual-pol.
Among those on the task are OU’s Alexander Ryzhkov and graduate student
Matthew Kumjian. Along with former OU grad student Scott Giangrande (now
at Brookhaven National Laboratory), they’ve published a variety of
recent papers on what dual-pol can do. One technique they developed for
classifying various types of precipitation has yielded rainfall
estimates up to twice as accurate as those derived from the current NWS
Doppler radars (Next Generation Radar, or NEXRAD, installed in the
1990s). “That’s the business case for dual-pol,” says Schlatter. “There
are some kinks to be worked out, but everyone is very confident that
quantitative precipitation estimates will be vastly improved.”
Dual-pol may also unlock some of the secrets of severe thunderstorms.
Kumjian has found distinct signatures associated with downdrafts near
the front and rear of a storm, which in turn shed light on the amount of
rotation being swept into potentially tornadic circulations. He’s also
working with colleagues on techniques that will help forecasters quickly
distinguish small hail from large and identify areas where snow is
melting or sleet is forming. “Observing these signatures should help
boost the confidence of operational meteorologists making warning
decisions,” says Kumjian.
Postscript: I was in Oklahoma City at the NBC affiliate (1977-82) at the time that Doppler radar was being tested in was later became known as NEXRAD. It is truly amazing how much research and work has gone into developing this technology. I can say that the people working in this environment understand that their work is pioneering and will ultimately save lives.
