The next phase of remote sensing

There’s still plenty of life coursing through the dual beams of the airborne Electra Doppler Radar. But when ELDORA goes to work, Wen-Chau Lee spots the telltale signs of middle age. For instance, a motor failure caused ELDORA’s antenna and rotodome to slow down midway through a flight sampling cirrus clouds in Florida last summer.

NCAR’s Wen-Chau Lee, one of the principal investigators for the Electra Doppler Radar.

“ELDORA is in its eleventh year now,” he notes, “so we’re facing a few issues.”

Lee expects the airborne radar to remain a linchpin of field campaigns for years to come. By the 2010s, though, it will have reached its expected lifespan of 20 years. Because much of its hardware was custom-built by NCAR and the French government in the late 1980s, spare parts are virtually nonexistent.

The status of ELDORA, and how it might eventually be succeeded, were among the questions posed to remote-sensing specialists at a meeting hosted by NCAR on 23–25 April. The goal was to help NCAR’s Atmospheric Technology Division serve the hundreds of researchers who make use of ATD remote-sensing facilities allocated through a community-based panel.

Organizers were pleasantly surprised at the turnout—an “overwhelming response,” says Lee. “We basically gave the community two months’ notice. Originally we’d anticipated about 40 people.” Instead, more than 130 registered, including about 35 from outside Colorado.

Keeping ELDORA up to date is a matter of keen interest to many. It’s one of only three airborne, dual-Doppler radars in the weather research community. The others, aboard two NOAA P-3s, are used for hurricane hunting as well as research. Among this group, ELDORA’s unique aspects include its ability to detect returns from clear air and its 300-meter (1000-foot) resolution.

Built for the NCAR/NSF Electra, ELDORA debuted in 1993. After the Electra retired in 2001, engineers and technicians grafted the radar onto a P-3 operated by the Naval Research Laboratory. From its new home, ELDORA has successfully served three field experiments, including this year’s Bow Echo and MCV Experiment (see the spring Quarterly for details).

Thanks to the stamina of the NRL P-3, the radar can now sample phenomena of interest for over nine hours—“fifty percent longer than our typical mission on the Electra,” says Lee. “On almost every flight on BAMEX, we’ve used the full duration. Scientists are eager to use this new capability to study hurricanes and their genesis in the Atlantic and the eastern Pacific.”

The Navy’s contract to host ELDORA is due for a five-year renewal early in 2006. By 2010, though, “the P-3 will be aging,” notes Lee, and the community will need to decide how to proceed. Among the options discussed at the workshop for ELDORA and its potential successors:

• Adding a lidar to sense water vapor. Results from the International H20 Project, held in 2002, show that with the help of water-vapor data “you learn a lot more from the dual-Doppler wind field.”

• Providing real-time, quality- controlled, dual-Doppler wind data, so that researchers can observe storm dynamics during flight and adjust course accordingly. Attendees from France’s National Center for Scientific Research and NOAA showed some promising data-processing techniques to automatically remove aircraft motion and mitigate ground-clutter signals.

• Improving the data-processing software, an issue that spanned all of the workshop groups. “Most scientist-written software does not conform to programming industry standards,” said Lee at the workshop. He stressed the need for portable, user-friendly code that can handle multiple instruments in a modular format. Such code would allow for collective effort on tasks such as producing the real-time dual-Doppler wind fields discussed above.

How hard is it raining? That basic question is still a tough one for meteorologists to answer. Ground-based radar is now the tool of choice for real-time estimates of rainfall rates. However, there is no gold standard for the multistep process of converting raw radar data into rainfall accumulations, nor any widely accepted techniques for determining local or system-wide errors.

“Basically, we provide rainfall estimates to users, but without an error bar associated with them,” says Frederic Fabry (McGill University). At the workshop, Fabry urged the community to address this issue for the dual- polarization radars now being refined at NCAR, NOAA, and elsewhere. “As long as errors remain uncharacterized,” he said, “radar will remain a semi-quantitative tool.” One user pointed out that a blend of the rainfall-estimate algorithms developed at NCAR and the National Severe Storms Laboratory could give better results than either algorithm on its own.

On the hardware front, NCAR is in the home stretch of a major upgrade to S-Pol, ATD’s portable, shippable community radar. Technicians are adding a second transmitter/receiver duo, this one in the millimeter-wavelength range compared to the original wavelength of 10 centimeters. The second wavelength should yield finer-scale detail on the distribution of cloud droplets and ice particles, says NCAR’s Jothiram Vivekanandan. According to modeling studies, he says, “a combination of the two wavelengths should be useful for studying cloud droplet spectra and their evolution to a raindrop spectrum.”

Like other dual-polarization radars, S-Pol can distinguish rain from hail—a task made easier by target-recognition software developed at NCAR. “One of the merits of this technique,” says Vivekanandan, “is that a forecast meteorologist may not be required to know the intricacies of interpreting polarimetric radar data.” The procedure is now being put to use in Montreal, home of the first dual-polarization radar being used in North America to support public weather warnings, according to Fabry.

The workshop also included sessions on wind profilers. Despite their widespread use in both research and prediction, profilers have suffered at the budget-cutting table, most recently in Washington’s move to zero out funding for NOAA’s National Profiler Network. At press time, efforts to rescind the cut were under way in Congress.

“No institution has a funded mandate to pursue [profiler] development,” said Kenneth Gage (NOAA Aeronomy Laboratory) at the workshop. “Some of it’s being done in the private sector, and that’s very laudable . . . but the effort is very diffuse.”

The potential “show-stopper” for profiler research, said Gage and others, is that forecast models can’t easily incorporate profiler data to get a better outlook. Modelers, he said, “don’t feel the profilers add a lot of value to a numerical forecast . . . If models can’t assimilate the data, then I think it’s a serious problem.” Rapidly updated mesoscale models show potential for incorporating profiler data more easily, he added.

Like human forecasters, profilers may be most valuable when the weather is at its most extreme. “Forecasters think very highly of the profilers,” says Gage. Experts at the meeting recommended more work on proving profilers’ utility in these worst-case situations, said NCAR’s Stephen Cohn. “That’s when profilers have the most impact and the most benefit.”
While modelers are still coming to terms with wind profilers, researchers have embraced them. NCAR’s Integrated Sounding Systems (ISS), consisting of radiosondes, a surface weather station, a profiler, and an acoustic sounder, has been used in more than 40 field projects since 1992.

Workshop participants discussed three areas for potential ISS upgrades: improving the time and height resolution of the wind profiler, adding a sodar (sound-based radar) to fill in a profiler blind spot below about 150 meters (500 feet), and providing a quick-to-deploy ISS. “I’d like to see a truly mobile ISS,” said Howard Bluestein (University of Oklahoma). He’s working with ATD engineers this summer to test a new version with a 15-minute set-up time.

Some of the ideas that percolated at the workshop may get refined in Seattle this August when the American Meteorological Society holds its 31st Conference on Radar Meteorology, organized by Lee and Bradley Smull (NOAA). For the first time, the meeting will be collocated with the 32nd AMS Conference on Broadcast Meteorology. Joint sessions on nowcasting and severe weather will allow speakers from both conferences to share their experiences from different perspectives. Lee hopes to build a dialogue between radar specialists and TV meteorologists, the latter being some of the most avid and visible users of weather radar.

The next phase of remote sensing

NCAR-hosted workshop compiles a vision for the future

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