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A new look at moisture

REFRACTT tests out radar technique on the Front Range


by Bob Henson

Wilson and Roberts

NCAR's Jim Wilson (left) and Rita Roberts discuss refractivity data. (Photos by Carlye Calvin.)

When TV viewers take in the bright reds and yellows that show heavy rain on a radar display, they're only seeing part of the picture. Pools of water vapor in the atmosphere's lowest kilometer help determine where and when heavy rain will develop. Yet water vapor can't be tracked by television meteorologists or by NOAA's National Weather Service (NWS) using operational radars as they're now configured.

A technological breakthrough could make the invisible water vapor visible to these radars over the next few years. To test the concept, four Doppler radars have been using refractivity data to track low-level moisture across northeast Colorado in unprecedented detail. Over the last two summers, the Refractivity Experiment For H2O Research And Collaborative operational Technology Transfer (REFRACTT) has depicted the pools of water vapor that slosh back and forth across the High Plains. The idea is to help pin down the locations where storms might rage a few minutes to a few hours later.

The broader goal is to help the NWS decide whether refractivity ought to be included in the next upgrade to the national NEXRAD network of Doppler radars. If the refractivity-based technique passes muster, the data it provides could benefit researchers as well as the hundreds of forecasters who monitor thunderstorms and warn the public about them.

Frederic Fabry (McGill University) hatched the scheme behind REFRACTT while he was a postdoctoral researcher in NCAR's Advanced Study Program during the mid-1990s. Fabry's idea was to measure how changes in atmospheric moisture and density affect the speed of radar signals. The changes in signal speed are subtle, so the technique requires a fixed target—such as a silo or a power line—in order to determine exactly how the signal speed varies across different weather conditions.

Frederic Fabry

Frederic Fabry.

"Much of the early work was spent developing a robust code to implement the technique," says Fabry. "Although the idea is simple in concept, extracting the information out of the fixed targets is quite a challenge. This is the main reason why few other researchers have ventured into this area, even though there has been considerable interest in it."

After years of work, Fabry and his colleagues at McGill and NCAR developed a solid set of algorithms to measure refractivity and extract moisture data from it. The result is a map much like any other radar display, but one that shows water vapor instead of blobs of rainfall or bundles of wind (see images at right).

NCAR tested the refractivity algorithms during field projects in 2000 and 2002 using the transportable Doppler radar S-Pol. This summer, in a broader test, REFRACTT researchers pulled data from S-Pol (stationed just south of Boulder) as well as other radars across the Front Range operated by Colorado State University, the University of Oklahoma, and the Denver-Boulder
NWS office.

"In 2005 we demonstrated that we could obtain refractivity fields from all the radars and get good results," explains NCAR's Rita Roberts, the lead principal investigator for REFRACTT. "This summer we've been running multiple radars, ‘mosaicing' the refractivity data in real time, and putting them in a display that forecasters can use." The data were cross-checked against observations from a variety of other tools, including radiometers, radiosondes, and ground-based sensors that intercept Global Positioning System signals and calculate moisture along slanted vertical paths.

This summer didn't yield quite as much severe weather as usual across northeast Colorado, but the REFRACTT scientists and local NWS forecasters were nonetheless intrigued by the moisture displays. During the torrid heat of early June, bright patches of crimson denoting areas of extremely low moisture flowed off the mountains on hot, dry west winds. In contrast, the monsoon of early July brought surges of green (indicating high moisture content) westward and upslope. These general patterns are well known, but the high-resolution details in the moisture displays went far beyond traditional data sources. Surface weather stations are separated by 50 km (30 mi) or more, and vertical moisture profiles produced by twice-a-day radiosonde launches are typically separated by hundreds of miles.


Stationed just south of Boulder at NCAR's Marshall Field Site, the S-Pol radar scans the skies during REFRACTT.

"Low-level moisture is the key to our weather here, especially during the summertime," says Larry Mooney, meteorologist in charge at the Denver NWS office. "We're really excited about the REFRACTT data. I think it's a great example of how you can move technology into the operational realm pretty quickly if you're committed to it."
Though forecasters and researchers alike are upbeat about REFRACTT, it remains to be seen if and how soon the technique becomes operational (which could take five years or more, says Roberts) and how the new data would be used in short-term weather prediction. "Refractivity is a complex field to interpret, and forecasters may not have the time to digest the information properly given how busy they are," says Fabry. "We may find that measuring refractivity is very useful for short-term forecasts, but that the information needs to be combined with other clues in such a way that can be done only by a numerical model."

Distance is another consideration. The lowest beam of a standard weather radar skims 0.5° above the horizon. Even at that low angle, Earth's curvature means that the beam overshoots even the tallest objects beyond about 50 km (30 mi). Thus, the refractivity technique can only provide data within a couple of counties of each radar. However, many NWS radars are positioned close to the urban areas where the majority of Americans work and play (and dodge storms).

The REFRACTT scientists will analyze the data in depth this autumn and make their pitch to the NWS. But the project is already a success in another sense. In order to make REFRACTT possible, Roberts and colleagues convinced the NWS to allow an external connection to its Level I radar data—the rawest and most detailed form. Until now, it's been difficult for researchers to access Level I data due to security concerns. The approval is a major step, says Roberts: "It's going to open up a lot of doors."

Meanwhile, the UK Met Office and the University of Reading are conducting a feasibility study on whether their operational weather radars might be able to acquire refractivity data. Initial tests are encouraging, says Malcolm Kitchen of the Met Office's Observing Methods Technology Centre. "The work on refractivity is one of the most exciting things we have done in recent years," says Kitchen. "Something about the possibility of our older radars operating way beyond their design specifications catches the imagination."

1July 2006

14June 2006
These two images show refractivity data for northeast Colorado as compiled for REFRACTT by one or more of the radars involved in the project. Cooler colors indicate greater amounts of water vapor in the lower atmosphere (roughly the lowest 3,000 feet).  The image on the left is from 1:54 p.m. CDT on 1July 2006, when moisture had pooled against the Front Range (green band).  At right is an image from 2:37 p.m. CDT on 14 June 2006, when hot, dry air was moving east from the foothills across the plains. Major roads and highways are visible as faint lines. (Click on images to enlarge)


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