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NCAR's S-Pol radar proves fieldworthy

by Anatta
UCAR Communications

NCAR's portable dual-polarization S-Pol radar lived up to expectations this past summer by distinguishing between rain and hail, something the National Weather Service's nationawide network of single-polarization radars (WSR-88D, also known as NEXRAD) is unable to do. On 12 July, heavy rains flooded Buffalo Creek in the mountains near Denver, while at the same time hail pelted the Colorado plains. The S-Pol, being field tested at Front Range Airport just east of Denver, clearly distinguished between the shapes of the raindrops swelling Buffalo Creek and those of the hailstones pounding the eastern prairie.

"Measuring heavy rains accurately is important for anticipating flash floods," explains NCAR scientist Jim Wilson (Atmospheric Technology Division), who heads the S-Pol precipitation experiment. "Hail can fool the WSR-88D into 'thinking' it's raining harder than it actually is, thereby introducing uncertainty into the issuance of flash flood warnings." In fact, the Buffalo Creek storm did produce a flash flood that washed away a road and left two people dead.

NCAR's dual-polarimetric radar, S-Pol. (Photo by Carlye Calvin)

S-Pol's signal is polarized into alternating horizontal and vertical orientations. Small raindrops tend to be round, so their reflection is similar in both orientations; hailstones may be irregular in shape, but since they tumble as they fall they average out to a round shape as well. Large raindrops, on the other hand, flatten into toroid shapes that reflect more of the horizontally polarized signals than the vertically polarized ones.

To quantify the advantages of dual over single polarization, NCAR is planning a series of experiments over the next few years in various seasons and locations around the United States. Anticipating promising results from the current refinement of the now-15-year-old technology, scientists at the National Severe Storms Laboratory (NSSL) are already working on how to add it to the existing WSR-88D network, which was installed around the country in the early 1990s. Colorado State University and NSSL use dual-polarimetric radar data and are collaborating with NCAR on the S-Pol tests. CSU will deploy the S-Pol in early 1997 northeast of Denver.

S-Pol is NCAR's second dual-polarization radar. Its predecessor, CP-2, was expensive to set up because it required the construction of a concrete pad at each new site. By contrast, S-Pol can be placed on a base of four seatainers--the same ones it's shipped in--for assembly at any stable, accessible site in the world. The 28-foot aluminum dish can withstand winds up to 50 miles an hour and can be covered with a radome in more severe weather. An improved antenna provides more accurate measurements than CP-2 did, and a new data processor uses modern digital technology.

For information on deploying the radar, contact Jeff Keeler, ATD Remote Sensing Facility (303-497-2031 or keeler@ucar.edu).

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Last revised: Tue Apr 4 10:18:15 MDT 2000