COLUMBIA — In 1992, St. Louis became one of the first 10 National Weather Service offices in the country to install the then-revolutionary Doppler radar system. Two decades later, the Doppler radar in St. Louis is undergoing its first significant update since it was installed.
Later this month, the Doppler radar at St. Louis that covers the Columbia area will undergo an upgrade to incorporate a new technology called dual-polarization radar. As a result, radar data from the Weather Service in St. Louis will be unavailable for two weeks beginning Feb. 27.
By providing the capability to identify the shape of precipitation particles in the atmosphere, the new technology will improve the ability of forecasters to differentiate between each type of precipitation falling, as well as the rate and volume. It can also make it easier to confirm tornadoes on the ground.
The key difference between the current single-polarization radar and the new dual-pol radar involves the orientation of the waves emitted from the radar.
Doppler radar assesses atmospheric conditions by transmitting pulses into the atmosphere that rebound off precipitation particles such as rain or snow, sending back a return signal to be evaluated.
Current Doppler radar systems only transmit pulses in a horizontal orientation, which is useful for providing information about how wide particles are. The new dual-pol radar transmits two pulses—one horizontal, and one vertical -- that provide information for both the width and height of particles.
Neil Fox, associate professor of atmospheric science at MU’s School of Natural Resources, said each form of precipitation in the atmosphere has a unique shape. He said smaller raindrops are spherical, like balls, but large raindrops are "shaped more like burger buns, and the larger they are the flatter they get.”
“If we’re trying to tell the difference between heavy rain and hail, at the moment that’s quite difficult,” he said. “But with the dual-polarization, we can tell the difference between the large raindrops you get in heavy rain and spherical hailstones.”
Snow tends to be shaped like a classic snowflake—a flat, hexagonal shape that falls with the flat axis oriented horizontally. When meteorologists look at the return signal on the updated radar systems, they will be able to observe a large difference between the horizontal and vertical signal strengths, which will help differentiate between snow and rain.
With current Doppler radar systems, there are multiple techniques that can help determine what type of precipitation is falling, but each requires a significant amount of guesswork. Whether precipitation is falling as rain or snow is predicted based on whether temperatures are above or below freezing, but hail or sleet are much more difficult to identify with the current radar.
In general, the volume and rate of rainfall is determined by the return signals, known as reflectivity. Higher reflectivity values indicate more rain.
There are multiple equations forecasters use, however, to convert reflectivity to rainfall rate, Fox said, and that can lead to uncertainty.
“Dual-polarization really removes that uncertainty because it gives you a lot more information on how many large drops there are and how many small drops there are, so we can do a much better job of figuring out exactly how much rain there is,” he said.
The increased certainty about the volume and rate of rainfall is expected to have significant benefits in terms of flash flood prediction.
Dual-pol technology may also have benefits in terms of determining when tornadoes have touched ground. Current radar provides information about circulation in the atmosphere that can indicate a tornado, but dual-pol Doppler can confirm that a tornado is occurring because the debris that tornadoes lift into the air has a different shape than the rain or hail that accompanies tornadoes.
Jim Kramper, warning coordination meteorologist for the Weather Service in St. Louis, said dual-pol is expected to provide new benefits but is not revolutionary compared to the original Doppler's impact in the 1990’s.
“It’s like going from a very nice Ford and stepping up to a Lincoln—you’re getting more options with this model,” said Kramper.
The upgrades will be mostly internal. The radar displays that can be viewed on various Internet systems will not change for the most part. However, there may be more detail in certain situations, for example in cases of heavy snow or rain, Kramper said.
Local emergency managers who issue severe weather warnings will possibly reap the new system’s benefits as well.
“Hopefully, it will enable us to get warnings out quicker, and if we can get warnings out more ahead of time," emergency managers "will have more time to react," Kramper said. "As much lead time as we can give them, the better."
The full benefit of the new system, however, cannot be fully determined until forecasters and meteorologists are able to use the system in real-time.
The update in St. Louis is part of the National Weather Service’s plan for a Weather-Ready Nation. Every Weather Service radar site will receive the upgrade, which began in 2011, by the end of 2012. St. Louis, along with Kansas City, is within the first third of offices receiving the updates.