Snow, sleet and freezing rain can all blend together in what is commonly referred to as a "wintry mix." New radar technology at the National Weather Service is helping forecasters detect exactly which form of winter precipitation is occurring during storms this winter.
"A few years ago, our radar was upgraded with dual-pol technology," said Hayden Frank, a senior forecaster with the National Weather Service in Taunton, Mass. "The main idea is it can detect different sizes and shapes within the [precipitation] particle."
Installed in early 2012, forecasters have now had one full winter using the dual-pol technology on their WSR-88D radar in Taunton. Learning the relationships between these different precipitation particle shapes can be a valuable tool in determining exactly where snow is transitioning to rain, and vice versa.
"With just conventional Doppler radar, we were never ever able to differentiate except from reports," said Frank. "Now we can actually see it on the radar."
Because dual means two, the dual-pol radar now sends out signals in two planes, one oriented horizontally and one vertically. This creates a better cross-section analysis of each precipitation particle.
Wet snow, dry snow, sleet, hail, raindrops and even insects all have a unique shape and size. The dual-pol radar can give the forecaster insights as to what type of precipitation is occurring, and areas where more than one may be occurring.
"We have to dissect it because they are just algorithms and their not necessarily correct, but knowing the different particles and seeing the shapes and sizes are different, allows us to detect what type of precipitation is falling."
For example, a regular snowflake may produce a different shape and size than a partially melting snowflake. When a dual-pol radar senses a region with varying shapes, a variable called the correlation coefficient changes, and produces a complex image on the map for the forecaster to interpret.
"Where you have these uniform colors, it's all snow," Frank explains while showing a sample radar image. "Where you have these greenish colors, you have what we call the melting layer. There was rain mixed with snow, or just transitioning to just plain rain."
Conventional radars shown on television reports, websites and weather apps typically smooth out a wintry mix into a shade of pink or purple. The benefits of the complex dual-pol technology is not shown to the viewer, but remains a valuable behind-the-scenes tool for meteorologists to assess storms in western Massachusetts this winter.
"The technology has really improved, and it's helping us issue better forecasts," said Frank.
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