Dual-polarization Doppler radar essential to winter forecasting

While several factors affect forecasting wintry weather, in the past five years technology upgrades have really improved our ability to tell you what type of precipitation is falling.

Since we’re located in the central U.S., Missouri typically experiences all types of precipitation between November and March.

But outside conditions can be deceiving, especially if temperatures are very close to the freezing line or are changeable through the lower and middle levels of the atmosphere. Since the early 1990s, meteorologists have had next generation radar data available to detect precipitation and wind.

However, it is not necessarily reliable in showing what type of precipitation is falling.

That’s where new, dual-polarization Doppler radar comes in.

The new radar upgrades started nationwide across the country in 2011, and the University of Missouri just installed a dual-pol radar outside Columbia this past spring.

The one major difference between dual-pol and conventional radar are the pulses sent by the radar.

Conventional radar only sends out a horizontal pulse or beam, which only measures the horizontal dimensions of the target, limiting our ability to tell what the radar is seeing.

Dual-polarization radar sends out horizontal and vertical pulses that measure the size, shape, and variety of the targets they hit, allowing us to better determine the precipitation type.

Specifically for winter, dual-pol can detect whether heavy rain, snow, hail, or sleet is falling, and identify the rain/snow line.

One of the dual-pol products we use frequently during the winter is correlation coefficient.

In this example, a winter weather event is shown with a transition from rain to snow along the white line you see on your screen.

In the upper left corner, its hard to tell whether rain or snow is falling on conventional radar.

However, as shown in the upper right corner, correlation coefficient can help us out.

Dark red colors to the left of that white line indicate highly correlated or similarly shaped precipitation, meaning in that location, snow is falling.

To the right of the line, there is less correlation, indicating different size and shaped rain drops.

Later in this event, on the bottom left corner image, it’s still hard to tell whether rain or snow is falling on conventional radar.

Differential reflectivity, on the bottom right, shows a distinct difference.

On the left of the white line, you can see that the radar returns are similar in color, meaning similar shaped particles are falling, indicating snow. On the right, those values are less uniform, indicating different-sized rain drops.