Ask the meteorologist: What is dual pole radar?

Question: I hear WRAL tout its dual-pole radar? What is that, and why is it a big deal?

Answer: What you hear us say is "dual pol," it is short for dual polarization (also known as polarimateric). This is why we refer to the WRAL radar as the Dual Doppler Radar.

Dual-pol radar sends and receives pulses in both the horizontal AND vertical orientation. A radar that is not a dual-pol only sends and receives pulses in the horizontal orientation.

In order for a radar to determine the size and shape of an object in the air, it MUST have the capability to send and receive pulses in both the horizontal and vertical orientation. With dual-pol, we can get the size, shape and variety of objects.

This can give us a very high confidence, for example, as to whether an area in a thunderstorm is composed of heavy rain or hail, and can also give us a reasonably good estimate as to whether the precipitation particles within the beam are principally rain, wet snow, dry snow, snow pellets (also called graupel) or whether the echoes are most likely non-meteorological targets such as birds, bugs, or in extreme cases, tornado debris.

A dual-pol radar can "see" the different shapes of hydrometeors (raindrops, hailstones, snow flakes, etc.). The classic example of this is the raindrop, which when very small is almost a perfect sphere that scatters similar amounts of energy at either polarization, versus very large raindrops, which take on an oblate shape (on average) that has been likened to that of a hamburger bun. This shape makes the large droplets more reflective of horizontally polarized radiation than vertically, so that a differential measurement of the two polarizations can easily distinguish between an area with predominantly small droplets and an area with much larger ones. More importantly, since large hailstones, even through they can have irregular, nonspherical shapes individually, tend to tumble as they fall, their average shape is much closer to spherical and therefore they can be discriminated from heavy rain that might produce a similar reflectivity signature on single polarization radar by the fact that, unlike the large raindrops, the horizontal and vertical polarization reflectivities are almost the same.

The National Weather Service recognizes these benefits of dula-pol radar:

• Improved accuracy of precipitation estimates, leading to better flash flood detection
• Ability to discern between heavy rain, hail, snow and sleet
• Improved detection of non-meteorological echoes (e.g. ground clutter, chaff, anomalous propagation, birds and tornado debris)
• Detection of aircraft icing conditions
• Identification of the melting layer (e.g. bright band)

You can see how this level of precision plays into our ability to more specifically forecast weather on the way to the Triangle and hlep you understand the conditions as they happen to keep you safe.