What is Dual-Pol?
A Special Gust Blog by Greg Schoor
Radar Program Leader-NWS Baltimore-Washington
A major upgrade is being installed in all National Weather Service (NWS) radars across the nation. Dual-Polarization technology basically takes each sweep of the radar from 2-D, to 3-D. Before Dual-Pol, a radar beam was transmitted only in the horizontal plane, so the beam could only receive data about falling precipitation in one direction. With February’s upgrade to our local NWS radar, forecasters can now get information about precipitation not only in the horizontal but in the vertical as well, telling us much more about what we are looking at. Dual-Pol is a vital part of the Weather-Ready Nation effort by the NWS. Creating a Weather-Ready Nation requires providing the media, government and other key partners with the most accurate warnings and forecasts to protect lives and property.
What are the functions of Dual-Pol data compared to the conventional radar data?
Conventional radar products only allow forecasters to see the “brightness” of clouds and precipitation (raindrops, hail, snowflakes, etc.). The larger the raindrop or hailstone, the “shinier” it will appear to the radar. On a radar screen, the most reflective objects correspond to the more intense colors on the color wheel, red, pink, bright purple. Conversely, light precipitation will have the less alarming colors, light greens and blues, maybe even gray for the lightest drizzle or flurries.
The image below shows the difference between a beam that a conventional radar would emit, compared to one that is equipped with Dual-Polarization technology. Conventional radar beams only emit a frequency in the horizontal plane, while a dual-polarized beam sends and receives information in both the horizontal and vertical planes – creating a 3-dimensional view. The two images on the right show this in terms of individual precipitation forms, a snowflake, hailstone, and raindrop.
Dual-Pol offers the ability to dissect this “brightness” or “reflective” idea even further. Since the beam is now polarized, meaning that it sends out waves not only in the horizontal plane, but the vertical, we can now see the actual shapes of falling precipitation and how they change direction as they fall toward the surface. For the most part, raindrops fall straight without rotating or make too much random motion, while snowflakes are completely different. Snowflakes are flat and change direction constantly as they float toward the surface. Even hailstones, especially large hailstones, will “tumble” as they fall, rotating and changing direction because they are not perfectly round.
Dual-Pol products can show these characteristics in a number of ways to meteorologists. The radar can then take all of that information and make its best estimate of what type of precipitation is in the sky. In the lower right of the below Dual-Pol radar image is an example of summer thunderstorms over southern Pennsylvania and north central Maryland from the NWS Dual-Pol radar in Pittsburgh. Since the radar beam shoots out at an angle away from the ground, far away from the radar, the beam for the northern Maryland storms is high within the tops of the clouds. Up there, it is well below freezing and the radar can see the irregular shape of the ice crystals in the cloud – the blues. However, it can also tell the difference between those ice crystals and the red that signifies hail in the storm.
Finding Hail: In terms of severe storms, most of our typical summertime storms are pulse thunderstorms, lasting only 20-30 minutes on average. Unlike supercells (which we do see around here on occasion – i.e. La Plata, MD April 2001) which can last anywhere from an hour to several hours, pulse thunderstorms form within a few minutes, develop their core of hail and wind, and dissipate shortly thereafter. When these storms contain hail and damaging winds, NWS forecasters need to be able to quickly see it to help get the warning out before any of that hail or damaging wind reaches the ground.
Winter Precipitation: Dual-Pol technology gives meteorologists a new set of highly sophisticated tools to better diagnose and determine precipitation types at all levels of the lower atmosphere. Winter precipitation can be especially challenging, as the heavily populated I-95 corridor is often the transition zone between rain and snow. The zone of mixed precipitation in-between can be a relatively short distance and change quickly, so having tools that give forecasters better clues as to where the in-cloud melting/freezing layers are and how they are changing will be crucial to providing better service for our region for years to come.
Better rainfall estimates: Conventional radar can only rely on assigning rainfall rates to how reflective the precipitation is. This is an issue if hail is present within an area of rain – hail appears very bright like torrential rain, but causes no flooding. Dual-Pol radars automatically account for areas of rain that have hail mixed in and give a more realistic estimation for how much rain has fallen. This is crucial since flooding is the #1 weather killer. We need to have accurate rainfall estimation to provide the best possible flood warnings and help people get out of harm’s way.
More in Part 2 tomorrow