On a lazy Sunday morning, I asked my Twitter followers what is something they would like to see me write about in this space. Andrew Hatter suggested that I do something on dual-polarization radar. As a perused my previous articles over the past 5 years, I really had not written much on it. That’s my bad, y’all. Dual-polarization is a critical tool of the National Weather Service and is being used to protect us every day.
First, I have to give a little bit of “weather radar 101.” Many of us that teach about weather radar often mention Radio Detection and Ranging (RADAR) as a by-product of the second World War. During that time, it became very clear that pesky “precipitation related” noise on radar was hampering the intended military needs of the radar. However, the National Weather Service website points out that, “the fundamental principle underlying all radars was first observed in 1886 by the physicist Heinrich Hertz when he found that electromagnetic waves could be reflected from various objects, and even focused into beams by appropriate reflectors.” For a deep dive into the history of weather radar, visit this link.
At this point, let me fast forward to the 1950s. The WSR-57 radar system was ushered into service by the United States Weather Bureau and the U.S. Navy. This was an S-band (~10 cm wavelength) system that was the backbone of U.S. radar weather monitoring for over 30 years. It evolved from an experimental X-band system. In the 1970s, the WSR-74 system came on line. At this point, some readers may be saying, “What in the world are you talking about Dr. Shepherd with all of this alphabet soup talk?
Weather radars send a pulse of microwave energy into the clouds. Some of that energy is backscattered to the radar from the volume of scatters (raindrops, hailstones, insects, and so forth). The various frequencies often used in radar meteorology are presented in the graphic above. For example, the S-band frequencies are less susceptible to attenuation than lower wavelength bands and have extended range. If you have satellite television, you know what attenuation is when it rains heavily and your signal is lost.
Around the time I was heading off to college in the late 1980s, the National Weather Service, Department of Defense, and other partners upgraded the U.S. weather radar network to the WSR-88D “S” band system. This was a part of the National Weather Service modernization effort in the early 1990s. These radars introduced the vital life-saving capability of Doppler shifted phases. Since we know the phase (shape, position, form, frequency) that the radar beam was transmitted, if scatterers (raindrops, etc.) are moving they will shift the phase of the returning radar echo. From that information, we can determine velocity towards or away from the radar (radial velocity not true wind velocity by the way). This Doppler effect also explains why the pitch of the horn of train sounds different as it approaches and moves away from you. With such information, meteorologists are now able to detect rotation in storms that could be indicative of a tornado. In fact, most tornado warnings today are based on Doppler radar identification. My master’s degree work involved the use of early WSR-88D radar data to track and locate landfalling hurricanes.
Ok, we finally made it to dual-polarization. According to NOAA website, “Dual-polarimetric radar transmits and receives pulses in both a horizontal and vertical orientation.” This means the returned echoes contain information in both orientations about the raindrops, snowflakes, sleet, and hailstones. Better information about shape, size, and composition of the targets allows for improved discrimination of precipitation type (for example, Is it heavy rain or hail? or Is it snow or freezing rain?) There are even products that can be derived from dual-polarization radar that can help identify debris associated with a tornado in real-time and improve rainfall estimation.
The National Weather Service began the transition to dual-polarization radar in the early 2010s. According to the National Weather Service, the first upgraded “dual-pol” system (NWS WSR-88DP) at Vance Air Force Base near Enid, Oklahoma. Since this installation in 2011, the National Weather Service has upgraded over 150 operational radars across the U.S. and its territories.
The image above is a good example of how dual-polarization products (correlation coefficient (CC) and differential reflectivity (ZDR) complemented standard reflectivity products and Doppler rotation patterns (SRM) to confirm that the radar was sampling tornado debris being thrown into the air. The image below illustrates how dual-polarization radar helped forecasters better pinpoint the location of rainfall versus snowfall in New York.
For more on how dual-polarization is used in weather forecasting, visit this NOAA National Severe Storms Laboratory website.