Throughout the 1990s, the National Weather Service (NWS) began deploying a network of 160 Doppler weather radars across the United States and its territories. NEXRAD, or Next-Generation Radar, would provide meteorologists a wealth of weather information that was either absent from or more poorly resolved in the previous generation of non-Doppler radar technology. This included data on velocity, which especially aids meteorologists to see the speed and movement of droplets and other targets within thunderstorms. Velocity data also greatly improves detection of rotation within thunderstorms, an important precursor to tornado formation. NEXRAD also has greater spatial resolution and sensitivity than previous radars, enabling such phenomena as the passage of cold fronts and thunderstorm gust fronts to be resolved. Recently, another upgrade of the existing NEXRAD network to include new dual polarization technology (dual pol) has been completed. This technology looks to provide improvement in many areas, but for this blog, one important improvement is in discerning different types of precipitation.
How Conventional Radar Works - Source: Terry Schuur (NSSL)
At the basic level, all radar works by sending out high frequency radio waves, where as FM radio stations broadcast on the order of 100 MHz or 100 million cycles per second, the frequencies typically used in weather radars are on the order of 3000 MHz. When these high frequency radio waves sent out by the radar encounter an object, such as a water droplet, insect, or even wind farm, they scatter or reflect back some portion of the sent out radio signal back to the radar dish. Based on the amount of time it took for the reflected signal to return to the receiving dish, and the direction from whence it came, the location of this water droplet or other object can then be determined in relation to the radar site. In a nutshell, this is how the weather radar maps that most people are used to seeing are created.
What differs with the new dual pol radar is that instead of just vibrating (as all types of waves in the electromagnetic spectrum do) simply in the horizontal direction, the radio signal now alternatively vibrates in the horizontal and vertical directions. This is important because a water droplet and a hail stone will reflect back vertically polarized versus horizontally polarized radio signals in different ways. Thus, meteorologists are better able to distinguish the different types of objects (targets) based on this property. One example of this new ability to distinguish targets comes in the new radar product called Differential Reflectivity (ZDR). ZDR is basically the strength of the reflected horizontal radar signal minus the strength of the reflected vertical radar signal. Basically an object that is flatter (more like a hamburger bun) will have higher ZDR as they reflect the horizontal signal better, whereas round objects would tend to have a ZDR close to 0.
How Dual Polarization (ZDR) Works - Source: NWS
In a strong thunderstorm, using conventional "horizontal-only" radar it can sometimes be difficult to distinguish large rain drops from hail stones as these two objects can sometimes reflect horizontal radar waves in a similar manner. However, employing both horizontal and vertical radar waves and measuring how much each type of signal is reflected off of each object can help meteorologists better distinguish large rain drops versus hail. Large rain drops, while they are falling are resisted by the air they are falling through, thus they tend to form a flatter, hamburger bun-like shape, while hail as it tumbles around in a thunderstorm tends to form a more spherical shape. Thus, a ZDR display is better able to distinguish between large rain drops and hail in thunderstorms.
Of course there are many other areas where dual polarization radar can provide improvements, such as in detecting non-weather targets like insects or other ground clutter like wind farms, or in discerning snow versus rain, or even picking out the debris that a tornado picks up off the ground. However, one of the most basic of dual pol radar's new products, ZDR, is representative of the types of new data that meteorologists can glean from this update to NEXRAD.
...For more on the Dual Polarization upgrade check out these links: