![]() The insert in the images shows the dual polarization correlation coefficient (CC) product denoting the location of debris produced by the tornado. Radar reflectivity (left) and radar velocity (right) around time of an EF1 tornado west of Marion, Wisconsin. The NWS currently operates 159 of these NEXRAD systems and they have undergone several upgrades during 20+ years of operation, including a further increase in resolution, and most recently, an upgrade to dual polarization technology, which provides forecasters a new wealth of information about precipitation including size and shape, better amount estimates, ability to see different types such as rain, snow, and hail, and the ability to differentiate from non-precipitation returns like birds, bugs, and even tornado debris! Also, the addition of Doppler velocities increased the ability for severe weather detection and warning by allowing forecasters to see wind fields and possible rotation within thunderstorms. ![]() It increased the resolution of the data, allowing forecasters to see storms in much finer detail. The NEXRAD System provided marked improvements for the NWS. After being developed and tested through the 1980s by the NOAA National Severe Storms Laboratory and partners, the first of these Next-Generation Radar systems (NEXRAD) were deployed operationally beginning in 1992. This allowed forecasters not just to see location and intensity of the precipitation along with basic storm movement, but the movement of the precipitation and winds within the storm itself. NWS Wilmington (OH) WSR-57 Radar image of supercells with hook echoes during 1974 Super Outbreak (courtesy of NCDC)Īround the same time period, researchers began developing a new generation of radars that would incorporate the use of Doppler radar. These radars provided similar data but with newer and more reliable components. An updated version, the WSR-74, supplemented and replaced the older radars beginning in 1977. ![]() The technology was refined and in 1959 the NWS began rolling out its first network of radars dedicated to a national warning network. Navy donated 25 surplus radars to the NWS (then known as the Weather Bureau), marking the start of a U.S. Investigation into this phenomenon resulted in the discovery that these echoes were returns from the precipitation, unmasking a further use for the technology. Analysts noted in periods of heavy weather, the radar would return strange signals. But the use of radar for weather observations occurred by accident. The concept of RAdio Detection and Ranging (Radar)began in the late 1800’s and by World War II, radar was in use by militaries around the world, scanning for incoming airplanes. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.Doppler radar sends the energy in pulses and listens for any returned signal. Hail is a good reflector of energy and will return very high dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values. The higher the dBZ, the stronger the rainrate. Typically, light rain is occurring when the dBZ value reaches 20. The scale of dBZ values is also related to the intensity of rainfall. The value of the dBZ depends upon the mode the radar is in at the time the image was created. Notice the color on each scale remains the same in both operational modes, only the values change. The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). One scale (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). Each reflectivity image you see includes one of two color scales. The dBZ values increase as the strength of the signal returned to the radar increases. So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). "Reflectivity" is the amount of transmitted power returned to the radar receiver. The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan.
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