8/28/2023 0 Comments Past us doppler radar![]() This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. The frequency of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.). The ability to detect the "shift in the frequency" of the pulse of energy makes NEXRAD a Doppler radar. Computers analyze the strength of the returned radar waves, time it took to travel to the object and back, and frequency shift of the pulse. This reflected signal is then received by the radar during its listening period. A small portion of that scattered energy is directed back toward the radar. The radar emits a short pulse of energy, and if the pulse strike an object (raindrop, snowflake, bug, bird, etc), the radar waves are scattered in all directions. NEXRAD ( Next Generation Radar) can measure both precipitation and wind. In addition, the radar image will not show echos from precipitation that lies outside the radar's beam, either because the precipitation is too high above the radar, or because it is so close to the Earth's surface that it lies beneath the radar's beam. To determine if precipitation is occurring at greater distances, link to an adjacent radar. This image will not show echoes that are more distant than 143 miles, even though precipitation may be occurring at these greater distances. The maximum range of the base reflectivity product is 143 miles (230 km) from the radar location. Base reflectivity images are available at several different elevation angles (tilts) of the antenna the base reflectivity image currently available on this website is from the lowest "tilt" angle (0.5°). "Reflectivity" is the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. The Local Radar base reflectivity product is a display of echo intensity (reflectivity) measured in dBZ (decibels). 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.Precipitation intensity is measured by a ground-based radar that bounces radar waves off of precipitation. 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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