RDA Subcomponents

Figure 2-25.—WSR-88D system configuration. REFLECTIVITY.—Recall, some degree of energy is likely to be returned to the antenna as a result of backscattering, and that size, shape, and composition all contribute to a target’s ability to backscatter this energy. Stronger targets have higher levels of reflectivity and return more energy. Thus, stronger targets have higher reflectivity values; that is, higher dBZ levels. For example, large raindrops backscatter many times more power than drizzle and will produce reflectivity levels 5 to 10 dBZ higher. Because of its characteristics, hail tends to have exceptionally high reflectivity values. And, since the WSR-88D displays information in the form of echo intensity levels, a severe thunderstorm with hail will appear much more intense than an area of light rain or drizzle. VELOCITY.—Velocity is used to determine wind speed, direction, and target motion (perhaps the movement of a thunderstorm cell). The WSR-88D can detect small areas of convergence, divergence, and even circulation. This has proven valuable in alerting whole communities to the onset of strong surface winds and tornadic activity. SPECTRUM WIDTH.—Spectrum width data is another method of determining atmospheric motion. Spectrum width examines the degree of velocity variance ofparticles (scatterers) in the atmosphere that results from dispersion and mixing. Within a single sample volume of the atmosphere, many particles of varying size, shape, and composition move in different directions and speeds. Because of their differences, each particle moves about independently of the others. Within this single sample volume, the WSR-88D does not sense each individual motion, but rather a combination of all velocities for the overall sample. This combination of different velocities is what spectrum width measures. Spectrum width data is quite useful in determining atmospheric stability, and is a good indicator of environmental turbulence. By itself, it offers very little guidance. Combined with other products, the user can determine radar performance and reliability of the data being displayed. The concept of evaluating spectrum width data is still relatively new and operational uses are still being developed. RDA Subcomponents The RDA consists of all the hardware and software needed to transmit and receive radar energy. It also preprocesses raw data by filtering inconsistencies. The transmitter, receiver, and signal processor are housed together in a small building near the antenna, as 2-23