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Radar Line of Sight Graphs

The WSR-88D radar performs 360º scans of the atmosphere with elevation angles between 0.5º and 19.5º. The WSR-88D samples up to 14 different elevations angles for each complete scan of the atmosphere. The radar beam increases in height and in diameter as it moves away from the radar, with most of the beams energy at the beam center height. The bottom and top heights of the beam are defined as the points where there is a 50% reduction in the transmitted energy. Below the beam bottom height and above the beam top height the energy drastically decreases. The area between the beam bottom and beam top is referred to as the “beam”. There is reduced impact on the radar if the rotating blades are below the beam bottom, especially at “close” distances.

  Illustration of radar beam

  Illustration of the total elevation angle coverage of the radar

Any object within the beam reflects energy back to the radar. The WSR-88D uses a complex algorithm, called a clutter filter, to perform an analysis on the data to determine if the returned energy is from a desirable target (weather) or not (non-weather clutter). One factor used in the clutter filter process is the motion of the return. Clutter mitigation filters within the WSR-88D can not filter rotating wind turbines due to the motion of the blades. When the turbines are close to the radar, they penetrate more of the beam increasing the amount of energy returned to the radar, resulting in higher reflectivity values, potentially at multiple elevation angles.