DLOC Principles of Meteorological Doppler Radar:
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Previous Page: Section 5: Radar Beam Characteristics
Sidelobe contamination is the result of returned power from the lobes which are off to the side of the main power lobe or beam. The transmitted power within the main lobe is significantly greater than that of any sidelobe. For any given target, the WSR-88D's strongest sidelobes would return about 54 dB lower power than that from the main lobe. Thus, when both the main lobe and the sidelobes are sampling a precipitation area, any possible increase in returned power due to sidelobes is usually negligible. The exception would be during low elevation scans in the presence of a strong surface-based temperature inversion (i.e. during superrefractive conditions) resulting in sidelobe contamination due to AP clutter.
The most significant sidelobe contamination tends to occur with intense convection at close range. If the reflectivity gradients are large enough, a situation may occur where the main lobe is sampling clear air while the sidelobes are sampling the storm (Figure 9). In this case, a low reflectivity would be displayed along the azimuth corresponding to the main lobe.On a reflectivity product, sidelobe contamination generally appears as a "smearing" of low reflectivities from one azimuth to the next in a clockwise direction (The WSR-88D antenna always rotates clockwise).
Figure 9: (Click Image to Enlarge) Example of sidelobe contamination. A strong reflectivity gradient results in side lobes interacting with a target while the main lobe does not. The top right graphic is a real-world example from May 4, 2003. The core of reflectivity with this supercell has values greater than 70 dBZ, which produces a three-body scatter spike down radial (south of core), but also results in side lobe contamination in the azimuthal (to the west of the core) direction.
The beam pattern of the WSR-88D is such that sidelobe contamination will most likely occur from lobes which are 5° to 10° off the main lobe. Therefore, sidelobe contamination requires a reflectivity gradient of about 10 dBZ radial-1 sustained for at least 5° of azimuth.
NOTE: Figure 9 mentions both side-lobe contamination and three-body scatter spike (TBSS) data artifacts. Three-body scatter spikes are a result of multiple reflections of the radar pulse off of large hail and the ground. More about the TBSS will be discussed in DLOC Topic 7, Lesson 5.
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