THE IMPACTS OF THUNDERSTORM GEOMETRY AND WSR-88D BEAM CHARACTERISTICS ON DIAGNOSING SUPERCELL TORNADOES Steve Piltz – WFO Tulsa, OK Don Burgess – CIMSS Norman, OK
Sidelobe Contamination The 3D Beam (Seen on end)
Federal Meteorological Handbook 11- Part BDoppler Radar Theory and Meteorology 3.6 Data Contamination by Antenna Sidelobe Signal. An occasional source of data contamination is simultaneous reception of signals at comparable power levels through both the antenna pattern main lobe and its sidelobes. This occurs when the meteorological reflectivity gradient exceeds the two-way sidelobe/main lobe isolation.
Two-way path… 27 + 27 = 54 dB 27 dB Measurements of WSR-88D radiation patters are rare. Therefore we are discussing approximations at best. For example, if the side-lobe is sensing a 64 dBZ return, the main-beam would have to be sampling something around or less than 10 dBZ for side-lobe contamination to occur.
Two-way path… 27 + 27 = 54 dB 27 dB FMH 11: …in order to detect a signal through the first sidelobe, … the meteorological signal in the sidelobe must be stronger than the signal in the main lobe, by at least the two-way, first-sidelobe isolation. This requires a reflectivity gradient of greater than… 40 dB / degree sustained over about 2 degrees. This is a rather large value and seldom-encountered in practice.
NWS Training on Sidelobe Contamination 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. 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).
Which Sidelobe? Federal Meteorological Handbook 11- Part BDoppler Radar Theory and Meteorology
Vertical Reflectivity Gradients in Supercells Horizontal reflectivity gradients from weather targets sufficient to result in sidelobe contamination are rather rare. In the vertical they are less rare…if not common in supercells.
R X-Section White arrow (right): 0.4 degree: -1.0 dBZ and +66 kts Yellow arrow (right): 3.3 degree: 55 dBZ and +48 kts Base Vel X-Section
R X-Section Why little / no contamination to the reflectivity? Every -3 dB is a 50% drop of power. Discussing power differences around -54 dB means the actual power returned is very small. Base Vel X-Section
R X-Section But velocity data are collected. They inappropriately become a part of what the main beam samples, and the main beam’s data are contaminated. Base Vel X-Section
Note the precision with which the Spectrum Width data “fit” into the low Reflectivity.
Time Continuity 21:11 UTC 21:06 UTC
R X-Section Yellow arrow (right): 1.4 degrees: -1.0 dBZ 3.3 degrees: 53.0 dBZ The 3D Beam (Seen on end) Base Vel X-Section
Spectrum Width 1.4 degrees 21:11 UTC Reflectivity 1.4 degrees 21:11 UTC
Key Points: Vertical reflectivity gradients in the “overhang” / inflow region of supercell thunderstorms can be sufficient for sidelobe data contamination. Warning forecasters should consider the possibility of velocity contamination when the vertical reflectivity gradient is around or more than ~50 dBZ. Forecasters should closely inspect any velocity data in the inflow of supercell when the associated reflectivity is less than 25 dBZ. Contaminated velocity data will have higher associated Spectrum Width. Velocity and Spectrum Width character changes in reflectivity gradient.
Needed work… The NWS should quantify the beam at each 88D in an effort to better understand this. A simple algorithm to flag areas where vertical reflectivity gradients exceed ~50 dBZ. Mesocyclone evolution needs to be studied further and training developed (Vortex2). NWS Impacts: Warning Decisions Pathcasts Assessing Spotter Reports
Developing mesocyclone…local vorticty max…where the inflow went vertical, or ??. (Too much low-level reflectivity for sidelobe contamination.)
Thank You Steve Piltz – WFO Tulsa, OK Don Burgess – CIMSS Norman, OK