Abstract
he three-body scatter spike (TBSS), an artifact caused by radar microwave scattering associated with large hydrometeors, traditionally has been utilized in the National Weather Service as an operational indicator of destructive hail. Severe weather warning strategies based on the TBSS were derived using a lower-bound reflectivity threshold of 5 dBZ. With recent WSR-88D workstation software and product upgrades, the operational display of very low value reflectivity data to -30 dBZ has allowed meteorologists to observe more subtle, but meaningful, atmospheric targets and artifacts. Since these upgrades, radar-based interrogations of thunderstorms over west Texas and South Dakota (among other places) have revealed weak TBSS signatures characterized by faint radar returns as low as -14 dBZ. A sufficient dataset of weak TBSS cases does not exist currently to support a robust statistical analysis. Preliminary observations, however, indicate that the artifact may occur prior to and during low-end severe hail measuring between 1.9 cm and 2.5 cm in diameter, especially when recognized to emanate from pulse-type convective storms sampled at close range over the Great Plains. These observations suggest that an extension to the current conceptual use of the conventional TBSS model, which associates the traditional signature to a likelihood of “very large hail,” should be investigated.
Highlights
The three-body scatter spike (TBSS) is a radar microwave scattering artifact occasionally observed in association with severe convection producing large and damaging hail
Radar analyses of five low-end severe thunderstorms bearing weak TBSSs over west Texas and South Dakota are provided in the form of animated four-panel reflectivity data
Detailed radar-based interrogations of five low-end severe storms over west Texas and South Dakota have revealed weak three-body scatter signatures characterized by reflectivity values as low as -14 dBZ
Summary
The three-body scatter spike (TBSS) is a radar microwave scattering artifact occasionally observed in association with severe convection producing large and damaging hail. Previous studies and diagrams by Wilson and Reum (1986 and 1988), showed that TBSS signatures defined by minimum reflectivity contours between -10 dBZ and -5 dBZ were detected with radars of varying wavelengths between 3 cm (X-band) and 10 cm (S-band) Some of these weak TBSSs, observed over Alabama, seldom were associated with surface hail. Since the 8-bit data became operationally available, close range radar-based observations of convective storms producing low-end severe hail (defined here as hail between 1.9 cm and 2.5 cm), indicate that weak reflectivity (less than the previous lower-bound reflectivity of 5 dBZ) TBSS signatures are detectable using the WSR-88D. Conditions in which the weak TBSS may be observed in association with more organized and significant severe storms will be briefly discussed
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