Wednesday, 8 November 2006
Pre-Convene Space (Adam's Mark Hotel)
Chris Smallcomb, NOAA/NWSFO, Reno, NV
The mid-level three body scatter spike (TBSS) Doppler radar signature generally is a 10-30 km long region of artifact echo aligned radially downrange from a highly reflective (>60 dBZ) echo core. Caused by non-Rayleigh radar microwave scattering or Mie scattering (Lemon 1998), the TBSS is widely used by operational NWS forecasters as an indication of very large hail within a severe thunderstorm. Established research and training material describe the radial velocity signature associated with the TBSS as generally weak inbound coupled with high (noisy) spectrum width values, due in part to the combination of horizontal and vertical air motions along the path of the radar return. As such, the radial velocity data associated with the TBSS are nearly always meaningless.
However, during recent severe convective events in the Lower Ohio River Valley, several examples of the TBSS exhibiting high inbound radial velocity signatures have been noted. In at least one case, the signature resulted in false mesocyclone and tornado vortex signature algorithm alerts from the Louisville/Fort Knox WSR-88D (KLVX). The cases presented demonstrate that the TBSS can contaminate velocity data, thereby making storm interrogation and the warning decision process more complicated. It is emphasized that forecasters need to be cautious in analyzing the placement of these features within the overall storm structure before making a warning decision, particularly when weighing whether to issue or upgrade to a tornado warning.
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