Wednesday, 6 August 2003
Three-Dimensional Radar and Total Lightning Structure of Mesoscale Convective Systems
Analysis of three-dimensional radar and total lightning characteristics for two mesoscale convective systems (MCSs) occurring in the Dallas-Fort Worth, Texas area during 7-8 April and 16 June 2002 are presented. This study utilizes WSR-88D Level II radar (KFWS), Vaisala Inc. Lightning Detection and Ranging II (LDAR II) VHF total lightning, and National Lightning Detection Network Cloud-to-Ground (CG) lightning observations to gain a better understanding of the structure and evolution of MCSs. More specifically, this research examines: 1) the co-evolving relationship between radar reflectivity structure, single Doppler velocity inferred kinematic features, and LDAR II VHF lightning sources and flash origins, 2) the evolution of the dominant CG lightning polarity and peak current in both the stratiform and convective regions, and 3) the LDAR II inferred gross charge structure relative to radar morphology. Both cases were symmetric MCSs, characterized by a leading convective line, a trailing stratiform region, and a transition zone separating them. As in earlier studies, CG lightning polarity in both MCSs was predominantly negative (~90%). Most CG lightning occurred in the leading convective line associated with enhanced (40-55 dBZ) reflectivity cores at low levels. In each MCS, the positive CG lightning percentage and positive peak currents were significantly higher in the stratiform region than in the leading convective line. Most VHF lightning sources and flash origins occurred at mid-to-upper regions (5-11 km AGL) in the rear portion of the convective line associated with mature cells that were characterized by moderate reflectivity (30-45 dBZ). Convective cells within the MCSs that exhibited enhanced (> 50 dBZ) radar reflectivities and weak echo regions had higher mean lightning flash origin heights than weaker cells. Relatively few VHF lightning flashes were initiated in the stratiform region. Line-normal composites of reflectivity and VHF lightning sources provide a revealing perspective on the co-evolving relationship between 3-D lightning, storm morphology and likely charge structure in the convective and stratiform regions. The composites reveal a classic dipole structure in the convective region with the largest VHF source density at upper levels (implying upper positive over negative charge centers). Interestingly, the VHF lightning sources associated with the inferred upper level positive charge center of the convective region sloped rearward (by ~ 40-75 km) and downward (by ~ 4-6 km) through the transition zone to the stratiform region, settling above the radar brightband at heights (temperatures) of 5 to 7 km AGL (-10° to -20° C). Also, multiple, layered VHF source density maxima (charge layers) in the stratiform region were sometimes observed (inferred) above the brightband.