Thursday, 7 October 2004
Horizontal and vertical (e.g., line-normal and line-parallel) composite cross-sections of Vaisalas Dallas-Fort Worth Lightning Detection and Ranging (LDAR II) VHF radiation source density (per km3 per hour, color shaded as shown below in line-normal view) and KFWS WSR-88D radar reflectivity (dBZ, contours every 5 dBZ) provide a unique perspective on lightning pathways within a leading-line, trailing-stratiform (LLTS) mesoscale convective systems (MCS) on June 16, 2002. The overwhelming majority of VHF lightning sources occurred within the leading convective line in a bi-modal pattern in the vertical with upper and lower maxima centered at about 9.5 km and 4.5 km AGL, respectively. As seen below, a persistent lightning pathway extended rearward in the line normal direction (by 40-50 km) and downward (by 4-5 km) from the upper VHF source maximum in the convective line, through the transition zone, and into the radar bright band of the stratiform region. This slanted pathway was similar to hypothetical trajectories taken by snow particles that were detrained at the top of the dissipating convection, moved rearward at about 10 m/s in the storm relative front-to-rear flow and downward at about 1 m/s. Over the stratiform bright band, VHF sources tended to be stratified into extensive horizontal layers located at three different levels from (1) 3.5 to 5 km AGL (7 to -2 deg C), (2) 5.5 to 7.5 km AGL (-6 to -19 deg C), and (3) 8 to 10 km AGL (-22 to -39 deg C). The lower two layers were typically between 25 and 65 km rearward of the convective line while the upper layer was between 65-110 km rearward of the line. Examples of the three-dimensional structure of individual lightning flashes, including positive CG lightning flashes in the stratiform region detected by the National Lightning Detection Network (NLDN), will also be presented. These results provide some of the first in-depth views of detailed three-dimensional lightning structure within a LLTS MCS and provide independent confirmation of gross charge structure as inferred from earlier E-field studies within MCSs.
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