Evolution of the total lightning structure in a leading-line, trailing-stratiform mesoscale convective system over Houston, Texas

摘要

Line-normal, vertical cross sections of Houston Lightning Detection and Ranging (LDAR) VHF radiation sources and radar reflectivity provide new insights into the three-dimensional total lightning structure and evolution of a leading-line, trailing-stratiform (LLTS) mesoscale convective system (MCS) over Houston, Texas, on 31 October 2005. Previous research examining only the mature stage of an MCS showed that the overwhelming majority of VHF lightning sources occurred in the convective region with a lightning pathway extending rearward and descending in altitude into the stratiform region. This descending pathway was most likely associated with small, charged ice particles advected from the convective line. Unlike previous research, the lightning pathway observed during the evolution of the MCS on 31 October 2005 initially extended rearward 40 km at a nearly constant height of 9–10 km. In less than an hour, the lightning pathway evolved into a sloped pathway, similar to that found by previous research, with a horizontal extent between 50 to 60 km and downward descent of 4 to 5 km. During the lightning pathway evolution, radar analysis showed an increase of reflectivity in the midlevels of the stratiform region, consistent with increased depositional and aggregational growth above the melting layer. Broadening (increased range of values) of radar reflectivity above the melting layer was most likely an indication of a strengthening mesoscale updraft in the stratiform region. This strengthening updraft may have contributed to an increase in the growth of the small, charge carrying ice crystals giving them a greater fall speed. In addition, the mesoscale updraft may have promoted an environment conducive to local stratiform region charge generation in the mixed phase region just above the melting layer.