Transient luminous events above two mesoscale convective systems: Storm structure and evolution

Abstract

Two warm‐season mesoscale convective systems (MCSs) were analyzed with respect to their production of transient luminous events (TLEs), mainly sprites. The 20 June 2007 symmetric MCS produced 282 observed TLEs over a 4 h period, during which the storm's intense convection weakened and its stratiform region strengthened. TLE production corresponded well to convective intensity. The convective elements of the MCS contained normal‐polarity tripole charge structures with upper‐level positive charge (<−40°C), midlevel negative charge (−20°C), and low‐level positive charge near the melting level. In contrast to previous sprite studies, the stratiform charge layer involved in TLE production by parent positive cloud‐to‐ground (+CG) lightning resided at upper levels. This layer was physically connected to upper‐level convective positive charge via a downward sloping pathway. The average altitude discharged by TLE‐parent flashes during TLE activity was 8.2 km above mean sea level (MSL; −25°C). The 9 May 2007 asymmetric MCS produced 25 observed TLEs over a 2 h period, during which the storm's convection rapidly weakened before recovering later. Unlike 20 June, TLE production was approximately anticorrelated with convective intensity. The 9 May storm, which also had a normal tripole in its convection, best fit the conventional model of low‐altitude positive charge playing the dominant role in sprite production; however, the average altitude discharged during the TLE phase of flashes still was higher than the melting level: 6.1 km MSL (−15°C). Based on these results, it is inferred that sprite production and sprite‐parent positive charge altitude depend on MCS morphology.