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 degrees C), midlevel negative charge (-20 degrees 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 degrees 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 degrees C). Based on these results, it is inferred that sprite production and sprite-parent positive charge altitude depend on MCS morphology.
