Characteristics of sprite-producing electrical storms in the STEPS 2000 domain

During the summer of 2000, a comprehensive field program, primarily funded by NSF, investigated the electrical, microphysical and dynamical characteristics of High Plains convective systems. A major focus was improving our understanding of positive cloud-to-ground (+CG) generation within the storms of this region. Given the primary resource of the Lightning Mapping Array (LMA), operated by New Mexico Tech, the STEPS program design was ideal for further investigating sprite producing +CGs and their parent storms. Sprites, over continental areas, appear almost exclusively associated with +CGs, but yet even in the most active storm, rarely do more than one in 5 +CGs generate a sprite or a related transient luminous event (TLE) such as a halo or elve. Moreover, only within certain phases of certain storms will +CGs generate TLEs. What is different about these storms and lightning discharges? In general, it appears that charge moment change (Qds) rather than peak current is the key metric for sprite production (Cummer and Lyons, 2004, 2005; Lyons et al. 2003). On any given night, there generally appears to be a minimum threshold for Qds which will generate sprites, a value which is considerably larger than for typical +CGs. We have assembled a database of over 2000 TLEs (mostly sprites) for both STEPS storms and others in the same region over a ten year period. The characteristics of the sprite parent lightning (polarity, peak current, Qds) and of the storm (type, cloud top temperature, radar reflectivity values and patterns) have been retrieved.

It is becoming clear that for summertime MCSs with trailing stratiform regions, the most likely source of TLEs in the continental US, rather specific criteria must be met for TLEs to occur. Cloud top temperatures must reach at least -55°C, with the -50°C area being >20,000 km2. The colder the cloud top, the more likely are TLEs. Radar reflectivities must be >55 dBZ within the storm core, and the contiguous echo area must be >10,000-15,000 km2 (at 10 dBZ). In addition, a substantial portion of the echo must consist of stratiform precipitation in the 20-40 dBZ range, preferably with bright band-like indications. However, these criteria for very cold cloud tops plus high peak reflectivities appear somewhat at odds with the accumulating evidence that the majority of sprites occur within the weak reflectivity stratiform precipitation region. In addition, STEPS LMA data suggest the majority of the discharges occur at very low levels (3-5 km AGL). However, when considered in light of our developing understanding of MCS morphology (Carey et al. 2005), and especially the role of the elevated front-to-rear circulation, the emerging model becomes physically consistent. It appears the intense (and deep) updrafts are required to supply the large amount of condensate within the stratiform area in which the massive laminae of positive charge accumulate which make possible the large Qds values distinguishing the sprite parent +CG.