Results from the SPRITES'99 and STEPS 2000 Field Programs

The Severe Storm Electrification and Precipitation Study (STEPS) is planned for the High Plains of eastern Colorado, western Kansas and southwest Nebraska from 22 May through 16 July 2000. STEPS is a multi-agency observational program designed for coordinated measurements of the dynamical, microphysical and electrical processes within several classes of severe storms, especially those producing positive cloud-to-ground lightning (+CG). Many of these storms are also the parents of middle atmosphere sprites and elves. Both phenomena are preferentially associated with +CG events, often with large peak currents. Yet not all large peak current +CGs generate optically detectable responses in the mesosphere. The Yucca Ridge Field Station plans to continuously monitor transient luminous events in the stratosphere and mesosphere above storms traversing the STEPS observational domain. STEPS provides the first opportunity to coordinate low-light and photometric detection of sprites and elves with the NLDN, ELF and VLF signatures, lightning video and the New Mexico Tech 3-D Lightning Mapping Array. The goal is to obtain a comprehensive characterization of those CGs which do, and do not, produce sprites and elves. In particular, it has been proposed that sprite-parent +CGs can be distinguished by unusual continuing current characteristics, large charge moments, and horizontally extensive "spider" lightning discharges. Such conditions appear to be favored only during particular phases of certain classes of convection. It is believed that sprites and elves are not commonly produced by supercells, though there is a possible linkage of blue jets to hail-generating storms. These connections will be extensively explored during the STEPS program.

The STEPS 2000 campaign is a follow on to the SPRITES'99 program (August 1999) at Yucca Ridge which obtained excellent data on the occurrence of sprites and elves associated with an exceptionally well organized MCC in Nebraska (18 August). Analysis of low-light video imagery, NLDN stroke data, and GOES infrared data reveal distinct spatial and temporal patterns. During the intensification stages of the MCC, CG flashes of both polarity were commingled in the high reflectivity cores at the leading edge of the system. As the MCC progressed through its mature stage, the positive and negative strokes became spatially separated, with the +CGs becoming associated with the continuously expanding trailing stratiform region. Moreover, those +CGs that were associated with sprites and elves were clustered along the westernmost portion of the trailing stratiform. The +CGs, including those associated with sprites and elves, were consistently associated with the coldest cloud tops. The parent lightning of sprites and elves also appear to be associated with a secondary reflectivity maximum (and perhaps radar bright band processes) in the trailing stratiform region.