Experiment

On August 14, 1998, the sferic system was armed between 23:07-23:42 UT (6:07-6:42 pm CDT). During this time interval, there was a large mesoscale convective system (MCS) over southern Texas not quite 2 hours remaining before local sunset after 8 pm CDT (see Figure 4.2). NEXRAD radar data (not shown) indicated that the areal coverage of base reflectivity ($>$15 dBZ) for this MCS was $>$66,000 km$^2$, which exceeds the mean of $\simeq$50,000 km$^2$ for nighttime sprite-producing MCSs (Lyons et al., 1999). Figure 4.3 shows National Lightning Detection Network (NLDN) data for southern Texas during the time period that the sferic system was armed. A horizontal bipolar pattern is clearly evident, which is a common feature of nighttime sprite-producing MCSs (Lyons, 1996; Boccippio et al., 1995).

Figure 4.2: The GOES-8 visible satellite image of the United States (top) at 23:15 UT, August 14, 1998, $\sim$5 min prior to the first sprite event. The diagonal line over the Atlantic Ocean on the right side of the image is the terminator, or ``sunset'' line. The GOES-8 infrared satellite data is shown at the bottom for the same time and region. The darkest blue colors correspond to cloud tops of about $-$65$^{\circ}$ C. A large MCS over southern Texas is evident in both pictures, with the infrared indicating an average cloud top temperature below $-$60$^{\circ}$ C over a large area. The images were obtained from http://www.rap.ucar.edu/weather/satellite.html.

Figure 4.3: National Lightning Detection Network (NLDN) data for a Mesoscale Convective System (MCS) over southern Texas is displayed for the time period 23:07-:42 UT on August 14, 1998. The five energetic +CGs discussed in this section are labeled.