Storm Development

Shortly after 18:00 UT on October 6, 1997, scattered thunderstorm cells formed along a roughly north-south corridor to the east of the Rio Grande in central New Mexico. Some of the cells clustered together, particularly in southern New Mexico, but the storms were not well organized as a whole. The storms in southern New Mexico organized into a mesoscale convective system (MCS) after 23:00 UT. The line of convection advanced eastward and was still active at 01:30 UT on October 7, as shown in Figure 5.1 by the line of -CG activity.

Figure 5.1: NLDN stroke data for southeast New Mexico between 01:30 and 4:30 UT on October 7, 1997. A line of convective cells, marked by -CG activity, decayed throughout the time interval. A significant number of sprite events were associated with the +CGs which trailed primarily behind the northern half of the convective line after 02:30 UT. The brighter colors and larger symbols correspond to larger peak current magnitudes.

Figure 5.1 shows that the convective line became less active after 02:30 UT with only infrequent -CG strokes occuring by 04:30 UT. In contrast, the +CG activity increased after 02:30 UT, particularly in the region behind the northern half of the line where significant -CG activity was located much earlier (not shown). These +CGs were centered $\sim$300 km away from the observation site at Langmuir Laboratory. The weak amplitude +CGs prevalent throughout the time period are probably IC events, as discussed in Section 3.4.

It was shown in Chapter 3 that sprite-producing +CGs were situated near the periphery of a sprite cluster in a Florida MCS. The center of the sprite clusters were generally located at a greater plan distance from the convective cells than were the +CGs, a fact which was attributed to the outward motion of the negative leaders and the decay of older channels. Thus, one can obtain a rough estimate of where the sprite clusters will be located in relation to their parent +CGs by comparing the +CG locations with the convective cell locations indicated by the $-$CG activity. The sprite-producing flashes analyzed here occurred between 03:00 and 04:00 UT. The +CGs were situated primarily to the northwest of the convective line. The observation site at Langmuir Laboratory was situated to the west-northwest of the +CGs, so it follows that the sprites will tend to appear both to the left of the +CGs and at a closer range. This latter effect will result in a systematic increase in height estimates obtained from the plan location of the NLDN CG data, relative to actual heights. This will be further demonstrated in this section by comparing sprite terminal altitudes estimated from the NLDN plan location with previous triangulated measurements of terminal altitudes.