SPRITES'99 Research Findings and Plans for August Campaign

7-10-99

Tom Nelson/Walt Lyons
Yucca Ridge Field Station and FMA Research have recorded a total of three days in 1999 to this date. Two of the days involved testing of equipment in preparation for the main emphasis in August of this year. They were 6/5/99, and 7/3/99. These two days were unique in that they were characterized by a high percentage of positive flashes as seen from the NLDN data feed. On 6/5/99, a large MCS developed in the Nebraska panhandle and moved to the ENE through north central Nebraska, south central to eastern South Dakota, and into western Minnesota.

The percentage of positive flashes was over 50% for at least 3-5 hours during which sprites and elves were seen a varying rates averaging 8-10 per minute. We estimate that the 4 tapes we ran contain between 600-1000 TLE's from this event along. The MCS structure was very similar to 7-24-96 in that there was a bow echo with a radar bright band to the west in the trailing stratiform area. Operations were carried out with both wide and narrow field-of-view lenses on the Xybion intensified cameras. The narrow lens was at 75mm giving a nominal 10 degree view of a portion of the storm.

On July 3, 1999, Dr. Russ Armstrong of Mission Research Corporation was testing photometers for the August campaign. Another large MCS developed in nearly the same area as 6/5/99, but moved much slower. Viewing distance was at 300-400km. Lightning data again showed large numbers of positive polarity flashes. Sprites and elves were seen and recorded at a rate at least as high as the 6/5/99 storm (8-10 per minute). At one point, the storm was producing 94% positive polarity lightning flashes as seen by the NLDN. Early in the storm's lifetime, loud pops were heard in the 1-10khz VLF range on an INSPIRE receiver designed by NASA. These pops were displayed on a Tektronix digital oscilloscope which showed the waveform of the received signal. With the scope set to see only the largest of the positives, a transition from singular large waveforms to slightly less large, but longer waveforms was seen near dusk. This was the first time real-time monitoring of the VLF waveform was done at YRFS. It will not be the last.

These two storms were unique for their high percentages of positive lightning. In 1998, researchers at YRFS noted that large storm systems in the central part of the US were apparently ingesting large amounts of smoke from the Central American fires. These storms also had high percentages of positive polarity lightning. By looking at the back trajectories of the air ingested by the storms on 6/5 and 7/3, we noted that the air had moved through areas of fires in the southwestern US (6/5) and the western US (7/3).

While no definitive conclusions can be reached at this stage in the research, it is apparent that the electrical structure of these storm systems was altered in some way to produce the high percentage of positive flashes.

AUGUST 1999 Campaign

FMA Research will be supporting the balloon launches from Ottumwa, IA, that will be conducted by the University of Houston. Since this is the 'dark of the moon' period, observations are expected to be unhindered by the light from the moon. This is also the peak of the MCS season in the high plains. Plans call for the balloon-borne instrumentation package to fly approximately 300km from the storm system to make measurements at about 100,000 ft AGL. For further information, see the project web site.

This page last updated on 7-10-99.
Webmaster, Liv Lyons (livlyons@frii.com)