ABSTRACT
On July 24 and August 1, 1996, storm systems over Kansas and the panhandles of Oklahoma and Texas produced over 400 transient luminous events (TLE's). These systems were recorded with image intensified cameras simultaneously from locations in Colorado and New Mexico. Regional radar composites, surface data, and GOES satellite imagery were also archived for these storm systems. Using video imagery obtained at the two sites above as a basis for analysis, locations of the center of the sprite or elve, it's minimum and maximum elevation, and preliminary maximum values for both areal and volumetric extent have been calculated. Differences in viewing perspective and location within the storm will be investigated using NLDN data, GOES IR imagery, and regional radar composite imagery. It is known from previous work (W.A. Lyons, J. Geophys. Res,, 1996, 29641-652) that most TLE's are closely associated with positive cloud-to-ground lightning data. Extensive analysis of the 24 July 1996 events has shown that the NLDN detected the parent CG of some 71.8% of the TLE's on videotape. Values for the offset between the attach point of the CG and the center of the TLE as well as their average amplitude and multiplicity by type of event will be presented.
The storm system of 24 July 1996 was perhaps the most energetic in terms of sprite/elve production of any system studied to date. Over 305 events were recorded on videotape, ranging from large, complex carrots and c-sprites, to the ubiquitous 'smudges' which are still being classified. The video cameras located at Yucca Ridge Field Station outside Fort Collins, CO, and Langmuir Lab outside Socorro, NM were the locations for this study. Image intensified cameras viewed the storm from two different locations at an angle of approx 90 degrees. Contitions for viewing were excellent, and several of the sprites between 3-4z were visible to the naked eye at YRFS.
A secondary storm system was viewed from Langmuir Lab and resulted in extreme close-up imagery being recorded from that location. A third system was located in western Mexico and was not viewed, but did have an impact on the electomagnetic recordings made at the MIT Schumann Resonance site in rural Rhode Island.
At YRFS, data were gathered from approximately 330UTC to 900UTC. NLDN data were analyzed for a period more closely related to the lifetime of the storm, 0 to 12 UTC. The storm start in Eastern CO and moved into the Kansas area by 02Z. An MCS had started to form several hours earlier, and by 03z had coalesced into a large system. GOES infrared imagery shows this system quite well, and the second and third systems.
24 July 1996 MCS Characteristics
| Type | Mesoscale Convective Complex with a Radar Bow Echo |
| Size at Peak | Approx. 65,000 km2 |
| Sprite Observations | 0300-0900 UTC |
| Number of CGs | 85,900 (0-12 UTC) |
| Percent of CGs | 5.2% |
| Avg. Peak Ampl. | -21 kA/ +31 kA |
| Total TLEs | 304 |
| Sprites 245 Elves 24 Both 35 |
|
| Avg. TLE Ampl. | +71 kA |
| Sprites +61 kA Elves +120 kA Both +107 kA |
| Hour | Sprite | Elve | Both |
|---|---|---|---|
| 3-4 | 9 | 3 | 1 |
| 4-5 | 16 | 8 | 5 |
| 5-6 | 52 | 2 | 3 |
| 6-7 | 49 | 3 | 11 |
| 7-8 | 57 | 5 | 4 |
| 8-9 | 60 | 3 | 11 |
An analysis of the radar reflectivity related to the TLE's has been done previously (T.E. Nelson, Master's Thesis, 1998, Mankato State University, Mankato, MN). For this work, an archive tape from the Vance Air Force Base WSR-88D radar was obtained and data manually extracted using the Dorada software form the University of Oklahoma Center for Computational Geosciences. The location of the NLDN parent CG was used as a starting point. Data were entered in a 25-cell grid surrounding the center point. Reflectivity from all available levels were entered, but analysis of only the lowest 4 levels was done due to lack of consistent returns above this. A modified composite reflectivity product was calculated by placing the highest values from any of the 4 levels on a single 25-cell grid. A true composite reflectivity product would have used all available levels.
Analysis of the grids was done for the center cell, the surrounding 9 cells, and the full grid. It was expected from published literature that the nominal values in the anvil region of an MCS would be less than 30 dBz. However, for this storm system, that is not the case. Sometimes in the anvil region, an area of subsidence develops in response to synoptic conditions. When this area reaches, and falls below, the 0C isotherm, the ice crystals begin to melt, forming a layer of liquid water on their surface. This combination is highly reflective at the radar beam wavelength. The increased reflectivity shows up clearly as a radar bright band in the regional composite imagery. By plotting the locations of the NLDN parent strokes with Maptitude, the distribution of events at the edges of the bright band is clearly seen. Although TLE's are not limited to this area, there is a high concentration. Some of the relevant data are presented below.
C O M P O S I T E R E F L E C T I V I T Y
S P R I T E E L V E S P R E L F
dBz 1 9 25 1 9 25 1 9 25
----------- ---------- -----------
0-20 0 0 0 0 0 0 0 0 0
21-25 2 10 28 0 0 0 0 0 0
26-30 7 81 243 1 13 33 3 26 82
31-35 13 119 336 2 10 40 4 39 123
36-40 22 188 552 4 38 100 3 29 60
41-45 8 64 171 0 2 7 1 5 20
46-50 0 7 23 0 0 2 0 0 1
The data above represent the total number of cells which had a reflectivity value in the range of the left column for all the analyzed events. There are three areas analyzed, the 1-cell, 9-cell, and 25-cell portions of the analysis grids described above. There are also three types of events analyzed.
Average Composite Reflectivity Values by Event Type
Sprite Elve Sprelf values are dBz
1-cell Avg 35.3 34.8 33.9
9-cell Max 37.6 37.6 36
9-cell Avg 35.2 35.1 33.6
9-cell Min 32.7 32.9 31.3
25-cell Max 39.2 40.4 37.7
25-cell Avg 35.2 35.4 33.5
25-cell Min 31.5 32.4 29.6
Extreme values were above 50dBz for all 25-cell cases. Sprites had 50dBz in 9-cell cases, and other types went to 45dBz. These values area clearly higher than the 30dBz, or less, values seen in other MCS with no bright band.
It was hoped that the extended area of 5 x 5 km (25 grid cells) would be, in some way, representative of the source area for the charge pool drained by the parent CG. A large area is needed, but is clearly beyone the scope of hand tabulation and analysis.
From video imagery, the location of the center of the TLE can be determined by plotting the event azimuths from YRFS and Langmuir. A mapping program, Maptitude, was used for the analysis. To offset any problems in determining the azimuth on the map, the forward geodetic point was calculated (one for each of the azimuths). These dots were then connected and the crossing represented the visual center of the event. The difference between the two points was analyzed with both the azimuthal difference and the range offset.
In analyzing the video imagery, the geographic azimuths were determined by overlaying a starfield map generated by SKYMAP. This map was resized in a copier and placed on a transparency which was then placed on the monitor and aligned using the star background. The azimuths of the center and edges as well as the top and bottom of the TLE could then be read directly off the map grid.
To determine the area of a TLE, the left, center, and right azimuths for the entire TLE were determined. For several events, the azimuths for individual elements of the event were measured. The top and bottom elevation were also measured, and the brightest elevation when possible. Combined, these azimuths and elevations from two sites are used to calculate the area and volume of the TLE.
55 individual events were analyzed for the paper. There are perhaps some 100 more events to be analyzed in coming months. Listed below are some of the more relevant data from this study based on 55 analyzed events. YRFS NMT Number of Events 55 55 Average Width of TLE 25 17 km Average Offset of Location of NLDN CG and Video Center 33 km Average area of TLE 502 km2 Largest Area 3306 km2 Smallest Area 57 km2 Average Vertical Extent 33 28 km (not elevation) Average Volume 20,350 km3 Average Amplitude 59.2 kA Maximum Elevation 105.8 108.2 km Minimum Elevation 32.7 31.4 km Average Elevation 71.1 73.9 km Maximum Range 701 836 km Minimum Range 493 505 km
For more information, contact Thomas E. Nelson at 970-204-0562 (H) or 970-568-7664 (W).