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The Time Between First Radar Echoes and First VHF Lightning Radiation Source Locations as an Indicator of Eventual Storm Intensity

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Wednesday, 26 January 2011
The Time Between First Radar Echoes and First VHF Lightning Radiation Source Locations as an Indicator of Eventual Storm Intensity
Washington State Convention Center
Patrick T. Hyland, University of Oklahoma, Norman, OK; and W. H. Beasley and S. A. Weiss

Storm electrification is one consequence of convection. It seems reasonable therefore to expect that the rate of electrification could serve as a proxy for intensity of convection and eventual storm intensity, at least under some circumstances. Further, we reason, the time between the first appearance of radar echoes above a predetermined threshold and the time of occurrence of the first VHF lightning source locations by a lightning mapping array could possibly serve as a proxy for the rate of electrification. To test these ideas, we analyzed data from 76 discrete storms in three different seasons (Winter, Spring, and Summer) to try to determine whether the time between the first radar echo and the first determinations of VHF radiation source locations by the Oklahoma Lightning Mapping Array could have value as a potential indicator of eventual storm intensity. We considered several possible indicators of storm intensity, including maximum echo top heights, height difference between echo tops and equilibrium level, height difference between echo tops and freezing level, number of CG lightning flashes per storm, average time between the first and last CG lightning flash, radar reflectivity increases within the storm core, and average time between the first radar echo and the highest radar echo. We also looked at the time between the first VHF radiation sources and the first CG lightning flash, the time between the first VHF radiation sources and the highest radar echo, and the characteristics of radar echoes in the time leading up to the first CG lightning strike.

We found that for more than half of the storms analyzed the time between the first radar echo and the first indication of VHF source locations was ten minutes or less. We found that in general there was only a slight tendency towards higher echo tops when the time between radar echoes and VHF source locations was shorter. Furthermore, the difference between maximum tops and equilibrium level and between maximum tops and freezing level did not show any dependence on the time between first echoes and first VHF sources. Similarly, there was no discernible pattern of dependence of the average number of CG flashes per storm on the time between first echoes and first VHF source locations. We did find that when the time between the first radar echoes and the first VHF source locations was short, it was more likely that these storms would experience radar reflectivity increases of at least 5-10 dBZ following the first CG lightning flash and that storms reached their maximum in radar reflectivity faster if they became electrified more quickly. In the majority of cases analyzed (55 percent), the time between the first radar echo and the first CG lightning flash was between 5 and 15 minutes. Interestingly, however, in approximately 70% of the cases analyzed, the time between the first indication of VHF lightning radiation source points and the first CG lightning flash was between 0 and 5 minutes. This interval is not as long as indicated in some previous studies and so bears further examination. It may be that our criterion for first indication of VHF source points may have been too stringent. Finally, in general agreement with several previous studies, approximately 94 percent of the storms had base-scan radar reflectivity values of 38 dBZ or greater at the time of the first CG lightning flash. The results of this study, though encouraging, are indeed mixed, suggesting the need for further refinement with regard both to the early indications of radar and VHF radiation activity and to indicators of storm intensity.