This study is mainly concentrated on examining the positive cloud-to-ground
lightning activity associated with Mesoscale Convective Systems. Six MCS events
which occurred during the O.K. PRE-STORM program in 1985 are studied. Data
indicating the location and polarity of the cloud-to-ground lightning flashes from a
lightning location network are analyzed in conjunction with the low-level echo
patterns as obtained from radar. Spatial and temporal characteristics of positive
cloud-to-ground flashes are identified from the data analysis. For all cases
examined, positive cloud-to-ground flashes were found most commonly in the
stratiform regions of the MCSs examined, and their frequency tended to peak
during the later stages of the storm lifecycle.
Two mechanisms responsible for the occurrence of positive cloud-to-ground
lightning flashes with the above spatial and temporal characteristics are discussed.
Based on the laboratory results, a 1-D charge generation model is developed. The
model results show that in-situ charging is unlikely to be the dominant
mechanism for charge generation in the stratiform region under normal
atmospheric conditions. Sensitivity studies show, however, that in-situ charging
processes strongly depend upon the liquid water, graupel and snow contents in the
cloud. Under favorable atmospheric conditions, in-situ charging may lead to a
significant charge generation. Hence, we cannot completely dismiss in-situ
charging mechanism. Analysis of wind fields from dual-Doppler radar in
combination with vertical profile of electric fields indicates that charge advection
from the convective region to the stratiform region of MCSs may be a potential
mechanism responsible for the occurrence of positive cloud-to-ground lightning
flashes in the stratiform region. / Graduation date: 1989
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28616 |
| Date | 05 August 1988 |
| Creators | Lu, Chungu |
| Contributors | Rutledge, Steven A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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