Three-Dimensional Radar and Total Lightning Characteristics of Mesoscale Convective Systems
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Date
2003-08-11
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Abstract
The radar and electrical characteristics of three linear leading convective/trailing stratiform midlatitude mesoscale convective systems (MCSs) that passed through Dallas-Fort Worth, Texas on the following dates are examined: 1) 7-8 April 2002, 2) 12-13 October 2001, and 3) 16 June 2002. Quantitative results from the April and June MCSs are presented, but data problems with the October MCS restricted partitioned analysis to qualitative results. The convective line produced ~69% and ~93% of the total cloud-to-ground (CG) lightning flashes in the April and June MCSs, respectively. The convective line CG flash rate averaged 12.3 flashes min-1 (53.6 flashes min-1) in the April (June) case study, and only 7.5% (2%) of these flashes were positive in polarity. Lightning Detection and Ranging (LDAR II) source data identified two main electrically-active regions present within the convective line in the following temperature layers: 1) 0 to -25 °C, and 2) -35 to -55 °C. The lower region (1) was most likely a combination of the main negative and the lower positive charge centers of the thunderstorm tripole, and the upper region (2) was most likely the upper positive charge center of this tripole. Convective echo volume aloft (≥ 30 dBZ, 0 to -40 °C) was strongly correlated to convective lightning activity, suggesting that the presence of strong updrafts and differential sedimentation caused convective line electrification via the non-inductive charging (NIC) mechanism.
The stratiform region CG flash rate averaged 2.2 flashes min-1 (4.5 flashes min-1) in the April (June) case study, and ~45% (~27%) of these flashes were positive in polarity. LDAR II source data identified one primary electrically-active layer (at -10 to -25 °C) that was sloped from the upper portions of the convective line rearward to just above the bright band in the stratiform region. A small and spatially distinct secondary electrically-active layer (at ~ -40 °C) was located towards the rear of the stratiform region. These two layers had smaller average source concentrations than the convective line had, resulting in significantly less lightning production in the stratiform region than in the convective line. Hydrometeor trajectory analyses using storm-relative vertical and horizontal motions determined from synthetic dual-Doppler results indicate that the these two stratiform region layers become electrified by a combination of 1) positive charge advection from the upper-positive convective charge center to the stratiform region and 2) stratiform in situ charging via NIC, likely creating an inverted dipole. This inverted dipole may explain why the +CG flash percentage was significantly higher in the stratiform region than in the convective region (where a normal dipole is present). In addition, stratiform echo volume aloft (≥ 25 dBZ, -10 to -40 °C) was strongly correlated to stratiform lightning activity, suggesting that differential sedimentation as a result of the presence of larger ice aggregates (i.e. Z > 25 dBZ) at these temperatures was required for stratiform region electrification via both charge advection and in-situ charging.
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mesoscale convective systems, lightning, radar
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Degree
MS
Discipline
Marine, Earth and Atmospheric Sciences
