P1.11
Characteristics of positive cloud-to-ground lightning
Scott David Rudlosky, Florida State Univ., Tallahassee, FL; and H. E. Fuelberg
Five years of cloud-to-ground (CG) lightning data (2002 2006) from the National Lighting Detection Network (NLDN) are examined to describe the nature of CG lightning in Florida. We focus on positive CG lightning (+CG; i.e., that which lowers positive charge from cloud to ground). +CG has received far less attention in the literature than total CG. Flash densities of +CG and total CG are computed on a 2×2 km grid during the warm season (May September), the cool season (remaining months), and the entire year. The maximum annual total CG flash density of 28.1 flashes km-2 yr-1 is found just north of Tampa. Relative minima of cool season CG flash density extend northeastward from Tallahassee and also south of the Tampa Cape Canaveral corridor. +CG flash densities are different from those of total CG, with greater values in the northwestern portion of the domain during both seasons. Specifically, relative maxima of +CG flash density are found near the Apalachicola National Forest in the panhandle (0.5 1.05 positive flashes km-2 yr-1), and near Naples in Southwest Florida (0.35 0.5 positive flashes km-2 yr-1).
The median peak current and number of return strokes (multiplicity) of +CG and CG flashes are found to be quite different during the cool season, but more similar during the warm season. The greatest peak current (~ 30 kA) and smallest multiplicity (~ 1.4) of +CG flashes occur during the cool season. The warm season is characterized by smaller peak current (~ 20 kA) and larger multiplicity (> 1.5) of +CG flashes. Since +CG lightning generally is thought to consist of a single return stroke, our warm season multiplicities of ~1.7 are unexpected. This value may represent cloud pulses that are misclassified by the NLDN as weak peak current +CG flashes, or it may actually describe characteristics of the +CG lightning.
The threshold for determining weak positive events recently was increased from +10 kA to +15 kA. An important finding is that more than 40 % of all positive events (> 10 kA) in Florida during June, July, August, and October range between 10 15 kA. The unusual warm season characteristics of +CG lightning suggest that numerous ambiguous events are retained in our dataset, even though we used the new threshold of +15 kA. Thus, the threshold value requires further study. Daily CG lightning patterns near Jacksonville and Miami are analyzed during March and July 2003. On a given day, there is more +CG lightning during March than July in both Jacksonville and Miami. Also, when lightning does occur, the percentage of positive flashes generally is greater in Jacksonville than Miami.
Poster Session 1, Lightning Applications, Studies and Technology in the Atmospheric Sciences
Tuesday, 22 January 2008, 9:45 AM-11:00 AM, Exhibit Hall B
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