88th Annual Meeting (20-24 January 2008)

Tuesday, 22 January 2008
Characteristics of positive cloud-to-ground lightning
Exhibit Hall B (Ernest N. Morial Convention Center)
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.

Supplementary URL: