Abstract
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. Our focus is on positive CG lightning (+CG; i.e., that which lowers positive charge from cloud to ground). Flash densities are computed on a 2×2 km grid for +CG and total CG during the warm season (May September), the cool (cold) season (remaining months), and the entire year. The maximum annual total CG flash density of 28.1 flashes km-2 yr-1 is located just north of Tampa. Relative minima of cool season CG flash density extend from Tallahassee northeastward and south of the Tampa Cape Canaveral corridor. +CG flash density shows a tendency towards greater values in the northwestern portion of the domain during both seasons. Two relative maxima of +CG flash densities are located 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 quite different during the cool season, but are 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 the smaller peak current (~ 20 kA) and larger multiplicity (> 1.5) of +CG flashes. Since +CG lightning is generally 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 may actually describe characteristics of the CG lightning. The threshold for classifying these weak positive events recently was increased from +10 kA to +15 kA. An important finding is that greater than 40 % of all positive events (> 10 kA) in Florida during June, July, August, and October ranged between 10 kA 15 kA. The unusual warm season characteristics of +CG lightning suggest that numerous ambiguous events are retained in our dataset, even when using the new threshold of +15 kA. Daily CG lightning patterns in Jacksonville and Miami were 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.
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