Pre- and Postupgrade Distributions of NLDN Reported Cloud-to-Ground Lightning Characteristics in the Contiguous United States

The National Lightning Detection Network (NLDN) underwent a major upgrade during 2002–03 that increased its sensitivity and improved its performance. It is important to examine cloud-to-ground (CG) lightning distributions before and after this upgrade becauseCG characteristics depend on bothmeasurement capabilities andmeteorological variability. This study compares preupgrade (1996–99, 2001) and postupgrade (2004–09) CG distributions over the contiguous United States to examine the influence of the recent upgrade and to provide baseline postupgrade averages. Increased sensitivity explains most of the differences in the pre- and postupgrade distributions, including a general increase in total CG and positive CG (1CG) flash densities. The increase in 1CG occurs despite the use of a greater weak 1CG threshold for removing am-biguous 1CG reports (post 15 kA versus pre 10 kA). Conversely, the average 1CG percentage decreased from 10.61 % to 8.65 % following the upgrade. The average 1CG (2CG) multiplicity increased from 1.10 (2.05) before to 1.54 (2.41) after the upgrade. Since true 1CG flashes rarely contain more than one return stroke, explanations for the greater than unity 1CG multiplicities remain unclear. Postupgrade results in-dicate that regions with mostly weak peak current 1CG flashes now exhibit greater average 1CG multi-plicities, whereas regions with mainly strong 1CG flashes now exhibit smaller average 1CG multiplicities. The combination of NLDN performance, meteorological conditions, and physical differences in first 2CG return strokes over saltwater producemaxima in2CGmultiplicity and peak current over the coastal waters of the southeast United States. 1