Cloud-to-ground lightning in the United States: NLDN results in the first decade, 1989-98

The physical and geographical characteristics of over 216 million cloud-to- ground lightning flashes recorded during the first decade (1989-98) of oper ation of the National Lightning Detection Network (NLDN) covering the entir e continental United States are presented. These characteristics include th e total cloud-to-ground flash density, the positive flash density, the perc entage of positive flashes, the first stroke negative and positive peak cur rents, and the multiplicity for negative and positive flashes. All analyses were done with a spatial resolution of 0.2 degrees corresponding to an app roximate resolution of 20 km. Flash densities were not corrected for detect ion efficiency; the measured values are presented. The maximum measured fla sh density is found to exceed 9 flashes km(-2) across Florida in the Tampa- Orlando-Cape Canaveral corridor, near Fort Myers, and between Lake Okeechob ee and the Atlantic Ocean. The mean monthly flash count peaks in July at ap proximately 5.5 million flashes. Positive flash density maxima, greater tha n 0.4 flashes km(-2) occur in southern Florida; Houston, Texas; and along t he Texas-Louisiana border. A broad region of relatively high positive densi ty also occurs throughout the Midwest. The mean monthly positive flash coun t peaks in June and July at approximately 240 000 flashes in each month. The annual mean percentage of lightning that lowered positive charge was hi ghest in the upper Midwest, exceeding 10% or 20% throughout most of the reg ion. High percentages are also characteristic along the West Coast. The ann ual percentage of positive lightning has increased from 3% in 1989 to appro ximately 9% in 1998. The authors believe the increase is the result of impr oved sensor detection capability in the past decade. The mean monthly perce ntage of positive lightning flashes ranged from 4% in August to 17% in Dece mber for the decade. The annual median negative peak current ranged from 30 kA in 1989, decreasing steadily to about 20 kA in 1998. The annual median positive peak current ranged from 55 kA in 1989 decreasing to about 22 kA i n 1998. The annual median peak negative and positive currents have approxim ately the same value since 1995, the first year after the NLDN upgrade. The monthly median first stroke peak currents for the decade peak in the winte r and reach a minimum in May (positive current) and July (negative current) . The mean monthly negative multiplicity for the decade ranges from 2.1 in February to 2.5 from June to October. The mean monthly positive multiplicit y is approximately 1.2 throughout the year. The diurnal variation of the ma ximum flash rate over land was examined and found to peak during 1200-2000 local time (LT) with an exception for the upper Midwest, which peaked durin g 2000-0400 LT. Over water surrounding the continental United States, the l ightning flash rate peaks primarily in the morning hours from 0400 to 1200 LT.