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The spatial and temporal distributions of cloud-to-ground lightning are examined over the contiguous United States from 1995 to 1999 using data from the National Lightning Detection Network. Annual flash density, annual lightning days, cumulative frequency distributions of daily flash counts, and annual and summertime diurnal distributions of lightning are documented. The spatial, annual, and summertime diurnal distributions of positive and negative polarity cloud-to-ground lightning are also documented. Over the same five-year period, the production of positive and negative lightning is examined over two case study areas located in the north- central United States and along the Gulf Coast, centered on Sioux Falls, South Dakota, and Fort Rucker, Alabama, respectively. Case studies include radar-lightning analyses of significant lightning episodes from 1996. Maximum flash densities and lightning days are found over coastal regions of the southeastern United States. Other prominent maxima are seen over parts of the southern Rocky Mountains and adjacent High Plains. Cumulative frequency distributions indicate that throughout the contiguous United States roughly 10% of the days with lightning accounted for 50% of lightning production. The majority of lightning was produced during summer (June-August) throughout the contiguous United States, except over the south-central United States and along and near the Pacific coast. Summertime lightning activity over the western and eastern United States exhibited a diurnal cycle with maximum frequencies in the afternoon to early evening. Over the central United States, summertime lightning activity was complex with significant longitudinal variations in daily activity and a tendency to occur at night. Over most of the contiguous United States, a larger fraction of negative lightning was produced during summer than positive lightning, and the diurnal cycle of positive lightning lagged the diurnal cycle of negative lightning by up to two hours during summer. The main exception to these behaviors occurred over an area in the north- central United States extending from the Colorado-Kansas border to western Minnesota. Over this area, positive lightning peaked during midsummer versus late summer for negative lightning, and the diurnal cycle of positive lightning also peaked up to several hours prior to the maximum in the diurnal cycle of negative lightning during summer. In addition, this area was characterized by maxima in the percentage of positive lightning and positive mean peak current. The maximum in the percentage of positive lightning over the north-central United States was caused by a dramatic increase in positive flash density to the east of the Rocky Mountains and a local minimum in negative flash density over the area described above. Results from the Sioux Falls case study indicate that positive lightning was produced primarily during summer in the hours around sunset by isolated storms and convective lines in various stages of mesoscale convective system (MCS) development. These convective events usually contained one or more storms that were characterized by predominantly positive lightning, high positive flash rate, and large positive peak currents. Negative lightning activity was produced later in the summer and throughout the night by more mature convective systems arranged in lines or clusters. Over Fort Rucker, positive and negative lightning was produced throughout the year, by diurnally forced storms during the warm season and by MCSs with areally extensive stratiform regions during the cold season. Diurnally forced storms (MCSs) were characterized by a low (high) percentage of positive lightning.

Cloud-to-Ground Lightning Activity in the Contiguous United States from 1995 to 1999