Characteristics of cloud-to-ground lightning activity in the contiguous United States from 1995-1997

The spatial and temporal distributions of cloud-to-ground lightning (CG) are examined over the contiguous United States from 1995-97 using data from the National Lightning Detection Network. Annual CG flash density, annual CG lightning days, cumulative frequency distributions of daily CG flash count, and annual and daily variations of CG lightning activity are documented. During the same three year period, the production of positive and negative CG polarity lightning is examined over the contiguous U.S.

Examination of total cloud-to-ground lightning activity (positive and negative polarity) revealed that local maxima in CG flash density and CG lightning days were collocated with elevated terrain in the western U.S., Gulf Coast, Florida peninsula, and the Gulf Stream. Cumulative frequency distributions of daily CG flash count indicate that a relatively large portion of cloud-to-ground lightning was produced on a relatively few number of days; this behavior was consistent throughout the contiguous U.S. The majority of cloud-to-ground lightning was produced during the summer (June-August) over most areas in the contiguous U.S. with the exceptions of the south-central U.S. and the Pacific Coast. Summertime CG lightning activity over the western and eastern U.S. exhibited a diurnal cycle with a well-defined time of maximum frequency occurring in the afternoon. Areas characterized by a strong diurnal cycle in summertime CG lightning activity were collocated with elevated terrain in the western U.S. and coastlines along the Gulf of Mexico and Atlantic Ocean. Over the central U.S., summertime CG lightning activity was complex with significant longitudinal variations in daily activity and a tendency for nocturnal cloud-to-ground lightning.

Examination of positive and negative polarity cloud-to-ground lightning activity revealed that a larger fraction of negative CG lightning was produced during summer than positive CG lightning over most areas in the contiguous U.S. with the exceptions of the northwestern and north-central U.S. During summer, positive CG lightning activity tended to lag negative CG lightning activity in time over most areas in the contiguous U.S. with the exception of an area in the north-central U.S. where positive CG lightning activity preceded negative CG lightning activity. This area was collocated with maxima in the percentage of positive CG lightning and in positive CG mean peak current.