Spatial and temporal variability of cloud-to-ground lightning over the continental U.S. during the period 1995–2010

We use 16 years (1995–2010) of data from the National Lightning Detection Network (NLDN) to examine the spatial and temporal variability of major cloud-to-ground (CG) lightning days (defined as the 80 days with the largest lightning activity) over the continental United States. Extreme lightning activity is concentrated over the central U.S. and west of the Appalachian Mountains. The largest frequency of major lightning days is concentrated during the summertime, with a tendency for these major days to have occurred in recent years. We also examine the presence of monotonic patterns over time in CG lightning flashes over the continental United States. Analyses are performed at the monthly scale (from April to September) and for total, negative-only, and positive-only flashes. The non-parametric Mann–Kendall test is used to examine the presence of monotonic patterns. The upgrades in NLDN during the study period complicate the separation between cloud-to-cloud flashes (CC) and the targeted CG lightning flashes. The results of the trend analyses are sensitive to the threshold used to discriminate between CC and CG flashes, in particular for positive-only flashes. The central U.S. is an area that exhibits statistically significant increasing trends independently of the selected threshold, while there is a general tendency towards decreasing trends over the Rocky Mountains. These results raise the question of whether the observed changes in lightning activity during the recent years are related to natural or human-induced changes in the climate system, and/or to inhomogeneities in the observational network.

► Analyses of spatio-temporal variability of major lightning days during 1995–2010.
► Major lightning days concentrated during summertime and over the central US.
► Increasing (decreasing) trends in lightning activity over the central US (Rockies).