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This work examines how to adjust the definition of dry lightning in order to optimize the correlation between dry lightning flash count and the climatology of large (>400km(2)) lightning-ignited wildfires over the contiguous United States (CONUS). The National Lightning Detection Network and National Centers for Environmental Prediction Stage IV radar-based, gauge-adjusted precipitation data are used to form climatic data sets. For a 13year analysis period over CONUS, a correlation of 0.88 is found between annual totals of wildfires and dry lightning. This optimal correlation is found by defining dry lightning as follows: on a 0.1 degrees hourly grid, a precipitation threshold of no more than 0.3mm may accumulate during any hour over a period of 3-4days preceding the flash. Regional optimized definitions vary. When annual totals are analyzed as done here, no clear advantage is found by weighting positive polarity cloud-to-ground (+CG) lightning differently than -CG lightning. The high variability of dry lightning relative to the precipitation and lightning from which it is derived suggests it would be an independent and useful climate indicator.

Plain Language Summary Many wildfires are started by dry lightning, which are flashes that occur with low or no precipitation. There is no uniformly accepted definition for dry lightning, which must include an upper threshold on precipitation, the length of the dry period preceding the precipitation, and the spatial resolution over which lightning and precipitation are colocated. This work attempts to find a definition of dry lightning over the continental United States using lightning, precipitation, and wildfire data sets. The lightning and precipitation are located in space and time to the hour and on a 0.1 degrees grid. The location and timing of wildfire initiation is much harder to identify, and so is left as annual/national totals to be targeted by the best definition of dry lightning. We found an upper precipitation threshold of 0.2 mm/h and a dry duration of 30 h defines dry lightning that best matches the wildfire data set.