Lightning and Climate Modification Within, Near, and Downwind of Urban Areas

The National Lightning Detection Network (NLDN) has recorded over 250 million cloud-to-ground lightning flashes in the continental United States in the last eleven years. The characteristics of these flashes: location, polarity, peak current, and multiplicity show significant variations with geographic terrain and urban areas. These variations occur on a scale of a few hundred kilometers down to a scale of a few kilometers. On the smallest scale, we see significant variations that we attribute to urban and industrial effects. For example, relatively high flash densities can be identified with the urban areas of Houston and Dallas, Texas, and with the refineries near Lake Charles, Louisiana. The percentage of positive flashes is observed to vary with urban areas and is, we believe, related primarily to pollution. Small-scale geographical effects on lightning flash density, on the other hand, are readily apparent. For example, there is a significant variation of flash density in the middle of New Mexico that we associate with the valley extending south from Albuquerque. The reliability of these observations is directly related to the location accuracy and detection efficiency of the NLDN.