Southern Louisiana: Compelling Evidence for Pollution Enhancing Cloud-to-Ground Lightning

Fourteen-years (1989-2002) of cloud-to-ground (CG) lightning data show a significant enhancement of lightning associated with Lake Charles and Baton Rouge, Louisiana. A peak density value of 7 flashes km-2 yr-1 exists on the western side of the Lake Charles urban area. The Lake Charles CG lightning enhancement is greatest during the summer season (June, July, and August), and for each season the greatest enhancement occurred during the late morning/afternoon (0900-1800 LT period). The percentage of CG flashes lowering positive charge to ground shows a relative minimum over Lake Charles and Baton Rouge (4-8%). The values of median peak negative current show a sharp difference between land and the Gulf of Mexico; inland values are near 24 kA, while over the Gulf waters immediately offshore the values are over 30 kA. A clear relationship between the CG lightning enhancements and the locations of sources of PM10 across Southern Louisiana suggests that pollution plays a key role in lightning enhancement. Urban effects can be neglected due to the small population of Lake Charles and the fact that most of the lightning enhancement is upwind (west) of the urban area. The Lake Charles anomaly is not associated with the sea breeze enhancement as it is 50 km inland from a nearly straight coastline. The observations of higher negative peak currents immediately off the Louisiana coastline and the existence of a relative minimum of negative peak current from the mouth of the Mississippi River southeastward into the Gulf gives support to the hypothesis that the surface conductivity influences the calculated negative current distribution. Land, fresh, and salt water have differing effects on the attenuation of a lightning discharge's electromagnetic signal as it travels over the particular surface. The signal is attenuated less over salt water (higher conductivity), making the negative peak current appear higher. Or alternatively, the peak current may be intrinsically higher over salt water, again because of the higher conductivity.