Cloud-to-Ground Lightning and Surface Rainfall during the Great Flood of 1993

Abstract

Relationships between cloud-to-ground (CG) lightning, as reported by the U.S. National Lightning Detection Network (NLDN), and surface rainfall, as reported by National Weather Service (NWS) cooperative observers, have been examined during the ?great flood? of 1993. The daily precipitation volume per reported CG flash (CGF) over the greater Upper Mississippi River basin (GUMRB) ranged from 4.0 ? 104 to 4.3 ? 106 m3 (CGF)?1 with a mean and a median of 4.6 ? 105 and 1.9 ? 105 m3 (CGF)?1, respectively, during June?August 1993. The monthly rain volume per reported CG flash ranged from 6.3 ? 104 to 2.1 ? 105 m3 (CGF)?1 with an overall mean of 1.8 ? 105 m3 (CGF)?1. Similar ratios were found for the Upper Mississippi River basin (UMRB) that is embedded within the GUMRB. For the entire summer season, there were about 6.5 ? 1011 m3 of rainfall over the GUMRB and there were 3.6 ? 106 CGF reported by the NLDN, which gives an overall seasonal mean of 1.8 ? 105 m3 (CGF)?1. If the lightning counts are corrected for an imperfect NLDN detection efficiency, then it is estimated that there were actually about 5.4 ? 106 CG flashes over the GUMRB, and therefore, the actual seasonal mean rain volume was about 1.3 ? 105 m3 (CGF)?1. The above values are remarkably similar to the summer mean of 1.1 ? 105 m3 (CGF)?1 obtained by Petersen and Rutledge over the midcontinental United States and are consistent with other studies on daily and storm scales. The above ratios are larger than, but still consistent with, an estimate of the excess stream volume per excess (reported) CG flash from the UMRB, 6.8 ? 104 m3 (CGF)?1, based on streamflow measurements at Keokuk, Iowa.