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

摘要

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 X 10~4 to 4.3 x 10~6 m~3 (CGF)~(-1) with a mean and a median of 4.6 x 10~5 and 1.9 X 10~5 m~3 (CGF)~(-1), respectively, during June-August 1993. The monthly rain volume per reported CG flash ranged from 6.3 x 10~4 to 2.1 x 10~5 m~3 (CGF)~(-1) with an overall mean of 1.8 x 10~5 m~3 (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 x 10~(11) m~3 of rainfall over the GUMRB and there were 3.6 x 10~6 CGF reported by the NLDN, which gives an overall seasonal mean of 1.8 X 10~5 m~3 (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 X 10~6 CG flashes over the GUMRB, and therefore, the actual seasonal mean rain volume was about 1.3 x 10~5 m~3 (CGF)~(-1). The above values are remarkably similar to the summer mean of 1.1 x 10~5 m~3 (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 x 10~4 m~3 (CGF)~(-1), based on streamflow measurements at Keokuk, Iowa.