Cloud-to-ground lightning flashes and debris-flow-generating rainfall in the post wildfire environment: An exploratory study of the Mitchell Creek debris flow in western Colorado, summer 2002

Abstract

Cloud-to-ground (CG) lightning flash parameters collected by the National Lightning Detection Network were analyzed in conjunction with rainfall observations near Mitchell Creek (MC) at the Coal Scam Wildfire site in western Colorado, USA. Nine thunderstorms produced significant CG flashes in the area surrounding MC from 28 June (fire containment) to 5 August 2002. A debris flow was generated at MC by one of these storms at ∼2058 LT on 5 August 2002. This study compares the CG flash parameters and rainfall characteristics of the 5 August thunderstorm with the eight thunderstorms (control group) that did not produce a hazardous hydrologic response at MC. The CG flash patterns and a synoptic analysis suggest that the 5 August thunderstorms occurred during a North American Monsoon "burst" period with a strong southwesterly surge of moisture advecting as far north as central Wyoming. The 5 August thunderstorm sequence was bimodal and more intense in terms of CG flash totals and rainfall rates than the control group. Moist southwesterly flow at lower levels and dry southeasterly flow in the upper troposphere may have enhanced orographically forced convection during the evening of 5 August. CG flashes and rainfall at spatial scales of 100 × 100 km, 50 × 50 km, and 25 × 25 km around MC confirm that the 5 August episode was more intense than any of the control group. The analysis concluded that the time of first flash and the number of consecutive 5-min intervals with CG flashes were temporally related with intense rainfall at the debris flow site. Additionally, there was a strong correlation between flash clusters within 40 km of MC and rainfall intensity at the site. The results of this study suggest that CG flash parameters may prove beneficial in modeling rainfall intensity thresholds in areas burned by wildfire. © 2003 by the American Geophysical Union.