The Intra-Cloud Lightning Fraction in the Continental United States

The first satellite in the Geostationary Operational Environmental Satellite R-series (GOES-R) will be launched in 2016 and will include the world's first Geostationary Lightning Mapper (GLM). GLM will provide continuous, spatially uniform Total Lightning observations that are expected to significantly improve our ability to nowcast severe weather. However, optimal use of changes in Total Lightning flash rate for individual storms may require knowledge of long-term regional differences in the relative contribution of intra-cloud (IC) and cloud-to-ground (CG) flashes to the total flash rate, reflected in the IC/CG ratio (or equivalently the IC fraction). Additionally, Carey and Rutledge (Journal of Geophysical Research, 1998) found a strong correlation between very high IC/CG ratios and severe storms, including, heavy rain, large hail and tornados.

Given the potential value of understanding long-term regional variations in the IC/CG ratio, we have expanded upon an earlier analysis by Boccippio and collaborators (Monthly Weather Review, 2001). They created the original IC/CG ratio climatology for the Continental United States (CONUS) using datasets provided by the U.S. National Lightning Detection Network™ (NLDN) and the Optical Transient Detector (OTD). These data sets were for the period of May 1995 through April 1999. Since 1999, the duration of the NLDN historical data set has more than doubled, and OTD data can be supplemented with data from the Lightning Imaging Sensor (LIS) which is on the Tropical Rainfall Measuring Mission satellite in a precessing low earth orbit. The Total Lightning data set in this study includes the OTD data from May 1995 to March 2000 and LIS from 1998-2010 which is limited between 38°South to 38°North. Preliminary findings support earlier results indicating factor-of-10 variations in the IC/CG ratio throughout CONUS. High values are seen in the Pacific Northwest and the border of Colorado with Kansas, Oklahoma and Nebraska. This second location is strongly correlated with positive lightning flashes and a high percentage of severe storms. This paper will also include intermediate analyses that will help bound the uncertainty in the IC/CG ratio and IC fraction.