Combined satellite- and surface-based estimation of the intracloud-cloud-to-ground lightning ratio over the continental United States

Four years of observations from the NASA Optical Transient Detector and Global Atmospherics National Lightning Detection Network are combined to determine the geographic distribution of the climatological intracloud-cloud-to-ground (CG) lightning ratio, termed Z, over the continental United States. The value of Z over this region is 2.64-2.94, with a standard deviation of 1.1-1.3 and anomalies as low as 1.0 or less over the Rocky and Appalachian Mountains and as high as 8-9 in the central-upper Great Plains. There is some indication that Z covaries with ground elevation, although the relationship is nonunique. Little evidence is found to support a latitudinal covariance. The dynamic range of local variability is comparable to the range of values cited by previous studies for latitudinal variation from the deep Tropics to midlatitudes. Local high Z anomalies in the Great Plains are coincident with anomalies in the climatological percentage of positive CG occurrence, as well as in the occurrence of large positive CGs characteristic of organized or severe storms. This suggests that storm type, morphology, and level of organization may dominate over environmental cofactors in the local determination of this ratio.