Mapping the impact of terrain on lightning incidence and multiple ground contacts in cloud-to-ground flashes

The climatological incidence of cloud-to-ground (CG) lightning is known to have large regional differences associated synoptic-scale variations in weather that impact the development and propagation of deep convection. At smaller spatial scales, terrain variations are known to play a significant role in the development of deep convection leading to thunderstorms. Convective clouds frequently build over the high terrain typically beginning in the late morning or early afternoon, driven by mountain-valley circulations. In addition to the enhancement of deep convection in complex terrain, there are flash-scale interactions between downward propagating leaders and terrain variations that can alter the nature and location of attachment to ground. This suggests that there may be terrain-related variations in not only the incidence (ground flash density) of CG lightning and the location of the first flash in a storm, but also in some of the physical parameters of CG lightning flashes. The work presented here focuses on demonstrating the impact of terrain on the nature of multiple ground contacts in CG flashes using data provided by the U.S. National Lightning Detection Network during the period of 2006 through 2011. A method for classifying a CG stroke as creating a new ground contact or occurring in a pre-existing channel is presented first. This method employs information that can be provided by modern lightning locating systems. This tool is then used to study the spatial distribution of the average number of ground contacts per flash and the dominant spatial separation distances, in 5x5 degree regions with differing terrain variations. These data are analyzed at sub-km spatial scales using the DTED digital elevation model dataset.