The Full-Wave Finite-Difference-Time-Domain (FDTD) electromagnetic (EM) simulation method can simulate large domains and produce time-marching solutions for arbitrary time-domain sources, making it ideal for many lightning applications. In this paper, a two-dimensional (2D) axial-symmetric and a three-dimensional (3D) FDTD model are developed that include irregular terrain based on real topographic data. The models open the door to a wide variety of advanced modeling applications in the analysis of the lightning EM field propagation over mountainous terrain and the location error of lightning location systems (LLSs) in the presence of in mountainous regions.