In this paper, lightning electromagnetic (EM) fields radiated by a lightning strike to the Gaisberg Tower in Salzburg, Austria, and propagating over irregular terrain are calculated using a 3D finite-difference time-domain (FDTD) method. With topographic height maps, 7 different propagation paths for a selected lightning strike to the Gaisberg tower are simulated and the resulting magnetic field (H-field) is compared with the received signal strength at seven sensors of the ALDIS (Austrian Lightning Detection and Information System) sensor network for that event. The return stroke is represented by a transmission line (TL) model and the current waveform recorded at the top of the Gaisberg tower was used as an input to it. The results of the 3D-FDTD simulation correlate well with recorded H-fields at sensor sites in mountainous environments. The results of recent studies, which show that a lightning strike to a mountain gives rise to a field enhancement could be verified by comparing the FDTD results to a set of existing sensor measurements. Further, 3D-FDTD simulations of the Gaisberg region have been performed to investigate the lightning EM field radiation pattern of the mountain for first and subsequent return strokes (RS). The results show that the amplitude enhancement of subsequent RSs exhibits a significant direction- dependence, which is caused by the surface structure of the mountain. For first RS fields, a smaller enhancement and more homogeneous radiation pattern is observed.