**5. Conclusions**

An OpenACC-aided graphics processing unit (GPU)-based finite difference time domain (FDTD) method was presented for the first time for the 3D evaluation of lightning radiated electromagnetic fields

along a complex terrain with arbitrary topography. The OpenACC (Open accelerators) directive-based programming model was used to enhance the computational performance and the results were compared with those obtained by using a CPU-based model. It was shown that OpenACC GPUs can provide very accurate results while being more than 20 times faster than CPUs. The presented results support the use of OpenACC not only for lightning electromagnetics problems, but also for large-scale realistic EMC applications in which computation time efficiency is a critical factor.

**Author Contributions:** Conceptualization, H.K.; methodology, S.M and H.K.; software, S.M; validation, S.M; resources, H.K.; writing—original draft preparation, S.M.; writing—review and editing, H.K., M.A., and F.R.; supervision, H.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Bakhtar Regional Electricity Company, Arak, Iran.

**Conflicts of Interest:** The authors declare no conflict of interest.
