Electromagnetic Fields at the Top of a Tall Building Associated With Nearby Lightning Return Strokes

We have calculated, using the finite-difference time-domain (FDTD) method for solving Maxwell's equations, the vertical electric field Ez and azimuthal magnetic field Hphi due to lightning return strokes in the presence and in the absence of a building at the field point. Strikes to both flat ground and tall objects of height h = 100, 200, or 500 m are considered. The magnitude of Hphi is not much influenced by the presence of either building at the field point or strike object, while the magnitude of Ez is significantly influenced by either of them. In the case of a lightning strike to flat ground, the magnitude of Ez at the top of the building (at the center point of its flat roof) of plan area Sb = 40 times 40 m2 and height hb = 20, 50, or 100 m located at horizontal distance ranging from 100 to 500 m from the lightning channel is about 1.5, 2, or 3 times, respectively, greater than that at the same horizontal distance on the ground surface in the absence of the building. The enhancement factor for lightning (transient) Ez due to the presence of the building at the field point is essentially not influenced by the presence of the strike object (up to 500 m in height) and is similar to the static electric field enhancement factor due to the presence of the same building in a uniform vertical electric field. The magnitude of the electric field at the corner of the building is about two to three times larger than that at the center point of its flat roof. The magnitude of Ez at the ground level in the immediate vicinity of the building is reduced relative to the case of no building, with this shielding effect becoming negligible at horizontal distances from the building exceeding twice the height of the building.