The Propagation Effects Of The Irregular Terrain On The Lightning Electromagnetic Field

In recent years, with the development of lightning detection technology, lightning monitoring and early warning system has been basically accomplished in China, especially for the department of meteorology and electric power systems. However, all of the lightning location theories assume the land surface is smooth, without considering the errors caused by the surface roughness. For the propagation of lightning return stroke, as the effect of rough ground, the parameters of lightning discharge has a large uncertainty. In this paper, a two-dimension fractional Brown motion (fBm) model is presented for describing the nature irregular terrain, and we analyze the propagation effects of three rough surfaces on the lightning-radiated electromagnetic field by using Barrick’s theory and Wait’s approximates. Also we discuss the time errors due to elongation of the propagation path of the lightning detecting signals caused by real irregular terrain compared with the ideal propagation path. The main results are as follows:(])Firstly, we analyze the propagation effect of the rough land surface on the lightning electromagnetic field. Compared with the subsequent return stroke, it is noted that the rough land has no or less effect on the magnitude of the vertical electric field for first return stroke. With the increase of the roughness, the extra attenuation of the vertical electric field and the rise time of the field waveform increases. And the influence of magnetic field is similar to the vertical electric field. For the horizontal electric field, we should consider the effect of the rough land when the roughness gets larger in the far field.(2)Else we discuss the propagation effect of the rough ocean on the lightning electromagnetic. The result shows that the rough ocean causes the increase on the rise time of the field waveform, but slightly effect on the magnitude of the vertical electric field. With the increase of the observation distance and roughness, the rise time of the field waveform increases. The influence of the vertical electric field spectrum is focus on the10MHz and above, which leading to a big extra attenuation. And the influence of magnetic field is similar to the vertical electric field. The rough ocean has an effect on the horizontal electric field, but the horizontal electric field is too small to be considered.(3)Further we study the propagation effect of the rough ocean-land mixed path on the lightning electromagnetic. On the lightning electromagnetic field, it is noted that the rough land has a larger influence than the rough ocean. While the electromagnetic spectrum above2MHz, the attenuation increase with the roughness increases. We can neglect the rough ocean effect when the electromagnetic propagation path is ocean to land except that the spectrum is more than1OMHz. For the horizontal electric field, the decrease of the roughness results in the increase of magnitude.(4)Then we assess the site errors of the Shenzhen lightning location system. Found that the Shenzhen lightning location system which uses four sites rectangle network has high detection accuracy. The minimum error is several meters, and the average of the site error is about1kilometers. (5)At last we analyze the real terrain around district of Baiyunzhang which remotely Sensed by the Satellite. It is noted that different azimuth and different observation distance lead to the differences of time-delay. Then we evaluate the lightning location system of this district using an elevation model, and found that the location error caused by real terrain is about several kilometers. When the lightning stroke points locate around the mountain, the stroke points which evaluated by the elevation model are close to the real stroke points.