Approximate calculation of the horizontal electric field at ground level from inclined lightning channel in frequency-domain

An approximate method to calculate the horizontal electric field at ground level from inclined lightning channel in frequency-domain is proposed based on the extension of Cooray formula. The validation results show that the deviation of the extended formula is closely related to the observation distance and the ground conductivity. The bigger the observation distance or the ground conductivity is, the smaller the deviation is. Moreover, the inclined angle of lightning channel and the azimuth angle of observation point have little influence on the accuracy of the extended formula, which could nearly be ignored.