Corona emission from raindrops in strong electric fields as a possible discharge initiation: Comparison between horizontal and vertical field configurations

The purpose of this work is to determine which electric field configuration (vertical or horizontal) produces corona emission from raindrops for the lowest ambient electric field. For that, a numerical modeling of the distortion of uncharged raindrops falling at terminal velocity in quiescent air in a horizontal electric field is performed. The results are compared with previous numerical study involving a vertical electric field. It is shown that the fall velocity is quite unaffected by ambient field intensities lower than 200 kV/m. The disruption and the corona onset fields are lower than those corresponding to the vertical field configuration and the larger the drop the larger the difference. For a given altitude the difference between the corona onset fields in both configurations can reach about 100 kV/m; meanwhile for a given ambient field intensity, the difference in altitude of corona emission can rise to about 1 km. These onset field intensities are, though, too high for allowing drop breakup and corona emission from an uncharged drop in permanent field conditions below 12 km height. An estimation of the critical onset field (disruption and/or corona emission) is carried out for charged drops in the horizontal field configuration. For a drop 2 mm in spherical equivalent radius carrying the quarter of its Rayleigh maximum net charge, the critical onset field is approximately equal to 290 kV/m below 7.4 km height and decreases down to 110 kV/m at 10 km. Given that present modeling does not take into account the effect of turbulence that could induce drop oscillations, the critical field intensities calculated here may be considered as upper limits.