On the Initiation of Upward Discharges Above Thunderstorms
A simple model of thunderstorm charging shows how upward discharges can be initiated out of the top of a cloud into clear air above the storm. The discharges are initiated between the upper positive charge of a normally electrified storm and the resulting screening charge of negative polarity attracted to the upper cloud boundary. The model was developed to predict the occurrence of intracloud and cloud-to-ground flashes in the storm, as a means of interpreting lightning mapping data to determine the charging currents in storms (Krehbiel, 200x). Although simple, the model is able to replicate average IC and CG flashing rates very nicely. To make the model more complete, the screening current to the top of the cloud was added. An unexpected result of doing this was that discharges were predicted to occur between the screening charge formed at the cloud boundary and the upper positive charge. The discharges would not occur if the screening charge was dissipated or removed. Since such discharges are not seen by lightning mapping observations, the implication is that the screening charge is indeed dissipated, probably by being turbulently mixed into the upper positive charge. If for some reason such mixing does not occur or is not sufficiently effective, the discharges would occur that would propagate vertically upward through the cloud top to an undetermined height above the cloud. The discharges are most likely to occur shortly after a negative CG discharge, which suddenly changes the net charge of the storm in a manner to enhance the electric field in the direction required for the initiation. For a normal-polarity storm in which the upper charge is positive, the upward breakdown would also be of positive polarity. This is consistent with the fact that we have never seen upward discharges with the Lightning Mapping Array - positive breakdown radiates very weakly in the VHF and is poorly detected by the system. However, an upward jet of negative polarity was recently found in looking at archived data from the STEPS project. The jet was initiated ~10 seconds after an intracloud discharge added negative charge to the cloud region below the initiation point. This observation is nicely consistent with what would be predicted by the model.
Paul Krehbiel
New Mexico Tech,Socorro, New Mexico, USA
国际会议
第13届国际大气电学会议(The 13th International Conference on Atmospheric Electricity)
北京
英文
2007-08-13（万方平台首次上网日期，不代表论文的发表时间）