Improved lightning model: Application to discuss the characteristics of upward lightning

A stochastic discharge parameterization scheme of upward lightning is presented and embedded into an existing fine resolution two-dimensional thundercloud electrification and lightning model. At present, our lightning model can simulate not only intra-cloud (IC) lightning and cloud-to-ground (CG) lightning but also upward lightning. By this improved model, 139 upward flashes originating from the tall structures were obtained. Out of these upward flashes, 67 cases occur without any nearby preceding lightning discharges (called self-initiated upward flashes), whereas 72 cases are triggered by the preceding lightning discharges (called nearby-lightning triggered upward flashes). The relationship between the initiation of self-initiated upward flashes and thundercloud electrical environment parameters is discussed. The results show that the initiation of self-initiated upward flashes is related to the weighted average of the lower charge heights (H) and the average of the lower charge density (D). As D increases, the critical H increases. In addition, the reason for IC lightning or upward lightning triggering upward lightning is investigated. IC lightning discharges or upward lightning discharges neutralize the main negative charge region in the cloud and enhance the influence of the lower positive charge region on the ground electric field.