We examine the lightning current of 81 upward lightning (UL) initiated from a windmill and its lightning protection tower in Japan under winter thunderstorms from 2005 to 2016. Among 81 UL, the negative, positive, and bipolar lightning account for 64%, 12%, and 24%, respectively. After checking the current waveforms during the initial stage (IS) of negative UL, we found that self-initiated negative UL have larger peak value and shorter rise time compared to other-triggered ones, although two types of negative UL show no significant difference in charge transfer and duration. Among 18 self-initiated negative UL, half of them are initiated from the windmill. On the other hand, only two of 27 other-triggered negative UL are from the windmill and both of them have smaller current peak value, shorter duration, and smaller charge transfer. According to whether or not the IS current waveforms exhibit pulsation, we have classified UL into type S (smooth), SP (transition from smooth to pulsation), and P (pulsation). We found that the rate of slow current rise for most type P UL is larger than 0.1 kA/ms, whereas for all type S UL the rate is smaller than 0.01 kA/ms. The type SP UL shows a slow current rise rate ranging from 0.01 to 0.1 kA/ms. We found that the current pulsation of type P UL corresponds to the stepwise propagation of a positive leader. Based on these results, we have discussed the effect of the lightning protection tower and the stepwise propagation of positive leaders. Key Points The lightning protection tower showed no meaningful protection for self-initiated negative upward lightning Self-initiated negative upward lightning have larger peak value and shorter rise time during initial stage Negative upward lightning tend to exhibit pulsation in current waveform under a larger rate of current rise
