The 3D Spatial and Temporal Evolution of K Process in Intra-cloud Flash

K process is a kind of discharge event in lightning. The study on evolution features of this event helps to increase understanding on the mechanism of lightning initiation and developing. 3D lightning imaging data of 3 intra-cloud flashes are used to describe and analyze spatial and temporal characteristics of K events and corresponding electric field's changing waveforms. These 3D location data are recorded by 2 VHF broadband interferometers at Conghua, Guangdong Province, in the summer of 2010, providing the developing image of lightning discharges with a temporal resolution of 5 μs and a spatial resolution better than 500 m. The VHF radiation of lightning is recorded by a high-speed oscilloscope with a sample rate of 1 GS·s^-1. Both fast and slow filed change antennas are employed in two broadband interferometer systems. Their decade time constants are 1 ms and 8 s, respectively. Changing waveforms of the electric field are record by an A/D card working synchronously with the oscilloscope used to record the VHF signal of lightning.Results show that K process is a kind of fast negative breakdown discharge and can be divided into 3 stages according to the distribution of VHF radiation sources located by broadband interferometers. In the first stage, negative recoil leaders occur under the initiation position of intra-cloud lightning, progress along the path of pre-existing positive leader, heading to the initiation region of lightning. In the second stage, some negative recoil leader can progress fast in the channel established by the previous negative leader of lighting initiation stage and induce a relatively large variation of electric field on the ground. In the last stage, the negative recoil leader reactivates the channel of negative leader in lighting initiation stage and facilitates the negative breakdown at the end of existing path. Speeds of the new air breakdown processes happen at the end of existing path are generally reduced to an order of 10⁴~10⁵ m·s^-1. The evolution speeds of 8 recoil leaders in the 3 intra-cloud lighting records are also calculated. The maximum, minimum and average value of the developing speeds of 8 recoil leaders are 3.1×10⁷, 3.1×10⁶ m·s^-1 and 1.6×10⁷ m·s^-1, respectively. The range of K process speeds is similar with that of dart leader but slower than return stroke.