  We have developed a three dimensional (3D) interferometric beamforming
technique for imaging lightning flashes using Very-High Frequency (VHF) radio
data recorded from several hundreds antennas with baselines up to 100~km as
offered by the Low Frequency Array (LOFAR). The long baselines allow us to
distinguish fine structures on the scale of meters while the large number of
antennas allow us to observe processes that radiate at the same intensity as
the background when using a time resolution that is close to the
impulse-response time of the system, 100~ns. The new beamforming imaging
technique is complementary to our existing impulsive imaging technique. We
apply this new tool to the imaging of a four stepped negative leaders in two
flashes. For one flash, we observe the dynamics of coronal flashes that are
emitted in the stepping process. Additionally, we show that the intensity
emitted in VHF during the stepping process follows a power-law over 4 orders of
magnitude in intensity for four leaders in two different lightning storms.
