A compact intracloud discharge (CID), also known as a narrow bipolar event (NBE) or narrow bipolar pulse (NBP) is an intensive form of lightning that produces radio waves and scarce visible light. Lasting only a few millionths of a second (typically 20 μs), these events are the most powerful known natural terrestrial source of radio waves in HF and VHF band."

Source : Wikipedia

To date, the physical processes behind CIDs have not been explained :

Despite the multi-year studies of CIDs and the significant volume of experimental data accumulated to date, the nature of this phenomenon is in many respects unclear."

Source : Radiophysics and Quantum Electronics, Vol.58, No.7, 2015

Also see Lightningplugin-autotooltip__plain plugin-autotooltip_bigLightning

High powered lightning discharges are happening somewhere on Earth around 100 times every second.

A possible mechanism for the very substantial electrical charges within the clouds was put forward in 1978. Following lab-based experiments, it was suggested that the charges arise from the static-electric interactions of graupel (slush) and ice crystals moving within the cloud (due to convection currents caused by widely differing air temperatures) .