Clusters of Compact Intracloud Discharges (CIDs) in Overshooting Convective Surges.

We observed five clusters of upper‐level compact intracloud discharges (CIDs) moving positive charge up over land and over water in Florida. The clusters each contained 3 to 6 CIDs, and the overall cluster duration ranged from 27 to 58 s. On average, the CIDs in a given cluster occurred 11 s apart and were separated by a 3D distance of about 1.5 km. All the clustered CIDs were located above the tropopause and were likely associated with convective surges that penetrated the stratosphere. The average periodicity of CID occurrence within a cluster (every 11 s) was comparable to the periodicity at which the average cluster area is expected to be bombarded by ≥1016 eV cosmic‐ray particles (every 5 s). Each of such energetic particles gives rise to a cosmic ray shower (CRS) and, in the presence of sufficiently strong electric field over a sufficiently large distance, to a relativistic runaway electron avalanche (RREA). We infer that each of our upper‐level CIDs is likely to be caused by a CRS‐RREA traversing, at nearly the speed of light, the electrified overshooting convective surge and triggering, within a few microseconds, a multitude of streamer flashes along its path, over a distance of the order of hundreds of meters (as per the mechanism recently proposed for lightning initiation by Kostinskiy et al., 2020, https://doi.org/10.1029/2020JD033191). The upper‐level CID clustering was likely made possible by the recurring action of energetic cosmic rays and the rapid recovery of the negative screening charge layer at stratospheric altitudes. Plain Language Summary: Compact intracloud discharges (CIDs) are non‐conventional lightning discharges inside the cloud that are characterized by a relatively small spatial extent (less than 1 km), very strong HF‐VHF (3–300 MHz) electromagnetic emissions, and characteristic wideband radiofrequency field pulses. It has been recently discovered, using space‐based detectors, that CIDs also produce optical signals, the so‐called blue flashes at cloud tops. Our understanding of the physics of CIDs is still poor. CIDs are usually solitary events, but we observed five clusters of CIDs, mostly at altitudes in the 18–20 km range, which were likely associated with kilometer‐scale convective surges that penetrated the stratosphere. Each cluster contained 3 to 6 CIDs, and the cluster duration ranged from 27 to 58 s. On average, the CIDs in a given cluster occurred 11 s apart and were separated by a 3D distance of about 1.5 km. Some CIDs within a cluster were essentially collocated, while being separated in time by up to several tens of seconds. We inferred that the CID clustering observed in our study was made possible by (a) the recurring action of omnipresent energetic cosmic rays and (b) the rapid recovery of the negative screening charge layer at stratospheric altitudes. Key Points: 5 clusters of 3–6 compact intracloud discharges (CIDs) moving positive charge up in convective surges overshooting the tropopause in Florida were examinedCID clustering was tight in both time and space: average time interval was 11 s and average 3D distance within a cluster was 1.5 kmCID clustering was made possible by recurring action of ≥1016 eV cosmic‐ray particles and rapid recovery of negative screening charge layer [ABSTRACT FROM AUTHOR]