  The Gamma-Ray Observation of Winter Thunderclouds (GROWTH) collaboration has
been performing observation campaigns of high-energy radiation in coastal areas
of Japan Sea. Winter thunderstorms in Japan have unique characteristics such as
frequent positive-polarity discharges, large discharge current, and low cloud
bases. These features allow us to observe both long-duration gamma-ray bursts
and lightning-triggered short-duration bursts at sea level. In 2015, we started
a mapping observation project using multiple detectors at several new
observation sites. We have developed brand-new portable gamma-ray detectors and
deployed in the Kanazawa and Komatsu areas as well as the existing site at
Kashiwazaki. During three winter seasons from 2015, we have detected 27
long-duration bursts and 8 short-duration bursts. The improved observation
network in Kashiwazaki enables us to discover that the short-duration bursts
are attributed to atmospheric photonuclear reactions triggered by a downward
terrestrial gamma-ray flash. Collaborating with electric-field and radio-band
measurements, we have also revealed a relation between abrupt termination of a
long-duration burst and a lightning discharge. We demonstrate that the mapping
observation project has been providing us clues to understand high-energy
atmospheric phenomena associated with thunderstorm activities.
