Terrestrial gamma ray flashes correlated to storm phase and tropopause height

We compare the seasonal and geographical occurrence of terrestrial gamma ray flashes (TGFs) with global lightning maps and find that only part of the difference can be explained by differences in tropopause altitude. The altitude hypothesis suggests either that TGFs are produced only in conjunction with the highest lightning or that only the highest events are seen from space because of the easier escape of gamma rays from the atmosphere. We find that the differences in atmospheric transmission due to seasonal and geographical differences in tropopause height play a major but not dominant role in reconciling lightning and TGF maps and that other factors are needed to explain the remaining local differences. In a second analysis, we use radio atmospherics data from the World Wide Lightning Location Network to study at what time in the evolution of a storm TGFs tend to be seen. We find that, on average, TGFs lag the peak flash rate of the associated storm by 38 min, although the range of lags is extremely wide, including some cases where the TGF leads the peak flash rate.