The Evolution of Lightning Flash Density, Flash Size, and Flash Energy During Hurricane Dorian's (2019) Intensification and Weakening

The two most distinct inner‐core lightning outbreaks in Hurricane Dorian (2019) are analyzed using the Geostationary Lightning Mapper (GLM). The first outbreak occurred during Dorian's intensification, including a rapid intensification (RI), and the second occurred during weakening. During RI, inner‐core lightning flash density increased as flashes concentrated inside of the radius of maximum wind (RMW). As weakening commenced, numerous flashes still occurred within the RMW, with a flash rate more than three times that during RI–a signal typically associated with strengthening. These flashes, however, were much smaller and less energetic than those during intensification. Evidence is presented that barotropic mixing and secondary eyewall formation increased the number of small, low‐energy lightning flashes in the inner core while simultaneously weakening the storm. The results suggest that flash area and energy from GLM could help distinguish between lightning outbreaks that correspond to intensification and those that correspond to weakening. Plain Language Summary Lightning can be a useful tool for forecasting hurricane intensity. Usually an increase in lightning within the storm means that it is likely to intensify. But sometimes even weakening hurricanes have large lightning outbreaks, so forecasters must carefully look at other data to see what a lightning outbreak really means for a hurricane's intensity. The Geostationary Lightning Mapper (GLM) is a new tool that provides a lot more information about lightning in hurricanes than was previously available. GLM is able to continuously detect the size and energy of lightning flashes, even over the open oceans. Here we show that the lightning flashes in Hurricane Dorian (2019) were larger and more energetic when the storm was intensifying than when it was weakening. We also argue that changes in the location of lightning flashes could help to identify processes that affect a storm's intensity. This information provides clues into how storm structure changes at peak intensity and can potentially help forecasters interpret whether a lightning outbreak signifies storm intensification or weakening. Key Points Lightning flashes during Dorian's rapid intensification were larger and more energetic than those during moderate intensification As Dorian weakened, lightning flashes were more numerous, but were smaller and less energetic than during intensification GLM observations reveal processes that affect hurric