[The eye of category 5 Hurricane Dorian from September 2nd, 2019 near Grand Bahama Island . From NASA/NOAA
] Instead of chasing storms, NASA researchers are using new weather prediction methods to see storms ahead of time.
By studying lightning, the team of scientists is working to develop new ways to help forecast the intensity of incoming hurricanes. Typically, an increase in lightning within the storm signals that the storm is likely to strengthen. But sometimes even weakening hurricanes have large lightning outbreaks, so forecasters must carefully analyze additional data to determine what a lightning outbreak really means for predicting a hurricane's intensity.
A team of scientists led by NASA researcher Patrick Duran recently published a study on the evolution of lightning flash density, flash size, and flash energy during Hurricane Dorian
. Duran and his team support NASA's Research and Analysis Program, Weather Focus Area, as part of the Short‐term Prediction Research and Transition Center at NASA's Marshall Space Flight Center in Huntsville, Alabama.
Duran and his colleagues used a new tool on National Oceanic and Atmospheric Administration's latest series of Geostationary Operational Environmental Satellites called the Geostationary Lightning Mapper (GLM) to capture information about lightning in hurricanes. GLM continuously detects the size and energy of lightning flashes, even over the open oceans.
"In this study," Duran said, "we were able to prove that the lightning flashes in Hurricane Dorian were larger and more energetic when the storm was intensifying than when it was weakening."
Using GLM, the team analyzed the two most distinct lightning outbreaks in the innermost part – or inner-core – of Hurricane Dorian. The first outbreak occurred during intensification, including a period of rapid intensification (defined as an increase in 30 kts (35 mph) in sustained winds over 24 hours). During rapid intensification, the number of inner-core lightning flashes increased as flashes concentrated inside of the radius of maximum wind – or the distance between the center of the cyclone and its band of strongest winds. The second outbreak occurred during weakening. As weakening continued, numerous flashes still occurred within the radius of maximum wind, with a flash rate more than three times that during rapid intensification – a signal typically associated with strengthening. These flashes, however, were much smaller and less energetic than those during intensification.
The GLM sensor provides continuous observations of lightning across most of the western hemisphere, including the Atlantic and Eastern Pacific tropical basins. The GLM sensor, effectively an optical event detector, measures changes in cloud top radiance produced by lightning. GLM's ability to detect not only flash location but average flash area and total optical energy enables the examination of lightning from a number of new perspectives.
[The visible satellite loop captured by the Geostationary Lightning Mapper depicts Hurricane Dorian, with lightning strikes indicated by the red "+" marks. The swirling clouds orbiting the inside eye of the storm are related to processes that affect a hurricane's intensity. In this case, scientists hypothesize that these swirling clouds represent mesovortices – small regions of spinning air that can help produce strong thunderstorms around the eye of the hurricane but can also cause processes that ultimately weaken a hurricane's maximum wind speed. Credits: NOAA/NASA
"We also argue that changes in the location of lightning flashes could help to identify processes that affect a storm's intensity," Duran said. "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."
"In the future," Duran said, "we will analyze a large number of storms to discover how lightning patterns differ between storms that intensify and those that weaken. We think that these patterns could be especially useful in identifying rapid intensification, which is very difficult to predict."
"We're still learning how to interpret and utilize the GLM in operational tropical cyclone analysis and forecasting," Stephanie Stevenson, meteorologist and programmer with the National Hurricane Center, said. "This study pushes us toward understanding how GLM's unique area and energy fields can be used in conjunction with lightning density to monitor a storm's evolution."