DISCUSSION: One way to map lightning strikes is to use ground-based networks that sense radio frequency emissions from lightning. This data is highly accurate, but the areal coverage may be limited. Furthermore, some ground-based networks are primarily sensitive to only cloud-to-ground lightning. But most lightning occurs cloud-to-cloud or within a single cloud. Hence, these networks may miss a large portion of total lightning flashes.
Another way to observe lightning is from satellites. In particular, the Geostationary Lightning Mapper (GLM) was first launched in 2016. There are currently two GLM instruments in orbit that combined provide constant coverage over nearly the entire western hemisphere. GLM is sensitive to optical emission from lightning. As long as the top of the cloud is illuminated, GLM can observe all types of lightning, including those that don’t strike the ground.
The U.S. National Weather Service uses three products that are derived from GLM data, and the picture (credit: NOAA/NESDIS/Scott Rudlosky) above illustrates these products. The bottom-right panel is an IR satellite image (not from GLM) that shows where the clouds are. In particular, the reds/blacks/whites show where the highest, coldest cloud tops occur. The panel in the top-left shows the flash extent density product derived from GLM (reds and yellows indicate a higher flash density). This product shows where lightning is occurring and how many flashes are occurring in a given area per time. This product can be used to identify when convection begins and where the strongest storms are located. In particular, a higher flash density generally corresponds with stronger convection (in this case, the coldest cloud tops). In addition, prior research indicates that severe weather (e.g., strong winds, hail, and/or tornadoes) are sometimes preceded by a strong increase in flash density with time. Thus, another use of the flash extent density product is to help provide severe weather warnings. The panel in the top-right illustrates the total optical energy product (magenta and yellowish colors indicate more energy). This product basically shows how dim or bright the cloud is. Brighter areas typically indicate deep clouds with lightning, while dimmer clouds are shallower clouds that are illuminated indirectly by lightning in nearby deeper clouds. Finally, the bottom-left panel provides an indication of the area covered by flashes. Typically, smaller flashes represent new or rapidly intensifying convection, while large flashes indicate dying convection.
In summary, GLM complements observations of lightning from ground-based networks. Products derived from GLM have numerous applications including severe weather nowcasting, distinguishing intensifying from weakening convection, identifying and warning areas where it may not be raining but may still be at risk of lightning, etc.
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©2021 Meteorologist Dr. Ken Leppert II