An overview of severe storm signatures in three-dimensional lightning mapping observations

Over the last few years, data from improved real-time lightning mapping systems have revealed potential signatures of severe weather that should be a useful supplement to the signatures in Doppler radar data. To understand these lightning signatures, one needs to know that much of the charge in thunderstorms appears to be produced by charge exchange during rebounding collisions between precipitating ice and small cloud particles in the mixed phase region, followed by differential sedimentation and advection to separate the charge systematically by polarity. As the charges separate, they produce lightning. Lightning channels tend to permeate the charge of each polarity.

As one would expect from such a charging mechanism, observations and numerical storm simulations suggest that lightning flash rates are proportional to precipitating ice mass and updraft mass flux in the mixed phase region. Studies have shown large, rapid increases in the flash rates of some storms 10-20 min before severe weather occurs, presumably due to intensification of the updraft leading to the severe weather. In the southeastern United States, this increase in flash rates is often followed by a decrease in flash rates a few minutes before severe weather occurs. Strong updraft pulses also sometimes produce inverted-polarity cloud flashes and positive cloud-to-ground lightning.

Because lightning tends to permeate parts of the storm that have advected from regions of charge generation, plots of the density of lightning channel segments reveal features of severe-storm structure similar to those revealed by radar reflectivity. Thus, for example, plots and movies of lightning density reveal rising maxima (“bubbles”) of lightning density indicative of new updrafts, overshooting storm tops, v-structure indicative of flow around a strong updraft at upper levels, and a transient minimum in lightning density (a lightning “hole”, similar to a bounded weak echo region in reflectivity) during early stages of a mesocyclone. Because three-dimensional lightning mapping systems continuously receive data from all regions of lightning activity, real-time mapping systems delineate such severe storm features with excellent temporal and spatial resolution.