From Extreme Flash Rates to Tropical Cyclones: Simulating Two Ends of the Spectrum

Lightning mapping systems have detected high rates of lightning initiation near the tops of convective cores, particularly in severe storms. Previous 3-D model simulations have reproduced similarly high flash rates using bulk (one or two moment) microphysics, but the details of the charge separation and sources of small ice crystals were not closely examined. New high-resolution simulations are better able to sufficiently resolve the steep vertical temperature gradients (on the order of 6-8 K/km) in deep convective updrafts and thus the droplet freezing process. The results suggest that high charge separation rates could occur as larger cloud droplets freeze homogeneously while smaller droplets remain liquid. This process far exceeds small ice particle production by vapor nucleation or ice multiplication. 

A new parameterization of ice splinter production during rain drop freezing is also being examined as a potentially important contributor to storm electrification in general, e.g., for tropical cyclones (TCs). New idealized TC simulations were performed to investigate potential linkages between lightning and TC intensity and structural changes caused by increases in wind shear or changing sea surface temperatures. In contrast to prior modeling work, the current TC simulations were able to produce realistic lightning patterns and rates in both the eyewall and the outer rainbands. The continental and tropical simulations use similar microphysical parameters except for the concentration of cloud condensation nuclei (low for maritime, higher for continental).