Electrical, Kinematic, and Microphysical Contrasts between Supercells
Exhibiting Normal and Anomalous Charge Structures in the Southeastern
United States
Abstract
Thunderstorms exhibiting anomalous charge structures (ACSs, i.e.,
anomalous storms) are comprised by more riming ice carrying net positive
versus negative charge, thought to result from increased mixed-phase
cloud liquid water content (LWC). Anomalous storms are rarely observed
in the United States (US) outside of the Great Plains (GP) region, where
environmental conditions that suppress warm precipitation efficiency and
support robust updrafts are thought to favor increased mixed-phase LWC.
Two rare anomalous supercells in the Southeastern (SE) US exhibited
similar charge structure characteristics as observed in GP anomalous
storms, including a deep positive (negative) charge layer associated
with riming (non-riming) ice in the lower (upper) mixed-phase region.
However, most characteristics associated with SE anomalous environments
were not consistent with those in the GP. A more rigorous evaluation of
hypotheses concerning ACS development and their observation in the SE
compared electrical, kinematic, microphysical, and environmental
properties between the two anomalous and two SE normal supercells (i.e.,
exhibiting normal charge structures). Similar metrics of warm
precipitation efficiency were observed in each. However, lower relative
humidity in the charging region of the SE anomalous storms uniquely
matched environmental characteristics in GP anomalous storms and
differentiated SE anomalous from normal environments, suggesting the
relative importance of saturation ratio alongside LWC in positive
charging of riming ice. Differences were also observed in charge region
characteristics and flash locations between a SE anomalous and normal
storm. As riming ice increased in the negative charge region of the ACS,
flash initiation locations were increasingly observed in stronger
updrafts and updraft gradients compared with the normal storm. The
evolution of microphysical characteristics of the negative charge region
in the anomalous storm suggested an increase in normal alongside
anomalous charging, indicating that variability in charging polarity may
impact spatial flash relationships with the updraft. Further work is
needed to diagnose whether particle-scale charging variability
influences flash rate relationships with convective parameters such as
updraft or graupel volume.