Microphysical, Dynamical, and Lightning Processes Associated with Anomalous Charge Structures in Isolated Convection

August 16, 2017

Brody Fuchs

Hosted by Steven Rutledge (advisor), Jeffrey Pierce, Brenda Dolan, Susan van den Heever, Richard Eykholt (Physics)

Abstract

Internal storm charge structures are linked to storm microphysics and dynamics. This study leverages available radar-based microphysical and dynamical information from recent field campaigns to investigate the processes that influence storm-scale charge structures. Nine normal polarity (mid-level negative charge) cases that occurred in northern Alabama, and six anomalous polarity (mid-level positive charge) cases that occurred in northeastern Colorado are studied in detail. The results suggest the presence of positively charged mid-level graupel in anomalous polarity storms, which is consistent with large amounts of supercooled liquid water (SCLW). Even though the normal polarity storms have more thermodynamic instability, the anomalous polarity storms have broader and stronger updrafts in addition to more robust mixed-phase microphysics. We expect the broader and stronger updrafts in anomalous Colorado storms are more resistant to dilution by entrainment. Using representative updraft speeds and warm cloud depths, the amount of time a parcel spends in the warm phase of a cloud was estimated for each storm observation. This metric is found to be the key discriminator between the two storm populations as the stronger updrafts and shallower warm cloud depths in Colorado lead to much shorter warm cloud residence time in those storms. We hypothesize this parameter strongly influences the amount of SCLW in the mid-levels because it impacts the loss of liquid water in the warm phase of the cloud via autoconversion and coalescence.

Using a recently developed automated flash clustering algorithm on multiple years of ground-based lightning mapping array (LMA) data, approximately 63 million lightning flashes were identified and analyzed from Washington DC, northern Alabama, and northeast Colorado. While LMA-based average annual flash density values in Washington DC (~ 20 flashes km-2 yr-1) and Alabama (~ 35 flashes km-2 yr-1) are within 50\% of corresponding satellite estimates, LMA-based estimates are approximately a factor of 3 larger (~ 50 flashes km-2 yr-1) than satellite estimates in northeast Colorado. By estimating the initiation and propagation of lightning channels with LMA data, we find that flashes were produced at lower altitudes in Colorado, compared to Alabama or Washington DC. This is a result of the storm charge structures in these regions as normal polarity storms (common in Alabama and Washington DC) produce systematically higher altitude flashes and anomalous storms (common in Colorado) produce systematically lower altitude flashes.