Impact of Kinematics, Microphysics, and Electrification on the Formation of Bounded Weak Lightning Regions in a Simulated Supercell Storm

This study investigates the impact of evolving kinematics, microphysics, and electrification on the development of bounded weak lightning regions (BWLRs; also known as "lightning-weak holes") in a simulated supercell storm.  The first 6 hours of evolution of the tornadic supercell storm which passed near Geary, Oklahoma, USA on 29-30 May 2004 are simulated with NSSL's three-dimensional COMMAS cloud model which includes airflow dynamics, bulk microphysics, hydrometeor and ion charging, and a discrete branched lightning parameterization.  The observed storm produced intense low-level mesocyclonic rotations and several tornadoes, as well as large hail and copious intracloud (IC) lightning as well as predominantly negative cloud-to-ground (CG) flashes.  The Geary supercell simulation details the evolution of the following storm characteristics from convection initiation (CI) through the storm's mature phase: (1) airflow; (2) cloud and precipitation (including hail); (3) the net space charge and the space charges on individual hydrometeor categories and ions; (4) the electrical potential; (5) the vector electric field; and (6) IC and CG lightning flashes.  As revealed by validation using radar and lightning mapping array (LMA) observations, several of the latter features have similar simulated and observed morphologies.

The time-varying vertical profiles of temperature, vapor mixing ratio, and horizontal winds in the simulated storm environment are all prescribed within the running model via a continuous base-state substitution (BSS) approach.  The BSS method manages the model's continuous assimilation of four observed storm-following mobile environmental soundings as the simulated storm tracks eastward during the 6-hour simulation period.  The storm environment changes significantly from a weakly sheared, moderately unstable air mass with high surface temperatures near a dryline at the time of CI to an environment characterized by a strong inhibiting temperature inversion, very large values of vertical wind shear and convective instability, and lower surface temperatures.  An example 3-D view of surface temperature (colorbar), the outer surface of cloud and precipitation (gray), and the positive (red) and negative (light blue) lightning leader density cores in the simulated storm at 3 hours is shown in the top panel of the attached image.

The bounded weak echo region (BWER) in the observed and simulated Geary storm locates the precipitation-free portion of the main supercell updraft which is hypothesized to also be relatively free of individual and net charges, thus hypothetically leading to absence of propagating flashes and the formation of simulated BWLRs.  For example, horizontal and vertical cross-sections of the negative lightning leader density (i.e., similar to readily LMA-observed negative breakdown) and space charge on graupel through the BWLR and BWER in the simulated storm at 3 hours are shown in the lower left and lower right panels of the attached image respectively (red contour = BWLR, green contour = 40 dBZ, yellow contour = 40 m s^-1 updraft, gray contour = cloud outline).  However, the simulated Geary storm exhibits a physically- and internally-consistent cycling behavior of its main updraft, low-level mesocyclone, and precipitation core that is manifested in a repeated cycle of formation followed by filling of the simulated BWERs and BWLRs.  We will explore the hypothesis that development of the low-level mesocyclone injects graupel-meltwater rain via the radar hook-echo into the base of the main updraft, leading to rapid vertical rainwater advection and midlevel freezing to form riming graupel that subsequently fills the BWER and rapidly initiates noninductive graupel-ice charging.  We further hypothesize that as the mixed-phase BWER fills with charging graupel, new charge centers begin forming in the midlevel updraft that support both lightning flash initiation and propagation and ultimately the infilling of the transient BWLRs.