Surprising sensitivity of lightning and related storm processes to environment on 4 June 2012 in West Texas

On 4 June 2012, during the Deep Convective Clouds and Chemistry field campaign, relatively disorganized cellular convection developed near the New Mexico-West Texas border and was observed by the West Texas Lightning Mapping Array (WTLMA). This convection contained an extremely unusual variety of charge structures in contemporaneous storms only tens of kilometers apart. Among boundary-layer state variables, the Texas Tech University Weather Research and Forecasting ensemble model's representation of relative humidity near 700 mb (below cloud base) correlated best to the observed distribution of charge. Because the surface RH did not directly correlate to the 700 mb pattern, this suggests that the parcels must have been modified before entering the cloud, impacting the microphysics within the cloud and thus electrification within the storms. Resulting differences among storms were also observed in other lightning characteristics such as flash rates and flash sizes and in overall storm evolution. For example, the storms over drier air were dominated by mid-level positive charge, which agrees with the theory of slower depletion rate of cloud liquid water promoting more positive charging to graupel. These storms had fewer cloud-to-ground flashes and were slower to intensify or cluster but were still relatively long lasting. As more organized storms developed along the outflows from the initial convection and in overturned air, the charge structures became more uniform across the region, overall flash rates increased and a few severe events occurred. This case allows for a detailed and unique examination of how relatively small changes in environmental conditions (a few °C in temperature and dew point) and forcing mechanisms can result in different storm-scale characteristics morphologically and electrically.