Search

On the afternoon of 4 June 2012 several single and multicell storms developed within range of the West Texas Lightning Mapping Array (WTLMA). Some of these storms had anomalous charge structures (primarily positive charge in the mixed-phase region) and some normal (primarily negative at mid-levels), even through they existed concurrently. None of the expected controls on storm charge polarity explained the net charge differences; however, there had been previous convection in the regions that had normal polarity storms. A WRF ensemble suggested that this previous convection may have moistened the mid-levels of the atmosphere especially near cloud base. In this study we investigate whether entrainment can enhance positive graupel charging in the mid-levels and produce anomalous charge structures. Idealized storm simulations in these environments provided two important insights. First, the updrafts' effective supercooled liquid water and rime accretion rates, important factors to electrification, reached their maximum values midway through the mixed-phase, meaning the inverted tripole charge structure can occur purely through non-inductive charging---an adjustment to the accepted paradigm. Second, through entrainment favoring smaller cloud droplets the rime accretion rates in the most active charging regions were more influenced by the magnitude of dry air at cloud base than the surface or instability characteristics. One tested electrification parameterization even captured different resulting charge structures but not at the heights or timing observed. The limiting factor of electrification within the simulations was cloud droplet freezing. Interpolations towards a colder, still reasonable freezing level produced profiles of updraft charging regions matching the different observed charge structures.

The airborne dust prevalent in West Texas and many other arid areas is known to become charged through triboelectric charging, which can be observed through measurements of the atmospheric electric field. Laboratory studies have focused on predicting and understanding the extremely near-surface correlations between dust composition, wind speeds and electrical charge, but few observational studies have occurred outside of the saltation layer or with relatively high frequency observations. An observational station was deployed on the western side of Lubbock, Texas in order to examine the correlation of the wind speed and electric field within the suspended dust layer using a downward-facing, reciprocating-shutter electric field mill at two meters and an ultrasonic anemometer at 1.68 meters above the surface. Measurements were taken at a frequency of one hertz and filtered to produce a three decibel power decrease at 7.1 seconds. Data were examined from eleven blowing dust events during 2012 in addition to 22 electrical fair-weather cases for comparison. While the diurnal fair-weather electric field pattern and its fluctuations were similar to that expected from the global electrical circuit and the electrode effect with an average value of -110.4 volts per meter, greatly different values were associated with the presence of blowing dust. During blowing dust events the electric field had large fluctuations and reached positive values over ten kilovolts per meter with total event averages up to two kilovolts per meter, indicating large amounts of negatively charged dust suspended above the two-meter measurement height. At low frequencies the correlation between the electric field and the wind speed is expected to be positive based on previous research, where increased winds loosen more dust from the surface which charges predictably and creates stronger electric fields. This study confirms the positive relationship through correlation coefficients up to 0.81 for five minute average values and by mostly positive covariance values in each case with respect to averaging durations greater than ten minutes. However, the covariance for each case was found to be either negative or near zero with respect to averaging durations less than thirty seconds, suggesting a higher frequency negative correlation. This was confirmed by negative correlation coefficients up to 0.23 in magnitude for fluctuations of less than thirty seconds. The minimum length of averaging window necessary to resolve a positive covariance differed in each case and appears to have some dependence on the concentration of blowing dust, approximated by the visibility. No time lag between the wind speed and the electric field was resolved, suggesting a direct linkage. A basic charge model analysis found that this phenomenon is likely due to the vertical rearrangement of charge from the expected steady-state profile and the inclusion of positively-charged soil from the surface.