Polarimetric Radar and Electrical Observations of Deep Moist Convection across northern Alabama during the DC3 Experiment

The Deep Convective Clouds and Chemistry (DC3) experiment seeks to examine the relationship between deep moist convection (DMC) and the production of nitrogen oxides (NO_X) via lightning (LNO_X). The focus of this study will be to examine integrated storm microphysics, kinematics and lightning properties of DMC across northern Alabama (NA) during the field phase of the DC3 campaign through use of polarimetric radar and lighting mapper platforms. UAHuntsville's Advanced Radar for Meteorological and Operational Radar (ARMOR) and the National Weather Service Weather Surveillance Radar (WSR-88D) located at Hytop, AL (KHTX) comprises the multi-Doppler network across NA. The National Aeronautical and Space Administration's (NASA) north Alabama Lightning Mapping Array (NA LMA) in conjunction with Vaisala's National Lightning Detection Network (NLDN) allow for a thorough depiction of total lightning. We explore the ability of radar inferred kinematic (e.g., updraft volume) and microphysical (e.g., precipitation ice mass, graupel volume) measurements to parameterize flash rates for estimation of LNO_X production in cloud resolving models without explicit lightning. Two storms have been analyzed so far; 21 May 2012 (S1) and one on 11 June 2012 (S2).

ARMOR and KHTX multi-Doppler wind synthesis revealed that the onset of lightning coincided or slightly lagged an increase in updraft strength within the mixed-phase region (0 °C to -40 °C) of DMC for both case days. Additionally, polarimetric radar information from ARMOR and KHTX confirmed the presence of millimeter sized ice particles, which were generated as a result of the stronger upward vertical motions within the updraft on both case days. As expected from the non-inductive charging (NIC) mechanism, the evolution of updraft volume (w > 3 and > 5 m s^-1) and radar reflectivity (Z) estimates of precipitation ice mass (M) within the mixed-phase zone are well correlated to the trend of lightning flash rate. However, a useful radar-based flash rate parameterization must provide accurate quantification of rates in addition to proper trends.

For S1 and S2, the difference reflectivity (Z_dp) was used to estimate Z associated with ice and then a single Z-M relation was employed to calculate M in the mixed-phase region. Using this approach it was estimated that S1 produced an order of magnitude greater M than S2. Despite this difference, the peak flash rates and storm total flash counts for both storms were nearly identical. Expectations based on the NIC theory suggest that the precipitation ice mass to lightning flash rate (F) ratio (M/F) should be stable from storm-to-storm, all else being equal. Further investigation revealed that the mean mixed-phase Z was 11 dB higher in S1 compared to S2, suggesting larger diameters and lower concentrations of ice particles in S1. Reduction by an order of magnitude of the intercept parameter (N₀) of an assumed exponential ice particle size distribution within the Z-M relation for S1 only resulted in a proportional reduction in S1's inferred M and therefore a similar M/F for both storms. This example illustrates the sensitivity of precipitation ice mass calculations to the form of the Z-M relation generally and to variability of N₀ specifically and thus demands closer inspection of methods employed. Ongoing analysis is exploring the tuning of N₀ within the Z-M relation by the mean Z in the mixed-phase zone. A related approach is to use polarimetric fuzzy-logic methods to identify classes of ice (e.g., aggregates, graupel, and hail) and apply distinct Z-M relations for each class. The relative benefits of each approach for estimation of M will be explored. The suitability of these M estimates and other radar properties (graupel or updraft volume, ice fluxes) for parameterizing F (and hence LNO_X) will be investigated on different storm types, including various cells across NA on 18 May, 21 May and 11 June 2012.