Comprehensive Characterization of Lightning Parameters during the
Houston Environmental Aerosol Thunderstorm Project (HEAT)

 

Abstract

The Houston Lightning Anomaly (HLA) was discovered by analysis of National Lightning Detection Network (NLDN) cloud-to-ground (CG) lightning flash data which indicated a >50% enhancement in flash densities over and downwind of Houston (Orville et al. 2001). Houston’s declining air quality has engendered multi-agency investigations of this sub-tropical megacity's impacts on its environment. The Houston Environmental Aerosol Thunderstorm Project (HEAT) is an exploration of the influences of pollutants, changes in land use, energy balance and moisture fluxes imposed by a major city, one whose convective climate is also substantially modulated by complex sea breeze circulations. Lightning integrates the complex interplays of microphysical, dynamical and charge generation/separation mechanisms within convective storms. Thus, systematic changes in lightning’s properties represent yet another in a series of indications that large urban areas significantly influence regional convective processes. But before one can explain any such effects, they must first be adequately documented. We propose contributing fundamental lightning measurements, complementing those already planned, to quantify the HLA as thoroughly as possible. The end product will be a comprehensive database of lightning parameters for HEAT researchers. We contend that a robust description of HLA lightning (the raison d’etre for HEAT) will be fundamental to investigating its underlying physics and quantifying its impacts (NOx production, infrastructure damage, etc.).

Intellectual Merit: The HLA has been defined to date only by NLDN flash density. HEAT plans to deploy a 3-D lightning mapping array (the LDAR-II) plus an enhanced NLDN. This instrumentation will measure in-cloud (IC) flash originating points, durations, altitudes, charge removal volumes, and peak current. However, a complete characterization of the CG component is lacking, and a comprehensive HLA assessment will require additional metrics, now made possible by applying new, cost-effective technologies. We propose that available remote sensing systems (developed under NSF support) should provide additional stroke-level lightning metrics, including charge moment change (ΔMq), charge lowered to ground (Q), continuing current moments and amplitudes, independent estimates of peak currents, and true stroke multiplicities and intervals.

New RF lightning characterization methods developed at Duke University [first applied during the 2000 Severe Thunderstorm Electrification and Precipitation Study (STEPS)] demonstrated by using ELF/VLF transient analysis techniques that exceeding ΔMq thresholds, not peak current, defined which +CGs induced mesospheric sprites (Hu et al. 2002; Lyons et al. 2003b). This lead to recent discoveries in which operational measurements of the impulse ΔMq (first 2 ms) for virtually all strokes in different storm types revealed unexpected patterns in ΔMq (and implicitly, Q) unrelated to peak current (Cummer and Lyons 2004). Within HEAT’s LDAR-II coverage, Q can also be directly computed, arguably a more HEAT-relevant parameter than peak current because it reflects lightning and electrification processes on a larger scale.

Many strokes contain continuing currents which are unmeasured by conventional methodologies. Yet these are major contributors to charge removal and may likewise be highly variable within the HLA. ULF transient analyses will estimate stroke continuing current durations and intensities.

The NLDN detection efficiency (DE) will be <1.00 for flashes and substantially less than unity for the component strokes. Is it possible that the HLA consists of more flashes but fewer total strokes? Lightning High Speed Imagers (HSI) will document the true stroke multiplicity (and calibrate the NLDN-derived statistics). The HSI will also visually corroborate the ULF results an also detect ICs misidentified as NLDN CG strokes.

Products will include an impulse ΔMq and estimated Q climatological database for all HLA NLDN-detected strokes for summers 2004-2005-2006. A one month intensive field program (August 2005) will deploy a HSI to determine true stroke multiplicities, multi-point attachments and detect misidentified ICs (for calibrating the NLDN) and quality-control ULF-derived continuing current durations and intensities. A secondary goal will be to ascertain whether the NLDN-detected discontinuity in median and large (>200kA), negative (but not positive) CG peak currents between land and salt water in the Gulf is a network artifact or a heretofore unreported characteristic of salt water CG discharges (Lyons et al. 1998). Also, low-light cameras will search above nocturnal Gulf storms for giant jets, believed generated by maritime deep convection.

Broader Impacts: This two year, collaborative research effort leverages the lightning-related expertise of two universities and a private-sector research group. It involves graduate student training (Duke), undergraduate meteorology major participation (FMA, UNC) and evaluates advanced instrumentation funded by NSF. We have planned a strong public outreach effort (FMA, UNC). FMA will document HEAT activities using HD video, assembling highlights for news and documentary media use. We will produce a professional-quality DVD summary presentation suitable for lay and professional audiences. HEAT findings will be added to www.Sky-Fire.TV, FMA’s NSF-funded informal science education outreach web site. Once the companion results from other HEAT investigators emerge, a follow-on, in-depth data analysis effort may be proposed as warranted.