Changes in lightning characteristics over the conterminous United States (CONUS) are examined to support the National Climate Assessment (NCA) program. Details of the variability of cloud-to-ground (CG) lightning characteristics over the decade 2003-12 are provided using data from the National Lightning Detection Network (NLDN). Changes in total (CG 1 cloud flash) lightning across part of the CONUS during the decade are provided using satellite Lightning Imaging Sensor (LIS) data. The variations in NLDN-derived CG lightning are compared with available statistics on lightning-caused impacts to various U.S. economic sectors. Overall, a downward trend in total CG lightning count is found for the decadal period; the 5-yr mean NLDN CG count decreased by 12.8% from 25 204 345.8 (2003-07) to 21 986 578.8 (2008-12). There is a slow upward trend in the fraction and number of positive-polarity CG lightning, however. Associated lightning-caused fatalities and injuries, and the number of lightning-caused wildland fires and burn acreage also trended downward, but crop and personal-property damage costs increased. The 5-yr mean LIS total lightning changed little over the decadal period. Whereas the CONUSaveraged dry-bulb temperature trended upward during the analysis period, the CONUS-averaged wet-bulb temperature (a variable that is better correlated with lightning activity) trended downward. A simple linear model shows that climate-induced changes in CG lightning frequency would likely have a substantial and direct impact on humankind (e.g., a long-term upward trend of 18C in wet-bulb temperature corresponds to approximately 14 fatalities and over $367 million in personal-property damage resulting from lightning).
Sources of middle atmosphere nitrogen oxides, including transport from the troposphere and production in situ by energetic electrons, are currently not well characterized. Production of nitrogen oxides (NOx) in the middle atmosphere by transient luminous events (TLEs), such as red sprites and blue jets has previously been estimated from satellite observations and modeling studies. This is the first laboratory attempt to estimate NOx production by TLEs, following studies that have confirmed electrical similarities between laboratory discharges and TLEs. A pressure‐controlled chamber and high‐voltage power supplies simulated middle atmosphere discharges. Chemiluminescence NOx analyzers sampled NOx following the completion of the chamber discharges, which was used to calculate total NOx production for each discharge as well as NOx per ampere of current and NOx per Joule of discharge energy. Three different production efficiencies in NOx/J as a function of pressure pointed to three different production regimes: one for tropospheric pressures (100–500 mb), one for stratospheric pressures (1–100 mb), and one for upper stratospheric to mesospheric pressures (no greater than 1 mb). Discharges at jet‐like pressures are measured to produce 1.7 × 1016 to 6.40 × 1017 molecules of NOx per discharge, while discharges at sprite‐like pressure produce 6.97 × 1013 to 8.57 × 1013 molecules of NOx per discharge. Blue jets were calculated to produce 1.7 × 1022 to 7.4 × 1026 molecules of NOx, while red sprites were calculated to produce 6.8 × 1023 to 6.3 × 1027 molecules of NOx. On the basis of global sprite frequency estimates global annual NOx production by sprites is estimated to be between 7 × 1023 and 2 × 1028 molecules per second.
This study investigates the kinematic and microphysical control of lightning properties, particularly those that may govern the production of nitrogen oxides (NOX = NO + NO2) via lightning (LNOX), such as flash rate, type, and extent. The NASA Lightning Nitrogen Oxides Model (LNOM) is applied to lightning observations following multicell thunderstorms through their lifecycle in a Lagrangian sense over Northern Alabama on 21 May 2012 during the Deep Convective Clouds and Chemistry (DC3) experiment. LNOM provides estimates of flash rate, type, channel length distributions, channel segment altitude distributions (SADs), and LNOX production profiles. The LNOM‐derived lightning characteristics and LNOX production are compared to the evolution of radar‐inferred updraft and precipitation properties. Intercloud, intracloud (IC) flash SAD comprises a significant fraction of the total (IC + cloud‐to‐ground [CG]) SAD, while increased CG flash SAD at altitudes >6 km occurs after the simultaneous peaks in several thunderstorm properties (i.e., total [IC + CG] and IC flash rate, graupel volume/mass, convective updraft volume, and maximum updraft speed). At heights <6 km, the CG LNOX production dominates the column‐integrated total LNOX production. Unlike the SAD, total LNOX production consists of a more equal contribution from IC and CG flashes for heights >6 km. Graupel volume/mass, updraft volume, and maximum updraft speed are all well correlated to the total flash rate (correlation coefficient, ρ ≥ 0.8) but are less correlated to total flash extent (ρ ≥ 0.6) and total LNOX production (ρ ≥ 0.5). Although LNOM transforms lightning observations into LNOX production values, these values are estimates and are subject to further independent validation.
A model is introduced for estimating the nitrogen oxides (NO x 5 NO 1 NO 2 ) production from a lightning return stroke channel. A realistic modified transmission line model return stroke current is assumed to propagate vertically upward along a stepped leader channel of 0.1-cm radius. With additional assumptions about the initial radial expansion rate of the channel, the full nonlinear differential equation for the return stroke channel radius r(z, t) is solved numerically using Mathematica V9.0.1.0. Channel conductivity and channel air density are adjustable constants, and the model employs typical atmospheric profiles of temperature, pressure, and density. The channel pressure is modeled by a dynamic pressure expression. Channel temperature is extracted from the pressure by a minimization technique that involves a generalized gas law appropriate for high temperatures where dissociation and ionization are important. The altitude and time variations of the channel energy density are also obtained. Three model runs, each with different input parameters, are completed. Channel radii at sea level range from about 1.7 to 6.0 cm depending on the model inputs and are in good agreement with other investigators. The NO x production from each 1-m segment of the channel is computed using conservation of energy and equilibrium freeze-out-temperature chemistry. Because the NO x per meter of channel is computed as a function of altitude, extensions of the results to tortuous and branched channels are possible and lead to preliminary estimates of total return stroke NO x . These estimates are found to be smaller than the return stroke NO x values obtained from the NASA Lightning Nitrogen Oxides Model (LNOM).
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