[1] We use observations from two aircraft during the ICARTT campaign over the eastern United States and North Atlantic during summer 2004, interpreted with a global 3-D model of tropospheric chemistry (GEOS-Chem) to test current understanding of regional sources, chemical evolution, and export of NO x . The boundary layer NO x data provide top-down verification of a 50% decrease in power plant and industry NO x emissions over the eastern United States between 1999 and 2004. Observed NO x concentrations at 8-12 km altitude were 0.55 ± 0.36 ppbv, much larger than in previous U.S. aircraft campaigns (ELCHEM, SUCCESS, SONEX) though consistent with data from the NOXAR program aboard commercial aircraft. We show that regional lightning is the dominant source of this upper tropospheric NO x and increases upper tropospheric ozone by 10 ppbv. Simulating ICARTT upper tropospheric NO x observations with GEOS-Chem requires a factor of 4 increase in modeled NO x yield per flash (to 500 mol/ flash). Observed OH concentrations were a factor of 2 lower than can be explained from current photochemical models, for reasons that are unclear. A NO y -CO correlation analysis of the fraction f of North American NO x emissions vented to the free troposphere as NO y (sum of NO x and its oxidation products) shows observed f = 16 ± 10% and modeled f = 14 ± 9%, consistent with previous studies. Export to the lower free troposphere is mostly HNO 3 but at higher altitudes is mostly PAN. The model successfully simulates NO y export efficiency and speciation, supporting previous model estimates of a large U.S. anthropogenic contribution to global tropospheric ozone through PAN export.

[1] Time correlated high-speed video and electromagnetic data for 15 cloud-to-ground and intracloud lightning flashes reveal bursts of light, bright enough to be seen through intervening cloud, during the initial breakdown (IB) stage and within the first 3 ms after flash initiation. Each sudden increase in luminosity is coincident with a CG type (12 cases) or an IC type (3 cases) IB pulse in fast electric field change records. The E-change data for 217 flashes indicate that all CG and IC flashes have IB pulses. The luminosity bursts of 14 negative CG flashes occur 11-340 ms before the first return stroke, at altitudes of 4-8 km, and at 4-41 km range from the camera. In seven cases, linear segments visibly advance away from the first light burst for 55-200 ms, then the entire length dims, then the luminosity sequence repeats along the same path. These visible initial leaders or streamers lengthen intermittently to about 300-1500 m. Their estimated 2-D speeds are 4-18 Â 10 5 m s À1 over the first few hundred microseconds and decrease by about 50% over the first 2 ms. In other cases, only a bright spot or a broad area of diffuse light, presumably scattered by intervening cloud, is visible. The bright area grows larger over 20-60 ms before the luminosity fades in about 100 ms, then this sequence may repeat several times. In several flashes, a 1-2 ms period of little or no luminosity and small E-change is observed following the IB stage prior to stepped leader development.

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Houston. We suggest that the elevated flash densities could result from several factors, including, 1) the convergence due to the urban heat island effect, and 2) the increasing levels of air pollution from anthropogenic sources producing numerous small droplets and thereby suppressing mean droplet size.The latter effect would enable more cloud water to reach the mixed phase region where it is involved in the formation of precipitation and the separation of electric charge, leading to an enhancement of lightning.

Thunderstorm case studies and earlier observations are described which illuminate the relationship between cloud vertical development and the prevalence of intracloud (IC) and cloud-to-ground (CG) lightning. A consistent temporal evolution starting with peak IC activity changing to predominant CG activity and concluding with strong outflow (microburst) suggests that ice is responsible for both the electrical (i.e., lightning) and dynamical (i.e., microburst) phenomena. The IC activity is attributed to the updraft-driven accumulation of graupel particles in the central dipole region, and the subsequent CG activity to the descent of ice particles beneath the height of the main negative charge. The subsequent descent and melting of ice particles beneath the height of the 0øC isotherm are associated with the acceleration of the downdraft and outflow. The IC lightning precursor can provide a valuable short-term (5-10 min) warning for microburst hazard at ground level. INTRODUCTIONConvective storms are well recognized to produce two common types of lightning: intracloud (IC) and cloud to ground (CG). Some convective storms are recognized to produce strong downdrafts, often referred to as microbursts, which have been demonstrated to pose a severe hazard to commercial aviation [Fujita, 1985]. This study was initially concerned with a search for a practical short-term precursor to the microburst hazard in the cloud electrical development. In the course of this investigation, consistent relationships between the stage of the convective activity and the lightning type became apparent, and tied in closely with earlier electrical observations of thunderstorms and with contemporary observations of microburst precursors in Doppler radar observations [Campbell, 1988]. This paper is concerned with a description of these relationships and with an interpretation based on ice-phase microphysics. CASE STUDIESThe microburst-producing clouds in this study were almost invariably air mass thunderstorms in weakly sheared environments and exhibited peak lightning rates greater than or equal to three flashes/min. The IC lightning dominated over CG lightning in the early stages of vertical development. The observed temporal relationship between the peak lightning rate and the peak outflow velocity was very systematic in the storms studied. The evolution of total lightning rate, CG lightning rates (all events located within 20 km of the Doppler radar), and maximum differential velocity at the ground are recorded for several case studies, which are discussed briefly in chronological order. The velocity differential across all outflows reported here exceeded 20 m/s. A headwind-tailwind change of 10 m/s is of concern to commercial aviation, while the largest microburst velocity differentials observed by Doppler radars have been roughly 40-50 m/s. All lightning rates were tabulated in 1-min time intervals. All microbursts studied were classified as "wet" 13,213