Lightning activity in Brazilian thunderstorms during TROCCINOX: implications for NOx production

[摘要] During the TROCCINOX field experiment in January and February 2005, thecontribution of lightning-induced nitrogen oxides (LNOx) from tropical andsubtropical thunderstorms in Southern Brazil was investigated. Airborne tracegas measurements (NO, NO_y, CO and O₃) were performed up to12.5 km with the German research aircraft Falcon. During anvilpenetrations in selected tropical and subtropical thunderstorms of 4 and 18February, NO_x mixing ratios were on average enhanced by 0.7–1.2 and0.2–0.8 nmol mol⁻¹ totally, respectively. The relative contributionsof boundary layer NO_x (BL-NOx) and LNOx to anvil-NO_x werederived from the NO_x-CO correlations. On average ~80–90% ofthe anvil-NO_x was attributed to LNOx. A Lightning LocationNetwork (LINET) was set up to monitor the local distribution ofcloud-to-ground (CG) and intra-cloud (IC) radiation sources (here called"strokes") and compared with lightning data from the operational Braziliannetwork RINDAT (Rede Integrada Nacional de Detecção deDescargas Atmosféricas). The horizontal LNOx mass flux out of theanvil was determined from the mean LNOx mixing ratio, the horizontal outflowvelocity and the size of the vertical cross-section of the anvil, and relatedto the number of strokes contributing to LNOx. The values of these parameterswere derived from the airborne measurements, from lightning and radarobservations, and from a trajectory analysis. The amount of LNOx produced perLINET stroke depending on measured peak current was determined. The resultswere scaled up with the Lightning Imaging Sensor (LIS) flash rate(44 flashes s⁻¹) to obtain an estimate of the global LNOx productionrate. The final results gave ~1 and ~2–3 kg(N) per LIS flashbased on measurements in three tropical and one subtropical Brazilianthunderstorms, respectively, suggesting that tropical flashes may be lessproductive than subtropical ones. The equivalent mean annual global LNOxnitrogen mass production rate was estimated to be 1.6 and 3.1 Tg a⁻¹,respectively. By use of LINET observations in Germany in July 2005, acomparison with the lightning activity in mid-latitude thunderstorms was alsoperformed. In general, the same frequency distribution of stroke peakcurrents as for tropical thunderstorms over Brazil was found. The differentLNOx production rates per stroke in tropical thunderstorms compared withsubtropical and mid-latitude thunderstorms seem to be related to thedifferent stroke lengths (inferred from comparison with laboratory data andobserved lengths). In comparison, the impact of other lightning parameters asstroke peak current and stroke release height was assessed to be minor. Theresults from TROCCINOX suggest that the different vertical wind shear maybe responsible for the different stroke lengths.

[学科分类] 大气科学