NO<sub>x</sub> production by lightning in Hector: first airborne measurements during SCOUT-O3/ACTIVE

Abstract

Abstract. During the SCOUT-O3/ACTIVE field phase in November–December 2005, airborne in situ measurements were performed inside and in the vicinity of thunderstorms over northern Australia with several research aircraft (German Falcon, Russian M55 Geophysica, and British Dornier-228. Here a case study from 19 November is presented in detail on the basis of airborne trace gas measurements (NO, NOy, CO, O3) and stroke measurements from the German LIghtning Location NETwork (LINET), set up in the vicinity of Darwin during the field campaign. The anvil outflow from three different types of thunderstorms was probed by the Falcon aircraft: (1) a continental thunderstorm developing in a tropical airmass near Darwin, (2) a mesoscale convective system (MCS), known as Hector, developing within the tropical maritime continent (Tiwi Islands), and (3) a continental thunderstorm developing in a subtropical airmass ~200 km south of Darwin. For the first time detailed measurements of NO were performed in the Hector outflow. The highest NO mixing ratios were observed in Hector with peaks up to 7 nmol mol−1 in the main anvil outflow at ~11.5–12.5 km altitude. The mean NOx (=NO+NO2) mixing ratios during these penetrations (~100 km width) varied between 2.2 and 2.5 nmol mol−1. The NOx contribution from the boundary layer (BL), transported upward with the convection, to total anvil-NOx was found to be minor (<10%). On the basis of Falcon measurements, the mass flux of lightning-produced NOx (LNOx) in the well-developed Hector system was estimated to 0.6–0.7 kg(N) s−1. The highest average stroke rate of the probed thunderstorms was observed in the Hector system with 0.2 strokes s−1 (here only strokes with peak currents ≥10 kA contributing to LNOx were considered). The LNOx mass flux and the stroke rate were combined to estimate the LNOx production rate in the different thunderstorm types. For a better comparison with other studies, LINET strokes were scaled with Lightning Imaging Sensor (LIS) flashes. The LNOx production rate per LIS flash was estimated to 4.1–4.8 kg(N) for the well-developed Hector system, and to 5.4 and 1.7 kg(N) for the continental thunderstorms developing in subtropical and tropical airmasses, respectively. If we assume, that these different types of thunderstorms are typical thunderstorms globally (LIS flash rate ~44 s−1), the annual global LNOx production rate based on Hector would be ~5.7–6.6 Tg(N) a−1 and based on the continental thunderstorms developing in subtropical and tropical airmasses ~7.6 and ~2.4 Tg(N) a−1, respectively. The latter thunderstorm type produced much less LNOx per flash compared to the subtropical and Hector thunderstorms, which may be caused by the shorter mean flash component length observed in this storm. It is suggested that the vertical wind shear influences the horizontal extension of the charged layers, which seems to play an important role for the flash lengths that may originate. In addition, the horizontal dimension of the anvil outflow and the cell organisation within the thunderstorm system are probably important parameters influencing flash length and hence LNOx production per flash.

Cloud-resolving chemistry simulation of a Hector thunderstorm

“…Unlike the Brazilian (Huntrieser et al, 2008) and Australian (Huntrieser et al, 2009) thunderstorms, the AMMA MCSs showed a greater influence from boundary layer versus lightning NO x . The tropical and subtropical MCSs investigated during AMMA indicated LNO x production rates (70 moles flash −1 and 179 moles flash −1 , respectively) were comparable to those observed in similar air mass thunderstorms during TROCCINOX.…”

“…Based on the Tropical Convection, Cirrus, and Nitrogen Oxides Experiment (TROCCINOX) in Brazil, the LNO x mass production for tropical thunderstorms was of similar magnitude as that found for one of the BIBLE-C storms (∼ 70 moles flash −1 ), while a subtropical thunderstorm also analyzed during the campaign had a production rate of ∼ 140-210 moles flash −1 (Huntrieser et al, 2008). Huntrieser et al (2009) (Labrador et al, 2009), while later in the ACTIVE experiment (e.g., January 2006) enhancements in LNO x were found over widespread regions.…”

Direct satellite observation of lightning-produced NO<sub>x</sub>

“…But at least most events with low P are indeed tropical events, which might thus be less productive due to low wind shears. Huntrieser et al (2009) also proposed that warm rain processes, which might be dominant for the tropical morningtime thunderstorms of this study, may result in very short flash components. I.e., though the detected events show -by selection -high FRD, these flashes might generally be less productive in producing NO x compared to flashes for mixedphase precipitation processes with probably longer flash channels.…”

“…They propose that tropical thunderstorms have generally shorter flash lengths, and are thus generally less productive with respect to LNO x /flash, than subtropical thunderstorms, as a consequence of enhanced vertical wind shear of the latter. In Huntrieser et al (2009) correlations of P with vertical wind shear are reported using in-situ measurements from the TROCCINOX and SCOUT-O 3 campaigns.…”

“…There are indications that PE might be related to warm rain vs. mixed-phase precipitation processes (Huntrieser et al, 2009). Thus, in order to investigate the presence of hail for the detected lightning events, we also analyzed polarizationcorrected temperatures (PCT) from brightness temperature measurements of TMI on TRMM for the coincident overpasses as defined in Appendix A. PCT are calculated as defined in Spencer et al (1989) (for 85 GHz, see Eq.…”