Abstract
Abstract. During the "African Monsoon Multidisciplinary Analysis" (AMMA) field phase in August 2006, a variety of measurements focusing on deep convection were performed over West Africa. The German research aircraft Falcon based in Ouagadougou (Burkina Faso) investigated the chemical composition in the outflow of large mesoscale convective systems (MCS). Here we analyse two different types of MCS originating north and south of the intertropical convergence zone (ITCZ, ~10° N), respectively. In addition to the airborne trace gas measurements, stroke measurements from the Lightning Location Network (LINET), set up in Northern Benin, are analysed. The main focus of the present study is (1) to analyse the trace gas composition (CO, O3, NO, NOx, NOy, and HCHO) in the convective outflow as a function of distance from the convective core, (2) to investigate how different trace gas compositions in the boundary layer (BL) and ambient air may influence the O3 concentration in the convective outflow, and (3) to estimate the rate of lightning-produced nitrogen oxides per flash in selected thunderstorms and compare it to our previous results for the tropics. The MCS outflow was probed at different altitudes (~10–12 km) and distances from the convective core (<500 km). Trace gas signatures similar to the conditions in the MCS inflow region were observed in the outflow close to the convective core, due to efficient vertical transport. In the fresh MCS outflow, low O3 mixing ratios in the range of 35–40 nmol mol−1 were observed. Further downwind, O3 mixing ratios in the outflow rapidly increased with distance, due to mixing with the ambient O3-rich air. After 2–3 h, O3 mixing ratios in the range of ~65 nmol mol−1 were observed in the aged outflow. Within the fresh MCS outflow, mean NOx (=NO+NO2) mixing ratios were in the range of ~0.3–0.4 nmol mol−1 (peaks ~1 nmol mol−1) and only slightly enhanced compared to the background. Both lightning-produced NOx (LNOx) and NOx transported upward from the BL contributed about equally to this enhancement. On the basis of Falcon measurements, the mass flux of LNOx in the investigated MCS was estimated to be ~100 g(N) s−1. The average stroke rate of the probed thunderstorms was 0.04–0.07 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. For a better comparison with other published results, LNOx estimates per LINET stroke were scaled to Lightning Imaging Sensor (LIS) flashes. The LNOx production rate per LIS flash was estimated to 1.0 and 2.5 kg(N) for the MCS located south and north of the ITCZ, respectively. If we assume, that these different types of MCS are typical thunderstorms occurring globally (LIS flash rate ~44 s−1), the annual global LNOx production rate was estimated to be ~1.4 and 3.5 Tg(N) a−1.
Highlights
Deep convection influences the chemical composition in the upper troposphere (UT) in many ways
Further away from the convective core, about 100–150 km to the north, mixing with the ambient air continued in flight segments 1 and 7 and the mean mixing ratios of O3 increased to 64 nmol mol−1 and of carbon monoxide (CO) decreased to 96 nmol mol−1
In this study we presented measurements carried out over West Africa with the German research aircraft Falcon and with the Location Network (LINET) lightning location network during the African Monsoon Multidisciplinary Analysis (AMMA) field phase in August 2006
Summary
Deep convection influences the chemical composition in the upper troposphere (UT) in many ways. The Deutsches Zentrum fur Luft- und Raumfahrt (DLR) conducted (or participated in) several EU-funded airborne field experiments in the tropics focusing on lightning-produced NOx, as the “Tropical Convection, Cirrus, and Nitrogen Oxides Experiment” (TROCCINOX) in Brazil in 2004 and 2005, and “Stratospheric-Climate links with Emphasis on the Upper Troposphere and Lower Stratosphere” (SCOUT-O3) in Northern Australia in 2005 (Huntrieser et al, 2007, 2008, 2009; Holler et al, 2009). In addition to the Falcon aircraft, we used the Russian M55 Geophysica aircraft for trace gas measurements above the convective outflow in the tropical tropopause layer (=TTL, e.g., Highwood and Hoskins, 1998; Fueglistaler et al, 2009; Law et al, 2010) Both aircraft were based in Ouagadougou (12.4◦ N, 1.5◦ W) in Burkina Faso during the AMMA SOP-2a2 experiment.
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