[1] Airborne in situ measurements of NO, NO 2 , NO y , CO, CO 2 , O 3 , J(NO 2 ), and CN were performed in European thunderstorms during the field experiment EULINOX in July 1998. The measurements in the upper troposphere show enhanced NO x (= NO + NO 2 ) concentrations within thunderstorms and their outflow at horizontal scales from 300 m to several 100 km. The maximum NO mixing ratio measured inside a thundercloud close to lightning (the aircraft was also hit by a small lightning strike) was 25 ppbv. A regional NO x enhancement of 0.5 ppbv over central Europe could be traced back to a thunderstorm event starting $24 hours earlier over Spain. The fractions of NO x in thunderclouds which are produced by lightning and convectively transported from the polluted boundary layer are determined by using CO 2 and CO as tracers for boundary layer air. The analyses show that on average about 70% of the NO x increase measured in the anvil region was found to result from production by lightning and about 30% from NO x in the boundary layer. Thunderstorms are also strong sources of small particles. The peak CN concentrations measured within thunderstorm outflows (>30,000 particles STP cm À3 ) were distinctly higher than in the polluted boundary layer. The amount of NO x produced per thunderstorm and NO produced per lightning flash was estimated. The results imply that the annual mean NO x budget in the upper troposphere over Europe is dominated by aircraft emissions (0.1 TgN yr À1 ) in comparison to lightning production ($0.03 TgN yr À1 ). On the global scale, NO x produced by lightning (mean 3 TgN yr À1 ) prevails over aircraft-produced NO x (0.6 TgN yr À1 ).

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We have analyzed the lightning activity recorded during the Stratosphere‐Troposphere Experiment: Radiation, Aerosols, and Ozone (STERAO‐A) July 10, 1996, storm by the Office National d'Etudes et de Recherches Aérospatiales (ONERA) lightning VHF interferometer and the National Lightning Detection Network (NLDN) system. Both cloud‐to‐ground and total lightning activity were observed and studied for the entire 5‐hour life of the storm. The July 10 storm was a multicellular complex, which became unicellular during the last hour. It primarily exhibited high intracloud activity with only 1.5% cloud‐to‐ground flashes. The maximum value of the total flash rate was 58 flashes per minute. Cloud‐to‐ground (CG) flashes occurred after some intracloud flashes with a delay ranging from 3 to 26 min for the different cells of the storm. Our study revealed that measured flash duration ranged from 23 μs to 1.8 s. Flash duration, averaged over 5‐min periods, increased during the storm life. Short‐duration flashes (<1 ms) did not occur until 30 min after the initial flash in the storm when the 50 dBZ vertical profile reached 8 km mean sea level (msl). The short‐duration flashes were recorded in cells where high reflectivity reached high altitude. Detailed analysis showed that the ONERA and NLDN reports were temporally and spatially consistent in the measurement of the cloud‐to‐ground flashes. Finally, we developed a new technique to distinguish negative CG flashes from other flashes by identifying the VHF signature of the negative downward stepped leader‐return stroke process in the flash VHF signal.

UAVs are relevant for monitoring cliff faces. In this study, several flights are performed with various imaging angles (nadir, 20°, 30° and 40° off-nadir) to assess the impact of the imaging angle on the 3D cliff face reconstruction. Occlusions issues arising with sub-vertical cliffs make nadir surveys nearly irrelevant. The results obtained with 20°, 30° and 40° off-nadir imaging angles are satisfactory regarding texture restitution and accuracy with respectively 5.5 cm, 5.9 cm and 4.9 cm of error, higher tilting angles yielding better reconstructions on sub-vertical or overhanging parts of the cliff. This article also investigates other parameters affecting tiepoint detection on the cliff face, as the effective overlap, the UAV-cliff face distance and the cliff face illumination. Guidelines are provided for UAV survey parameterization, aiming at capturing the whole cliff face with a good trade-off between distance to the cliff, flight height and spatial resolution of the photographs. Highlights Impact of the UAV camera tilting angle on the 3D cliff face reconstruction  Because of occlusions, irrelevance of nadir pointing camera for sub-vertical cliffs  Satisfying accuracy and texture for both 20°, 30° and 40° off-nadir imaging angles  Other parameters impacting tiepoint detection in the reconstruction process