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Abstract. The EU HIBISCUS project consisted of a series of field campaigns during the intense convective summers in 2001, 2003 and 2004 in the State of São Paulo in Brazil. Its objective was to investigate the impact of deep convection on the Tropical Tropopause Layer (TTL) and the lower stratosphere by providing a new set of observational data on meteorology, tracers of horizontal and vertical transport, water vapour, clouds, and chemistry in the tropical Upper Troposphere/Lower Stratosphere (UT/LS). This was achieved using short duration research balloons to study local phenomena associated with convection over land, and long-durationCorrespondence to: J.-P. Pommereau (jean-pierre.pommereau@latmos.ipsl.fr) balloons circumnavigating the globe to study the contrast between land and oceans.Analyses of observations of short-lived tracers, ozone and ice particles show strong episodic local updraughts of cold air across the lapse rate tropopause up to 18 or 19 km (420-440 K) in the lower stratosphere by overshooting towers. The long duration balloon and satellite measurements reveal a contrast between the composition of the lower stratosphere over land and oceanic areas, suggesting significant global impact of such events. The overshoots are shown to be well captured by non-hydrostatic meso-scale Cloud Re- Weather Forecast Models (NWP) underestimating the overshooting process. Finally, the data collected by the HIBIS-CUS balloons have allowed a thorough evaluation of temperature NWP analyses and reanalyses, as well as satellite ozone, nitrogen oxide, water vapour and bromine oxide measurements in the tropics.

Abstract. HIBISCUS was a field campaign for investigating the impact of deep convection on the Tropical Tropopause Layer (TTL) and the Lower Stratosphere, which took place during the Southern Hemisphere summer in February–March 2004 in the State of São Paulo, Brazil. Its objective was to provide a set of new observational data on meteorology, tracers of horizontal and vertical transport, water vapour, clouds, and chemistry in the tropical UT/LS from balloon observations at local scale over a land convective area, as well as at global scale using circumnavigating long-duration balloons. Overall, the composition of the TTL, the region between 14 and 19 km of intermediate lapse rate between the almost adiabatic upper troposphere and the stable stratosphere, appears highly variable. Tracers and ozone measurements performed at both the local and the global scale indicate a strong quasi-horizontal isentropic exchange with the lowermost mid-latitude stratosphere suggesting that the barrier associated to the tropical jet is highly permeable at these levels in summer. But the project also provides clear indications of strong episodic updraught of cold air, short-lived tracers, low ozone, humidity and ice particles across the lapse rate tropopause at about 15 km, up to 18 or 19 km at 420–440 K potential levels in the lower stratosphere, suggesting that, in contrast to oceanic convection penetrating little the stratosphere, fast daytime developing land convective systems could be a major mechanism in the troposphere-stratosphere exchange at the global scale. The present overview is meant to provide the background of the project, as well as overall information on the instrumental tools available, on the way they have been used within the highly convective context of the South Atlantic Convergence Zone, and a brief summary of the results, which will be detailed in several other papers of this special issue.

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The results of side‐by‐side instrument intercomparisons of Fourier transform infrared (FTIR) spectrometers at Ny‐Ålesund, Spitzbergen (79°N, 12°E), during May/June 1995 and at Harestua, Norway (60°N, 10°E), during September/October 1994 are reported. The spectrometers were operated simultaneously recording atmospheric spectra in the midinfrared using the Sun as a source. The differences in vertical columns of HCl, HF, N2O, and HNO3 measured simultaneously by different instruments were as large as 5.8, 7.7, 2.8, and 4.3%, respectively, having mean absolute values 0.5, 1.6, 1.0, and 1.6%, respectively. These results were used to derive 1σ overall uncertainties of 9.7% for HCl, 7.7% for HF, 6.4% for N2O, and 14.3% for HNO3. Separate uncertainties are quoted for the precision of daily column measurements which exclude the contribution due to uncertainty in the line parameters. These are 7.7% for HCl, 5.7% for HF, 4.9% for N2O, and 6.8% for HNO3. These numbers are estimated for the local conditions at the intercomparison sites but will differ from site to site depending on the knowledge of local atmospheric conditions at the time of measurement.