The Wave-driven ISentropic Exchange (WISE) mission is a collaborative research project to investigate transport and mixing processes in the upper troposphere and the lower stratosphere (UTLS) over the Atlantic. Headed by Forschungszentrum Jülich GmbH and the Johannes Gutenberg University of Mainz, 14 measurement flights were conducted with the high-altitude research aircraft HALO in September and October 2017 from Shannon, Ireland. Further partners providing instrumentation were the Karlsruhe Institute for Technology (KIT), the German Aerospace Center (DLR), the universities of Heidelberg, Frankfurt am Main, and Wuppertal, and the German National Metrology Institute. The scientific flights were supported by a team of about 90 persons.

The main objective of this project is to study the relation between chemical composition and the dynamical structure of the UTLS with a focus on the following topics: (i) interrelation of the tropopause inversion layer (TIL) and trace gas distribution, (ii) role of planetary wave breaking for water vapor transport into the extratropical lower stratosphere, (iii) role of halogenated substances for ozone and radiative forcing in the UTLS region, and (iv) occurrence and effects of sub-visual cirrus (SVC) in the lowermost stratosphere.

The special issue mainly includes papers from the airborne measurement campaign itself but is also open to other studies close to the topic. This includes both advanced scientific analyses of observational and measured data during the campaign as well as related climatological studies, but also manuscripts based on instrumental developments.

The Aerosol Chemistry Model Intercomparison Project (AerChemMIP) is endorsed by the Coupled-Model Intercomparison Project 6 (CMIP6) and is designed to quantify the climate and air-quality impacts of aerosols and chemically reactive gases. Specifically, the aim of AerChemMIP is to answer four scientific questions. 1. How have anthropogenic emissions contributed to global radiative forcing and affected regional climate over the historical period? 2. How might future policies (on climate, air quality, and land use) affect the abundances of NTCFs and their climate impacts? 3. How do uncertainties in historical NTCF emissions affect radiative forcing estimates? 4. How important are climate feedbacks to natural NTCF emissions, atmospheric composition, and radiative effects? This special issue is to be devoted to papers that analyse targeted simulations with CMIP6 climate models to address these questions as well as papers that evaluate these models against observations. It also includes papers from other CMIP6-related activities (e.g. RFMIP, DAMIP), containing contributions to the understanding of climate forcing and feedbacks involving aerosols, reactive gases, and associated biogeochemical cycles.

The ISTP11 AMT special issue aims at collecting original papers from the 11th Edition of the International Symposium on Tropospheric Profiling (ISTP11) as well as additional contributions fitting well within the scope of tropospheric profiling. The main topics will cover new algorithms, instrumental techniques and synergy to retrieve atmospheric profiling of state parameters and consituents (clouds, aerosols and trace gazes), improved understanding of tropospheric processes thanks to innovative dataset of tropospheric profiles and the use of experimental data to improve numerical weather prediction (model evaluation, advances in modelling and data assimilation).

The last 50 years have enabled significant progress in our ability to describe and model both the oceanic and atmospheric environments of the Indian Ocean and their interactions with the sea floor and coastal regimes. However, our understanding of the major processes of the Indian Ocean is still far from complete and is rudimentary in many respects. This SI compiles various synthesis articles about the recent results of the 2nd International Indian Ocean Expedition Program (IIOE-2: https://iioe-2.incois.gov.in/). The synthesis articles will review and highlight results of physical, biogeochemical, ecological, geological, coastal and atmospheric studies from all parts of the Indian Ocean. Moreover, the SI will serve to illustrate some of the key questions that are relevant for future research on the grand challenges in the Indian Ocean system.

In order to improve our current knowledge on the budgets of the two most important anthropogenic greenhouse gases, CO₂ and CH₄, a co-ordinated measurement campaign in the central European region was successfully carried out in May-June 2018 with the German research aircraft HALO and two smaller Cessna aircraft as its main experimental platforms. The goal of CoMet is to combine a suite of state-of-the-art airborne active (lidar) and passive remote sensors (spectrometer) with in situ instruments to provide regional-scale data about greenhouse gases (GHGs) which are urgently required for their accurate modelling.

During the intensive observation period, HALO research flights were performed along extended latitudinal transects to capture GHG gradients, over known regions of strong emissions, and over the ground-based remote-sensing sites of the Total Carbon Column Observing Network (TCCON). While HALO provides a larger-scale picture, the two Cessna aircraft concentrated on a region of prime interest: the Upper Silesian Coal Basin in Poland, which, due to hard coal mining activities, is one of the hot spots of anthropogenic methane emissions in Europe. Here, a variety of ground-based instruments additionally supported the CoMet mission: FTIR spectrometers, wind lidars, mobile vans, and small drones provided valuable information to quantify CH₄ emissions from coal mining activities. A model infrastructure (regional inverse modelling, chemistry-climate modelling with regional refinement) has been employed in forecast mode to optimize flight strategies and is now used to hindcast the campaign period to evaluate GHG fluxes and transport. The CoMet mission is also part of validation activities for existing passive remote sounding GHG satellites (GOSAT, Sentinel-5P, and OCO-2) and preparation of the first active CH₄ satellite mission MERLIN. For that reason, the CHARM-F lidar developed at DLR as an airborne MERLIN demonstrator was one of the key instruments. In addition, CoMet is intended to investigate methodologies for the synergistic combination for GHG measurements using lidar and passive remote sensing.