The purpose of this special issue is the compilation of modelling and observational studies in connection with five international field deployments (AEROCLO-sA, CLARIFY, LASIC, ORACLES, and NaFoLiCA) that focus on the interactions of natural and anthropogenic aerosols with radiation, clouds, and regional climate in the South Atlantic Ocean and the southern African region. These deployments, based in Namibia, Ascension Island, and São Tomé, took place between 2016 and 2018 and support a significant number of investigations extending beyond just the individual science teams. The airborne and ground-based observations, as well as the related satellite measurements and climate modelling studies, address all aspects of aerosol–cloud–climate interactions, including the link of aerosol properties to meteorological fields and dynamical processes that influence aerosol emission and transport. The projects also target the advancement of remote sensing of aerosols for complex scenes over land, sea, and clouds. The special issue will be open to all submissions, with complementary goals to the five mentioned deployments, so as to encourage the exchange of ideas from inside and outside the science teams of all projects.

We conducted the Fifth International Workshop on Ice Nucleation (FIN) to (1) understand the microphysics of how particles nucleate ice, (2) determine the number of ice forming particles as a function of atmospheric properties such as temperature and relative humidity, (3) measure the atmospheric distribution of ice forming particles and (4) ascertain the role of anthropogenic activities in producing or changing the behaviour of ice forming particles. To accomplish these goals we held three distinct workshops on the topic of atmospheric ice nucleation. The first was an intercomparison of instruments to determine the composition of ice forming particles in a controlled laboratory setting. This took place in autumn 2014 at the location of the last ice nucleation instrument intercomparison: the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber located at the Karlsruhe Institute of Technology. The second was an intercomparison of instruments used to determine cloud formation conditions. This activity also took place at AIDA and was conducted in spring 2015. Because ice nucleation predominantly takes place at the low temperatures found at high altitude, a critical requirement for the third workshop was a facility that offers access to free-tropospheric air masses with minimal local particle sources. We used the Desert Research Institute’s recently renovated Storm Peak Laboratory for this workshop in autumn 2015.

This special issue aims to highlight advances in spectroscopic techniques for applications in atmospheric science. It will highlight cutting-edge measurements of atmospherically relevant species, including aerosol, isotopologues, and trace gases, using optical cavity-based techniques. Such techniques can include cavity-enhanced methods for sensitive absorption and extinction measurements as well as advances in frequency comb techniques. Techniques based on both broadband and laser sources from the ultraviolet to infrared will be considered. Papers based on both laboratory and field measurements are welcome.

In this special issue papers resulting from two major combined field campaigns shall be aggregated: (i) the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD), and (ii) the Physical feedbacks of Arctic boundary layer, Sea ice, Cloud and AerosoL (PASCAL). These two concurrent campaigns took place in the vicinity of Svalbard in May and June 2017. They were designed to study processes important for explaining Arctic amplification, and, in particular, for investigating the role of microphysical and dynamical properties of Arctic low- and mid-level, mixed-phase clouds, and their interactions with atmospheric radiation and aerosol particles. Ground-based, ship-borne, tethered balloon, aircraft, and satellite observations have been combined. The research vessel (RV) Polarstern, an ice floe camp (erected close to the icebreaker) including an instrumented tethered balloon, and the two research aircraft, Polar 5 and Polar 6, were jointly operated. Polar 5 served as a mobile remote sensing observatory looking at the clouds from above, whereas Polar 6 operated as a flying in situ measurement laboratory mostly sampling inside the clouds. The permanent ground station of Ny-Ålesund observed the clouds from below, applying similar but upward-looking remote sensing equipment as Polar 5. Some of the flights were performed underneath respective satellite tracks. In this special issue we compile a number of papers reporting about the results of the observations conducted during ACLOUD/PASCAL within the framework of the (AC)³ project (http://www.ac3-tr.de/).

The Hydrological cycle in the Mediterranean Experiment (HyMeX, https://www.hymex.org/) programme is a 10-year concerted effort at the international level started in 2010 with aims to advance the understanding of the water cycle, and with emphases on the predictability and evolution of high-impact weather events, as well as on evaluating social vulnerability to these extreme events. The special issue is jointly organized between the Atmospheric Chemistry and Physics, Hydrology and Earth System Sciences, Ocean Science, Natural Hazards and Earth System Sciences, Atmospheric Measurement Techniques, and Geoscientific Model Development journals. It aims at gathering contributions to the areas of understanding, modelling, and predicting at various timescales and spatial scales of the Mediterranean water cycle and its related extreme events, including cyclones, heavy precipitation, flash floods and impacts, drought and water resources, strong winds, and dense water formation. The special issue is not limited to studies conducted within HyMeX: any multiscale or multidisciplinary approaches related to the Mediterranean water cycle are encouraged.