With the launch of the novel Aeolus wind profiling mission, a wealth of direct wind and atmospheric backscatter profile observations has become available across the globe, revealing new information on atmospheric dynamics and optical properties to the benefit of a number of applications. These applications include, for example, improvements in numerical weather prediction (NWP), general circulation models (GCMs) and their included parameterized processes, such as land and ocean drag, convection, stratosphere–troposphere interaction and atmospheric waves, air quality and air pollution transport, etc. In this special issue, new data products and their validation as well as studies using the Aeolus data for application in meteorology, air quality, and climate studies are discussed. Topics vary from observation interpretation to GCM or NWP model diagnostics, parameterization, and data assimilation experiments. This is the first special issue on scientific analysis of Aeolus mission data, including inter-comparisons with different collocated wind profiling, aerosol and cloud in situ and remote-sensing measurements, and atmospheric models. Contributions concerning Aeolus data calibration, processing, and tools for data access and exploitation are also provided.

The absorption of energy by water vapour keeps the average temperature on Earth at a level that makes life possible. However, the average temperature increased in recent decades due to increased greenhouse gas emissions. This anthropogenic increase is amplified by the water vapour feedback. Water vapour is further an important component of the water cycle and indirectly of the energy cycle. The role of water vapour in these cycles in a changing climate needs to be fully understood and quantified. Thus, high-quality observations of water vapour and the characterisation and validation of associated uncertainties are of high importance for climate applications. Topics covered around atmospheric water vapour include, among others, retrieval development, data records, uncertainty characterisation and validation, inter-comparisons, climate applications, and model validation.

This special issue originates from the GEWEX Water Vapor Assessment (G-VAP, http://gewex-vap.org) and is open for all submissions within scope, not just from G-VAP.

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 Chemistry and Aerosol Mediterranean Experiment (ChArMEx) special issue will be simultaneously presented in the ACP and AMT journals. It aims at gathering experimental and modelling contributions to the field of atmospheric chemistry in the Mediterranean region and its impacts on regional air quality and climate, both in the recent past, present and future decades. It addresses natural and anthropogenic emissions of tropospheric reactive species, source apportionment, chemical transformations, transport processes, atmospheric deposition, aerosol optical properties and interactions with radiation, water vapour and clouds, variability and trends, and future conditions following climate change and increasing anthropogenic pressure.

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.

Sand and dust storms (SDSs) are extreme meteorological phenomena that generate significant amounts of airborne mineral dust particles. SDSs play a significant role in different aspects of weather, climate and atmospheric chemistry. In addition, SDSs represent a severe hazard for life, health, property, the environment and the economy, which is aligned with several Sustainable Developed Goal (SDG) targets established by the United Nations (UN). Understanding, managing and mitigating SDS risks and effects requires fundamental and cross-disciplinary knowledge and filling current knowledge gaps on the dust cycle. Recent developments in Earth observation have led to big advances in the observational capabilities of desert dust particles. Research products show the possibility of exploring the wide dimensional range of desert dust particles by integrating different observational techniques on the ground, while satellite observations are currently providing aerosol description with finer and finer spatiotemporal resolution. Experts on dust observations and modelling participating in the EU-funded COST Action InDust (www.cost-indust.eu, CA16202) have identified critical scientific gaps in our current knowledge of dust aerosol measurements, modelling, processes and effects and have also identified a critical mass of scientists working towards addressing them. Such gaps include at least issues such as - the need for improvement of surface-based dust aerosol measurements (in situ, sun photometric, lasers), including the use of new technologies, improved algorithms and combined approaches; - the improvement of the global coverage on aerosol dust observation through improvement of satellite-based retrievals; - advancements in dust aerosol modelling (sources, mechanisms, deposition); - the presentation of intensive campaign results with a focus on dust aerosols; - dust mineralogy; - the assessment of the dust particle size distribution with an emphasis on giant particles; - enhancements regarding dust aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture, ocean life). This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts from at least the COST InDust community (0000108:0000100 scientists from 52 countries). A list of papers to be submitted has been created including results related to SDS-WAS, ACTRIS/EARLINET activities, long-term measurement analysis of measured and modelled data, dust retrieval algorithm and techniques, dust effects, and foreseen results of the ESA Aeolus Cal/Val activity “ASKOS” (Cabo Verde, June - July, 2020).

The project Effect of Megacities on the Transport and Transformation of Pollutants on the Regional and Global Scales, EMeRGe, was created to experimentally investigate the transport and transformation of air plumes coming from major population centres, MPCs. It comprises a set of unique airborne measurement campaigns carried out in two parts of the world having MPCs of very different characteristics and emissions. The two measurement campaigns using the HALO research aircraft (www.halo-spp.de) were conducted in Europe in summer 2017 and in East Asia during the intermonsoon period in March-April 2018. The initial scientific team of EMeRGe, comprising scientists from four German universities and four research centres in 2016 (www.iup.uni-bremen.de/emerge/), was extended within the EMeRGe international membership, incorporating more than 50 research partners from 16 different countries. The combined and synergetic set of information obtained from airborne and ground-based data, satellite products, and their comparison with models enables the patterns of atmospheric composition and meteorological parameters observed during the campaigns to be constrained and thereby our understanding of the emissions, their transport, and physico-chemical transformation of the outflow from MPC emissions to be tested and assessed. The EMeRGe special issue aims to publish the scientific results achieved by the project EMeRGe. This comprises contributions of the partners of the national and international EMeRGe scientific community. It solicits all relevant submissions addressing the EMeRGe science objectives.

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.

The version 8 spectral data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), released in January 2019, are supposed to be the final set. Besides the ORM reprocessed version 8 level-2 MIPAS data distributed by ESA, a research processor has been run at IMK and IAA to generate temperature and trace gas concentrations. The IMK–IAA data product includes a larger list of atmospheric constituents and covers a wider altitude range in the case of the special observation modes than the operational data. The processing is based on independent algorithms. This special issue collects articles which 1) document the retrieval strategies used to derive these distributions along with uncertainty estimates, 2) report validation studies, and 3) use the data to answer current questions in atmospheric research.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) has been measuring atmospheric middle-infrared limb-emission spectral radiances from the European Polar Satellite ENVISAT from 2002 to 2012. Several algorithms have been developed by the scientific community to process MIPAS data up to Level 2, which is to retrieve the vertical distribution profiles of many atmospheric minor constituents. Among these algorithms, the one developed by the European Space Agency (ESA) is of utmost importance as it delivers high-quality profiles of a large set of gases, retrieved from the measurements of the whole MIPAS mission. The papers included in the special issue deal with the most recent (and possibly the last) release of MIPAS/ENVISAT products obtained from the ESA Level 2 processor version 8. The papers deal with both the products and the algorithms employed to derive them from direct measurements. The algorithms are presented and assessed. The features of the Level 2 products are analyzed and the products themselves are validated by intercomparison with other independent co-located measurements.

This special issue includes papers based on recent field campaigns, including US Department of Energy Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA), NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), and the North Atlantic Climate System Integrated Study (ACSIS), and long-term measurements from observatories (e.g., the Atmospheric Radiation Measurement Eastern North Atlantic site on Graciosa Island in the Azores). Presently the responses of marine low-cloud systems to changes in atmospheric greenhouse gases and aerosols are major sources of uncertainty that limit our ability to predict future climate. Major contributions to this uncertainty derive from poor understanding of the cloud responses to aerosol changes and the natural aerosol state that is being perturbed by anthropogenic emissions. One overarching scientific objective of these recent field campaigns is to understand key processes that drive the properties and interactions of aerosol and clouds under representative meteorological and cloud conditions. Another important focus is to validate and improve ground-based retrieval algorithms using comprehensive in situ aircraft measurements. This will lead to high-quality, long-term datasets from the routine ground-based remote sensing, which allow greater statistical reliability in the observed properties and relationships among aerosols, clouds, and precipitation than is possible with the aircraft measurements alone.

Satellite limb measurements have become an essential tool to remotely sense the chemical composition of the Earth's atmosphere with a particular focus on the stratosphere. They enable combining near global sampling on a daily basis with vertical profiling capability and thus overcome some of the limitations of nadir or occultation measurements. Although the number of satellite limb instruments is currently declining, several missions are still operational (e.g., OSIRIS, SMR, ACE-FTS, MAESTRO, OMPS-LP and SAGE III) and several new missions are planned or will be launched in the near future (e.g., MATS, ICON, ALTIUS and several mini satellites). In addition, the data sets of several past limb sounders, particularly the Envisat instruments SCIAMACHY, MIPAS and GOMOS are still used for many scientific studies and new data products are developed based on measurements with these instruments.

This combined ACP/AMT special issue is open for submissions dealing with all aspects related to limb measurements (including limb scatter, limb emission and occultation) in the optical, IR and microwave spectral regions. This includes instrument development and characterization, retrieval algorithm development, validation studies, as well as scientific applications based on the retrieved data sets. We note that radio occultation measurements are outside the scope of the special issue.

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.

This ACP Special Issue seeks to report research advancements in the understanding of mercury sources, distribution pathways, levels, physicochemical characterization, measurement traceability, impacts on global ecosystems, and policy-relevant information presented at the ICMGP 2019 and being developed under the MercOx (Metrology of oxidized mercury) project and two ERA-PLANET initiatives – Integrative and Comprehensive Understanding on Polar Environments (iCURE) and Integrated Global Observing Systems for Persistent Pollutants (iGOSP). The Special Issue is open to all publications related to the scope of the Special Issue.

SKYNET is an international research network dedicated to aerosol—cloud—radiation interaction studies.
It consists of about 60 sites located all over the world. The main instrument at each site is the sun—sky radiometer, but to strengthen the ability of SKYNET, simultaneous measurements with other instruments such as pyranometers, pirgeometers, microwave radiometers, absorption meters, cloud cameras, lidars, MAX-DOAS, and instrumentation for in situ characterisation are also conducted for some selected sites.
This special issue will face issues related to the following topics: aerosol and cloud properties from radiometers; developments on instrumentation; aerosol radiative forcing and climate effects; intercomparison among radiometer networks; validation of aerosol and cloud properties from satellite and models; applications for air pollution studies; and applications for solar energy.

With a wide variety of aerosol optical, physical, and chemical properties, China offers a test bed for the study of aerosol processes and techniques leading to a better understanding of aerosol properties. Aerosol concentrations in the southeast of China are periodically among the highest in the world, while at other times they are relatively low. The prevailing aerosol chemical composition (e.g., sulfate, nitrate, black carbon, brown carbon, particulate organic matter, sea salt) also changes significantly with the quick evolution of both the economy and society, along with the government’s strong environmental actions. The complex mechanisms of the secondary formation of aerosols and haze-fog interaction are still not well understood in this important region. In areas other than the southeast of China, topography determines the aerosol concentrations, which are consistently high. Examples are Sichuan Province or the Guanzhong Basin, i.e. large basins surrounded by high mountains. Furthermore, the large deserts in the west and north of China are strong sources of dust particles influencing the aerosol properties over a large part of the country. There are however also large areas with relatively low aerosol loading, in particular in mountainous areas and over the high plateaus such as Tibet. Aerosol chemical properties in the center and east of China still need to be clarified both geographically and temporally, e.g. distinguish spectrally absorbing components like mineral dust and brown carbon from remote sensing observations. Anthropogenic emissions of aerosols and precursor gases have a large influence on aerosol, in regards to both chemical and physical properties, and consequently optical properties, and their effects can be augmented by the transport of aerosol from elsewhere, such as carbonaceous particles from biomass burning aerosol and dust particles emitted from the deserts. The complex mechanisms and intrinsic relations of aerosol chemistry and physics, and their impacts on optical properties, need better interpretation based on comprehensive observation and analysis. Weather conditions have a large influence on the formation of haze, and haze episodes occur more and more frequently. The influence of large-scale weather systems such as the monsoon or the Siberian high may result in inter-annual variations in different regions of China. In particular the progress of the monsoon from south to north during spring and summer, and back in the autumn, results in strong variations in aerosol content.

In this special issue we focus on the application of remote sensing techniques using both ground-based and satellite measurements to study the temporal and spatial variations in aerosol properties over China on different scales. Results from campaigns in the Taklamakan Desert (Kashi), on the Tibetan Plateau, and other remote areas such as the NE of China are contrasted with those from densely populated and industrialized urban areas in the east of China. The special issue aims to bring together results from different projects such as Kashi, MOST, and regional air quality studies. Contributions are solicited on aerosol optical, physical, and chemical properties inferred from remote sensing observations and related optical techniques, and the spatial and temporal variations in these properties across China, with emphasis on processes leading to the observed differences, including the influence of meteorological conditions and large-scale weather systems.

The 2019 Raikoke eruption emitted an estimated 1.5±0.2 Tg of volcanic sulfur dioxide (SO₂) into the atmosphere, which makes it the largest volcanic SO₂ perturbation in the upper troposphere and lower stratosphere since the 2011 eruption of Nabro. The 2019 Raikoke eruption triggered numerous discussions among scientists involved in the SPARC Volcano Response (VolRes) initiative and provides an excellent opportunity to test our understanding of processes involved in the formation, growth and removal of volcanic aerosols in the atmosphere. Remote sensing measurements from geostationary and low-orbiting satellites with ultraviolet, visible, and infrared sensors (HIMAWARI-8, OMPS/NPP, OMI/Aura, TROPOMI/SP5, IASI/MeteopA-B-C, AIRS/Aqua, CALIOP/CALIPSO, SAGE III/ISS) provided a wealth of information on the SO₂ concentration, ash and sulfate aerosol particle optical properties from the initial injection through to the growth and removal of aerosols. This eruption therefore provides an excellent opportunity to inter-compare satellite observations and validate the different retrievals for SO₂ and volcanic aerosol properties from new satellite sensors such as TROPOMI. After the initial injection into altitudes up to 15–16 km, the plume reached 25–26 km altitude in late 2019, which corresponds to a fast vertical ascent that appears unusual compared to previous eruptions. The fast vertical ascent of the 2019 Raikoke plume may suggest an influence of smoke aerosol from large wildfires that took place in Canada and Siberia around the time of the eruption. The veil of aerosol particles from Raikoke 2019 has noticeably reduced the transparency of the stratosphere and detailed information and measurements on the aerosol properties will be useful to constrain atmospheric and climate model simulations.

This inter-journal special issue of AMT, ACP, and GMD serves as a thematic collection for manuscripts arising from the work of Horizon 2020 integrated infrastructure project Eurochamp-2020; cf. https://www.eurochamp.org/ . As Eurochamp-2020 has project parts dealing with new instrumentation, with atmospheric chemistry and physics studies as well as with modelling, contributions are expected for all three journals as part of the inter-journal special issue.

StratoClim is a collaborative research project funded by the European Commission 7th Framework programme. The main objective of the project is to improve the understanding of key processes in the upper troposphere and stratosphere (UTS) for more reliable projections of climate change and stratospheric ozone. StratoClim focuses on the understanding of the microphysical, chemical, and dynamical processes that determine the composition of the UTS, such as the formation, loss, and redistribution of aerosol, ozone, and water vapour; how these processes are affected by climate change; and how they feed back on global climate.

The issue will include papers coming from all sectors included in the project: in situ and sonde measurement deployment, satellite data products and analysis, mesoscale and transport modelling, and CCM experiments and analysis. We expect a special emphasis on the results from the high-altitude research aircraft campaign in the Asian monsoon area carried out in 2017 deploying an innovative and comprehensive payload. Due to the expected contribution of instrumental manuscripts, the issue will also include AMT submissions.

Towards UNified Error Reporting (TUNER) is an emerging SPARC activity and an ISSI International Team. This project aims to provide consistent and inter-comparable error estimates for atmospheric temperature and composition measurements from space. Along with this, a consistent and inter-comparable characterization of spatial resolution and content of a priori information of the remotely sensed data shall be provided. This is important, because quantitative work with remotely sensed data – data assimilation, data merging, time series analysis, testing of hypotheses, etc. – depends largely on the adequate characterization of the data. Currently, multiple retrieval methods are used by the different instrument groups, and along with this various approaches to error estimation are applied. Resulting errors are not always inter-comparable. Some kinds of uncertainties are sometimes not reported at all. The different altitude resolutions and the different content of prior information in the data products are a particular problem. Within TUNER, data characterization methods currently in use are identified, their completeness and inter-comparability are assessed, recommendations for how unified data characterization should be performed and how data uncertainties should be reported are developed, and unified error estimates for some selected data products are provided. Beyond this, data users are instructed how to best utilize these metadata. This special issue is meant to include papers covering one of the following topics:

• methodical work related to the uncertainty assessment of satellite data on atmospheric temperature and composition;
• the error budget of satellite-borne atmospheric sounders;
• the use of results of validation studies to judge the adequacy of uncertainty estimates;
• recommendations for unified error estimates;
• tutorial articles on the correct use of error estimates and other diagnostic data and related topics.
This special issue is open to contributions from outside the related SPARC and ISSI TUNER teams, provided that the content fits within the scope of TUNER. Further details are available at http://www.imk-asf.kit.edu/english/304_2689.php.

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 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 Water Vapour Phase II (WAVAS II), a SPARC activity, started in 2008 (SPARC Newsletter No. 30 (2008) p. 16: SPARC Water Vapour Initiative, by C. Schiller et al.). The activity includes satellite assessment and in situ comparison components. This international activity encompasses:

1) Providing a quality assessment of upper tropospheric to lower mesospheric satellite records since the early 1990s
2) Providing, as far as possible, absolute validation against ground-truth instruments
3) Assessing inter-instrument biases, depending on altitude, location, and season
4) Assessing the representation of temporal variations on various scales
5) Including data records on isotopologues
6) Providing recommendations for usage of available data records and for future observation systems


The main objective of WAVAS II is to assess and extend our knowledge and understanding of measurements of the vertical distribution of water vapor in the upper troposphere and middle atmosphere (UT/MA), where water has small concentrations, but significant radiative impact. This is a follow-up of the SPARC WAVAS activity, whose report was published in 2000 (SPARC Report No. 2 (2000) Upper Tropospheric and Stratospheric Water Vapour. D. Kley, J.M. Russell III, and C. Philips (eds.). WCRP-113, WMO/TD – No. 1043). Information gained from this activity will improve our ability to estimate long-term changes with associated uncertainties in UT/MA water as well as make recommendations as to what data would be most valuable for model validation and how such data should be used.

Papers will be accepted for this special issue according to the following guidelines, independent if they originate from the WAVAS II activity or other activities.

Guidelines for submissions:

• Papers covering existing UT/MA satellite water vapour measurements;
• Papers discussing comparisons of UT/MA satellite measurements, including discussion of quantities derived from these measurements, such as seasonal cycles, estimates of transport, or estimates of drifts, trends and variability.
• Papers discussing merging of water vapour measurements will be considered, although this topic is not specifically part of the WAVAS-II activity.
• Model papers that incorporate the datasets discussed and the uncertainty estimates resulting from the WAVAS-II activity will also be considered for inclusion.

The prediction of the winter weather over complex terrain is quite challenging due to the highly variable nature of winds, visibility, and snowfall. As a World Meteorological Organization (WMO) World Weather Research Program (WWRP) Research Demonstration Project (RDP) and Forecast Demonstration Project (FDP), ICE-POP 2018 (International Collaborative Experiments for PyeongChang 2018 Olympic and Paralympic winter games) was held in the PyeongChang region from November 2017 to April 2018 with contributions from 29 agencies from 12 countries. The region was quite unique for observing winter weathers that are influenced by cold air and warm ocean interaction, sudden uplifting by steep terrains near the coast, and modulation by complex terrains. The main scientific goal was to understand the precipitation processes in this unique region during the cold season and to evaluate/improve forecasting from numerical models based on intensive observations. Dense observational networks of upper air observation (eight soundings, two wind profilers, shipborne sounding, and dropsonde), remote sensing (three X-Pol radars, one Ku/Ka-Pol radar and three S-Pol, one S-band, two C-band, and three Doppler lidars), microphysical observation (2DVD, MASC, PIP, Parsivel, MRR, POSS, Pluvio), and surface stations (64 stations) were implemented, in particular, to observe the evolution of precipitation along and across atmospheric flows. The field experiment and real-time forecast demonstration ended and the second phase of the experiment has started for better understanding of the microphysical processes, their better representation in the numerical modeling, and further improvement of winter weather prediction through various international collaborations.

The main purposes of the special issue are

1) to document the scientific findings on the winter weather during the forecast demonstration project

2) to share scientific knowledge on processes of winter weathers that have been investigated with unprecedented dense observational networks,

3) to share current status and improved knowledge of forecasting of winter weathers, and

4) to document new retrieval and quality control methods of the operational and advanced instruments.

The special issue will include all manuscripts related to observational data, products, NWP modeling, researches on observational instrumentation, process/mechanism study, reanalysis, integration of observation and numerical modeling, and prediction of the winter weathers.

The absorption of energy by water vapour keeps the average temperature on Earth at a level that makes life possible. However, the average temperature increased in recent decades due to increased greenhouse gas emissions. This anthropogenic increase is amplified by the water vapour feedback. Water vapour is further an important component of the water cycle and indirectly of the energy cycle. The role of water vapour in these cycles in a changing climate needs to be fully understood and quantified. Thus, high-quality observations of water vapour and the characterisation and validation of associated uncertainties are of high importance for climate applications. Topics covered around atmospheric water vapour include, among others, retrieval development, data records, uncertainty characterisation and validation, inter-comparisons, climate applications, and model validation.

This special issue originates from the GEWEX Water Vapor Assessment (G-VAP, http://gewex-vap.org) and is open for all submissions within scope, not just from G-VAP.

The prediction of the winter weather over complex terrain is quite challenging due to the highly variable nature of winds, visibility, and snowfall. As a World Meteorological Organization (WMO) World Weather Research Program (WWRP) Research Demonstration Project (RDP) and Forecast Demonstration Project (FDP), ICE-POP 2018 (International Collaborative Experiments for PyeongChang 2018 Olympic and Paralympic winter games) was held in the PyeongChang region from November 2017 to April 2018 with contributions from 29 agencies from 12 countries. The region was quite unique for observing winter weathers that are influenced by cold air and warm ocean interaction, sudden uplifting by steep terrains near the coast, and modulation by complex terrains. The main scientific goal was to understand the precipitation processes in this unique region during the cold season and to evaluate/improve forecasting from numerical models based on intensive observations. Dense observational networks of upper air observation (eight soundings, two wind profilers, shipborne sounding, and dropsonde), remote sensing (three X-Pol radars, one Ku/Ka-Pol radar and three S-Pol, one S-band, two C-band, and three Doppler lidars), microphysical observation (2DVD, MASC, PIP, Parsivel, MRR, POSS, Pluvio), and surface stations (64 stations) were implemented, in particular, to observe the evolution of precipitation along and across atmospheric flows. The field experiment and real-time forecast demonstration ended and the second phase of the experiment has started for better understanding of the microphysical processes, their better representation in the numerical modeling, and further improvement of winter weather prediction through various international collaborations.

The main purposes of the special issue are

1) to document the scientific findings on the winter weather during the forecast demonstration project

2) to share scientific knowledge on processes of winter weathers that have been investigated with unprecedented dense observational networks,

3) to share current status and improved knowledge of forecasting of winter weathers, and

4) to document new retrieval and quality control methods of the operational and advanced instruments.

The special issue will include all manuscripts related to observational data, products, NWP modeling, researches on observational instrumentation, process/mechanism study, reanalysis, integration of observation and numerical modeling, and prediction of the winter weathers.

The 2019 Raikoke eruption emitted an estimated 1.5±0.2 Tg of volcanic sulfur dioxide (SO₂) into the atmosphere, which makes it the largest volcanic SO₂ perturbation in the upper troposphere and lower stratosphere since the 2011 eruption of Nabro. The 2019 Raikoke eruption triggered numerous discussions among scientists involved in the SPARC Volcano Response (VolRes) initiative and provides an excellent opportunity to test our understanding of processes involved in the formation, growth and removal of volcanic aerosols in the atmosphere. Remote sensing measurements from geostationary and low-orbiting satellites with ultraviolet, visible, and infrared sensors (HIMAWARI-8, OMPS/NPP, OMI/Aura, TROPOMI/SP5, IASI/MeteopA-B-C, AIRS/Aqua, CALIOP/CALIPSO, SAGE III/ISS) provided a wealth of information on the SO₂ concentration, ash and sulfate aerosol particle optical properties from the initial injection through to the growth and removal of aerosols. This eruption therefore provides an excellent opportunity to inter-compare satellite observations and validate the different retrievals for SO₂ and volcanic aerosol properties from new satellite sensors such as TROPOMI. After the initial injection into altitudes up to 15–16 km, the plume reached 25–26 km altitude in late 2019, which corresponds to a fast vertical ascent that appears unusual compared to previous eruptions. The fast vertical ascent of the 2019 Raikoke plume may suggest an influence of smoke aerosol from large wildfires that took place in Canada and Siberia around the time of the eruption. The veil of aerosol particles from Raikoke 2019 has noticeably reduced the transparency of the stratosphere and detailed information and measurements on the aerosol properties will be useful to constrain atmospheric and climate model simulations.

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) has been measuring atmospheric middle-infrared limb-emission spectral radiances from the European Polar Satellite ENVISAT from 2002 to 2012. Several algorithms have been developed by the scientific community to process MIPAS data up to Level 2, which is to retrieve the vertical distribution profiles of many atmospheric minor constituents. Among these algorithms, the one developed by the European Space Agency (ESA) is of utmost importance as it delivers high-quality profiles of a large set of gases, retrieved from the measurements of the whole MIPAS mission. The papers included in the special issue deal with the most recent (and possibly the last) release of MIPAS/ENVISAT products obtained from the ESA Level 2 processor version 8. The papers deal with both the products and the algorithms employed to derive them from direct measurements. The algorithms are presented and assessed. The features of the Level 2 products are analyzed and the products themselves are validated by intercomparison with other independent co-located measurements.

Sand and dust storms (SDSs) are extreme meteorological phenomena that generate significant amounts of airborne mineral dust particles. SDSs play a significant role in different aspects of weather, climate and atmospheric chemistry. In addition, SDSs represent a severe hazard for life, health, property, the environment and the economy, which is aligned with several Sustainable Developed Goal (SDG) targets established by the United Nations (UN). Understanding, managing and mitigating SDS risks and effects requires fundamental and cross-disciplinary knowledge and filling current knowledge gaps on the dust cycle. Recent developments in Earth observation have led to big advances in the observational capabilities of desert dust particles. Research products show the possibility of exploring the wide dimensional range of desert dust particles by integrating different observational techniques on the ground, while satellite observations are currently providing aerosol description with finer and finer spatiotemporal resolution. Experts on dust observations and modelling participating in the EU-funded COST Action InDust (www.cost-indust.eu, CA16202) have identified critical scientific gaps in our current knowledge of dust aerosol measurements, modelling, processes and effects and have also identified a critical mass of scientists working towards addressing them. Such gaps include at least issues such as - the need for improvement of surface-based dust aerosol measurements (in situ, sun photometric, lasers), including the use of new technologies, improved algorithms and combined approaches; - the improvement of the global coverage on aerosol dust observation through improvement of satellite-based retrievals; - advancements in dust aerosol modelling (sources, mechanisms, deposition); - the presentation of intensive campaign results with a focus on dust aerosols; - dust mineralogy; - the assessment of the dust particle size distribution with an emphasis on giant particles; - enhancements regarding dust aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture, ocean life). This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts from at least the COST InDust community (0000108:0000100 scientists from 52 countries). A list of papers to be submitted has been created including results related to SDS-WAS, ACTRIS/EARLINET activities, long-term measurement analysis of measured and modelled data, dust retrieval algorithm and techniques, dust effects, and foreseen results of the ESA Aeolus Cal/Val activity “ASKOS” (Cabo Verde, June - July, 2020).

Satellite limb measurements have become an essential tool to remotely sense the chemical composition of the Earth's atmosphere with a particular focus on the stratosphere. They enable combining near global sampling on a daily basis with vertical profiling capability and thus overcome some of the limitations of nadir or occultation measurements. Although the number of satellite limb instruments is currently declining, several missions are still operational (e.g., OSIRIS, SMR, ACE-FTS, MAESTRO, OMPS-LP and SAGE III) and several new missions are planned or will be launched in the near future (e.g., MATS, ICON, ALTIUS and several mini satellites). In addition, the data sets of several past limb sounders, particularly the Envisat instruments SCIAMACHY, MIPAS and GOMOS are still used for many scientific studies and new data products are developed based on measurements with these instruments.

This combined ACP/AMT special issue is open for submissions dealing with all aspects related to limb measurements (including limb scatter, limb emission and occultation) in the optical, IR and microwave spectral regions. This includes instrument development and characterization, retrieval algorithm development, validation studies, as well as scientific applications based on the retrieved data sets. We note that radio occultation measurements are outside the scope of the special issue.

The version 8 spectral data of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), released in January 2019, are supposed to be the final set. Besides the ORM reprocessed version 8 level-2 MIPAS data distributed by ESA, a research processor has been run at IMK and IAA to generate temperature and trace gas concentrations. The IMK–IAA data product includes a larger list of atmospheric constituents and covers a wider altitude range in the case of the special observation modes than the operational data. The processing is based on independent algorithms. This special issue collects articles which 1) document the retrieval strategies used to derive these distributions along with uncertainty estimates, 2) report validation studies, and 3) use the data to answer current questions in atmospheric research.

With the launch of the novel Aeolus wind profiling mission, a wealth of direct wind and atmospheric backscatter profile observations has become available across the globe, revealing new information on atmospheric dynamics and optical properties to the benefit of a number of applications. These applications include, for example, improvements in numerical weather prediction (NWP), general circulation models (GCMs) and their included parameterized processes, such as land and ocean drag, convection, stratosphere–troposphere interaction and atmospheric waves, air quality and air pollution transport, etc. In this special issue, new data products and their validation as well as studies using the Aeolus data for application in meteorology, air quality, and climate studies are discussed. Topics vary from observation interpretation to GCM or NWP model diagnostics, parameterization, and data assimilation experiments. This is the first special issue on scientific analysis of Aeolus mission data, including inter-comparisons with different collocated wind profiling, aerosol and cloud in situ and remote-sensing measurements, and atmospheric models. Contributions concerning Aeolus data calibration, processing, and tools for data access and exploitation are also provided.

This ACP Special Issue seeks to report research advancements in the understanding of mercury sources, distribution pathways, levels, physicochemical characterization, measurement traceability, impacts on global ecosystems, and policy-relevant information presented at the ICMGP 2019 and being developed under the MercOx (Metrology of oxidized mercury) project and two ERA-PLANET initiatives – Integrative and Comprehensive Understanding on Polar Environments (iCURE) and Integrated Global Observing Systems for Persistent Pollutants (iGOSP). The Special Issue is open to all publications related to the scope of the Special Issue.

This special issue includes papers based on recent field campaigns, including US Department of Energy Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA), NASA North Atlantic Aerosols and Marine Ecosystems Study (NAAMES), and the North Atlantic Climate System Integrated Study (ACSIS), and long-term measurements from observatories (e.g., the Atmospheric Radiation Measurement Eastern North Atlantic site on Graciosa Island in the Azores). Presently the responses of marine low-cloud systems to changes in atmospheric greenhouse gases and aerosols are major sources of uncertainty that limit our ability to predict future climate. Major contributions to this uncertainty derive from poor understanding of the cloud responses to aerosol changes and the natural aerosol state that is being perturbed by anthropogenic emissions. One overarching scientific objective of these recent field campaigns is to understand key processes that drive the properties and interactions of aerosol and clouds under representative meteorological and cloud conditions. Another important focus is to validate and improve ground-based retrieval algorithms using comprehensive in situ aircraft measurements. This will lead to high-quality, long-term datasets from the routine ground-based remote sensing, which allow greater statistical reliability in the observed properties and relationships among aerosols, clouds, and precipitation than is possible with the aircraft measurements alone.

With a wide variety of aerosol optical, physical, and chemical properties, China offers a test bed for the study of aerosol processes and techniques leading to a better understanding of aerosol properties. Aerosol concentrations in the southeast of China are periodically among the highest in the world, while at other times they are relatively low. The prevailing aerosol chemical composition (e.g., sulfate, nitrate, black carbon, brown carbon, particulate organic matter, sea salt) also changes significantly with the quick evolution of both the economy and society, along with the government’s strong environmental actions. The complex mechanisms of the secondary formation of aerosols and haze-fog interaction are still not well understood in this important region. In areas other than the southeast of China, topography determines the aerosol concentrations, which are consistently high. Examples are Sichuan Province or the Guanzhong Basin, i.e. large basins surrounded by high mountains. Furthermore, the large deserts in the west and north of China are strong sources of dust particles influencing the aerosol properties over a large part of the country. There are however also large areas with relatively low aerosol loading, in particular in mountainous areas and over the high plateaus such as Tibet. Aerosol chemical properties in the center and east of China still need to be clarified both geographically and temporally, e.g. distinguish spectrally absorbing components like mineral dust and brown carbon from remote sensing observations. Anthropogenic emissions of aerosols and precursor gases have a large influence on aerosol, in regards to both chemical and physical properties, and consequently optical properties, and their effects can be augmented by the transport of aerosol from elsewhere, such as carbonaceous particles from biomass burning aerosol and dust particles emitted from the deserts. The complex mechanisms and intrinsic relations of aerosol chemistry and physics, and their impacts on optical properties, need better interpretation based on comprehensive observation and analysis. Weather conditions have a large influence on the formation of haze, and haze episodes occur more and more frequently. The influence of large-scale weather systems such as the monsoon or the Siberian high may result in inter-annual variations in different regions of China. In particular the progress of the monsoon from south to north during spring and summer, and back in the autumn, results in strong variations in aerosol content.

In this special issue we focus on the application of remote sensing techniques using both ground-based and satellite measurements to study the temporal and spatial variations in aerosol properties over China on different scales. Results from campaigns in the Taklamakan Desert (Kashi), on the Tibetan Plateau, and other remote areas such as the NE of China are contrasted with those from densely populated and industrialized urban areas in the east of China. The special issue aims to bring together results from different projects such as Kashi, MOST, and regional air quality studies. Contributions are solicited on aerosol optical, physical, and chemical properties inferred from remote sensing observations and related optical techniques, and the spatial and temporal variations in these properties across China, with emphasis on processes leading to the observed differences, including the influence of meteorological conditions and large-scale weather systems.

The project Effect of Megacities on the Transport and Transformation of Pollutants on the Regional and Global Scales, EMeRGe, was created to experimentally investigate the transport and transformation of air plumes coming from major population centres, MPCs. It comprises a set of unique airborne measurement campaigns carried out in two parts of the world having MPCs of very different characteristics and emissions. The two measurement campaigns using the HALO research aircraft (www.halo-spp.de) were conducted in Europe in summer 2017 and in East Asia during the intermonsoon period in March-April 2018. The initial scientific team of EMeRGe, comprising scientists from four German universities and four research centres in 2016 (www.iup.uni-bremen.de/emerge/), was extended within the EMeRGe international membership, incorporating more than 50 research partners from 16 different countries. The combined and synergetic set of information obtained from airborne and ground-based data, satellite products, and their comparison with models enables the patterns of atmospheric composition and meteorological parameters observed during the campaigns to be constrained and thereby our understanding of the emissions, their transport, and physico-chemical transformation of the outflow from MPC emissions to be tested and assessed. The EMeRGe special issue aims to publish the scientific results achieved by the project EMeRGe. This comprises contributions of the partners of the national and international EMeRGe scientific community. It solicits all relevant submissions addressing the EMeRGe science objectives.

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 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).

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.

StratoClim is a collaborative research project funded by the European Commission 7th Framework programme. The main objective of the project is to improve the understanding of key processes in the upper troposphere and stratosphere (UTS) for more reliable projections of climate change and stratospheric ozone. StratoClim focuses on the understanding of the microphysical, chemical, and dynamical processes that determine the composition of the UTS, such as the formation, loss, and redistribution of aerosol, ozone, and water vapour; how these processes are affected by climate change; and how they feed back on global climate.

The issue will include papers coming from all sectors included in the project: in situ and sonde measurement deployment, satellite data products and analysis, mesoscale and transport modelling, and CCM experiments and analysis. We expect a special emphasis on the results from the high-altitude research aircraft campaign in the Asian monsoon area carried out in 2017 deploying an innovative and comprehensive payload. Due to the expected contribution of instrumental manuscripts, the issue will also include AMT submissions.

This inter-journal special issue of AMT, ACP, and GMD serves as a thematic collection for manuscripts arising from the work of Horizon 2020 integrated infrastructure project Eurochamp-2020; cf. https://www.eurochamp.org/ . As Eurochamp-2020 has project parts dealing with new instrumentation, with atmospheric chemistry and physics studies as well as with modelling, contributions are expected for all three journals as part of the inter-journal special issue.

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.

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.

Towards UNified Error Reporting (TUNER) is an emerging SPARC activity and an ISSI International Team. This project aims to provide consistent and inter-comparable error estimates for atmospheric temperature and composition measurements from space. Along with this, a consistent and inter-comparable characterization of spatial resolution and content of a priori information of the remotely sensed data shall be provided. This is important, because quantitative work with remotely sensed data – data assimilation, data merging, time series analysis, testing of hypotheses, etc. – depends largely on the adequate characterization of the data. Currently, multiple retrieval methods are used by the different instrument groups, and along with this various approaches to error estimation are applied. Resulting errors are not always inter-comparable. Some kinds of uncertainties are sometimes not reported at all. The different altitude resolutions and the different content of prior information in the data products are a particular problem. Within TUNER, data characterization methods currently in use are identified, their completeness and inter-comparability are assessed, recommendations for how unified data characterization should be performed and how data uncertainties should be reported are developed, and unified error estimates for some selected data products are provided. Beyond this, data users are instructed how to best utilize these metadata. This special issue is meant to include papers covering one of the following topics:

• methodical work related to the uncertainty assessment of satellite data on atmospheric temperature and composition;
• the error budget of satellite-borne atmospheric sounders;
• the use of results of validation studies to judge the adequacy of uncertainty estimates;
• recommendations for unified error estimates;
• tutorial articles on the correct use of error estimates and other diagnostic data and related topics.
This special issue is open to contributions from outside the related SPARC and ISSI TUNER teams, provided that the content fits within the scope of TUNER. Further details are available at http://www.imk-asf.kit.edu/english/304_2689.php.

SKYNET is an international research network dedicated to aerosol—cloud—radiation interaction studies.
It consists of about 60 sites located all over the world. The main instrument at each site is the sun—sky radiometer, but to strengthen the ability of SKYNET, simultaneous measurements with other instruments such as pyranometers, pirgeometers, microwave radiometers, absorption meters, cloud cameras, lidars, MAX-DOAS, and instrumentation for in situ characterisation are also conducted for some selected sites.
This special issue will face issues related to the following topics: aerosol and cloud properties from radiometers; developments on instrumentation; aerosol radiative forcing and climate effects; intercomparison among radiometer networks; validation of aerosol and cloud properties from satellite and models; applications for air pollution studies; and applications for solar energy.

The Water Vapour Phase II (WAVAS II), a SPARC activity, started in 2008 (SPARC Newsletter No. 30 (2008) p. 16: SPARC Water Vapour Initiative, by C. Schiller et al.). The activity includes satellite assessment and in situ comparison components. This international activity encompasses:

1) Providing a quality assessment of upper tropospheric to lower mesospheric satellite records since the early 1990s
2) Providing, as far as possible, absolute validation against ground-truth instruments
3) Assessing inter-instrument biases, depending on altitude, location, and season
4) Assessing the representation of temporal variations on various scales
5) Including data records on isotopologues
6) Providing recommendations for usage of available data records and for future observation systems


The main objective of WAVAS II is to assess and extend our knowledge and understanding of measurements of the vertical distribution of water vapor in the upper troposphere and middle atmosphere (UT/MA), where water has small concentrations, but significant radiative impact. This is a follow-up of the SPARC WAVAS activity, whose report was published in 2000 (SPARC Report No. 2 (2000) Upper Tropospheric and Stratospheric Water Vapour. D. Kley, J.M. Russell III, and C. Philips (eds.). WCRP-113, WMO/TD – No. 1043). Information gained from this activity will improve our ability to estimate long-term changes with associated uncertainties in UT/MA water as well as make recommendations as to what data would be most valuable for model validation and how such data should be used.

Papers will be accepted for this special issue according to the following guidelines, independent if they originate from the WAVAS II activity or other activities.

Guidelines for submissions:

• Papers covering existing UT/MA satellite water vapour measurements;
• Papers discussing comparisons of UT/MA satellite measurements, including discussion of quantities derived from these measurements, such as seasonal cycles, estimates of transport, or estimates of drifts, trends and variability.
• Papers discussing merging of water vapour measurements will be considered, although this topic is not specifically part of the WAVAS-II activity.
• Model papers that incorporate the datasets discussed and the uncertainty estimates resulting from the WAVAS-II activity will also be considered for inclusion.

The Chemistry and Aerosol Mediterranean Experiment (ChArMEx) special issue will be simultaneously presented in the ACP and AMT journals. It aims at gathering experimental and modelling contributions to the field of atmospheric chemistry in the Mediterranean region and its impacts on regional air quality and climate, both in the recent past, present and future decades. It addresses natural and anthropogenic emissions of tropospheric reactive species, source apportionment, chemical transformations, transport processes, atmospheric deposition, aerosol optical properties and interactions with radiation, water vapour and clouds, variability and trends, and future conditions following climate change and increasing anthropogenic pressure.