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.

Atmospheric ozone is one of the most important trace gases in the Earth's atmosphere due to its multiple roles: filtering harmful levels of ultraviolet (UV) solar radiation and shortwave absorption, being a greenhouse gas in the Earth's climate system, and being a powerful oxidant in the troposphere. Also, surface level ozone produces harmful effects on human health, ecosystems, and agricultural production. The latest findings and emerging research on ozone-related topics were presented at the 2021 Quadrennial Ozone Symposium (QOS) held online on October 3–9 2021. This special issue covers studies presented at the QOS and other relevant studies that are broadly related to the following subject areas:

1. stratospheric ozone science;
2. tropospheric ozone science;
3. ozone-depleting substances sources, sinks, and budget;
4. ozone, climate, and meteorology;
5. ozone monitoring and measurement techniques;
6. environmental and human health effects of atmospheric ozone and UV radiation.

This issue is open to all submissions within its scope.

The Clouds, Aerosol Monsoon Processes Philippines Experiment (CAMP2Ex) was a NASA-led study to understand the interdisciplinary relationships between aerosol lifecyle, convection and the radiation budget within the Southeast Asian boreal summer southwest monsoon system. Central to CAMP2Ex was a 25 August–5 October 2019 airborne campaign based at Clark, Philippines, including the NASA P3 and SPEC Inc. Lear 35 research aircraft outfitted with radars, microwave sounders, lidar, radiation, cloud microphysics, composition and state instrumentation, aligned with significant remote sensing data collections and informatics efforts. Operations were the pinnacle of a multi-year monitoring effort that included development of an aerosol and radiation monitoring site at the Manila Observatory, performed in partnership with the ONR-sponsored Propagation of Intraseasonal Tropical Oscillations (PISTON) R/V Sally Ride deployment and contributing to the International Years of the Maritime Continent (YMC) effort. Given its interdisciplinary nature, CAMP2Ex is generating papers on numerous classes of research. We request a Special Issue be granted to the CAMP2Ex team and its partners specifically as a venue for aerosol–cloud–radiation research.

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 EarthCARE satellite (Earth Cloud, Aerosol and Radiation Explorer) is a joint ESA–JAXA mission due for launch in 2023, carrying a Doppler cloud profiling radar (CPR), a high spectral-resolution atmospheric lidar (ATLID), a multi-spectral imager (MSI) and a broadband radiometer (BBR). A large number of cloud, aerosol, precipitation and radiation data products will be produced, some derived from individual EarthCARE instruments and some from the synergy of multiple instruments. This collection of papers will document the theoretical basis for the EarthCARE Level 2 algorithms and evaluate their performance. An innovative aspect that links a number of the papers together is the use of realistic 3D test scenes, 6000 km in length, with cloud, precipitation and aerosol fields from a high-resolution cloud-resolving model and an aerosol transport model, from which observations by the four Earth CARE instruments have been simulated using state-of-the-art instrument simulators. This approach has enabled these algorithms to be evaluated and compared on a common dataset. The special issue is limited to invited papers describing official ESA or JAXA retrieval algorithms and their evaluation, plus several closely related papers.

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 (http://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 (http://www.iup.uni-bremen.de/emerge/home/home.html), 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.

In April 2017, the Senate of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) established the Priority Programme “Polarimetric Radar Observations meet Atmospheric Modelling (PROM)” (SPP 2115). The programme is designed to run for 6 years and is now in the third funding year. The overall motivation of the SPP PROM is a better understanding and representation of cloud and precipitation processes in numerical weather prediction and climate models. Since 2015 the whole atmosphere over Germany has been monitored by 17 state-of-the-art polarimetric Doppler weather radar observations suitable to challenge the representation of cloud and precipitation processes in atmospheric models. Data assimilation merges observations and models for state estimation as a prerequisite for prediction and can be regarded as a smart interpolation between observations while exploiting the physical consistency of atmospheric models as mathematical constraints. However, considerable knowledge gaps exist both in radar polarimetry and atmospheric models, which impede the full exploitation of the triangle radar-polarimetry–atmospheric-models–data-assimilation. Objectives of SPP PROM and thus the envisioned publications within the special issue are (1) the exploitation of radar polarimetry for quantitative process detection in precipitating clouds and for model evaluation, (2) the improvement of cloud and precipitation schemes in atmospheric models based on process fingerprints detectable in polarimetric observations, (3) monitoring of the energy budget evolution due to phase changes in the cloudy, precipitating atmosphere for a better understanding of its dynamics, (4) the generation of precipitation system analyses by the assimilation of polarimetric radar observations into atmospheric models for weather forecasting, and (5) radar-based detection of the initiation of convection for the improvement of thunderstorm prediction.

Geostationary Environment Monitoring Spectrometer (GEMS) is the first instrument to observe air quality from a geostationary Earth orbit (GEO) successfully launched on 19 February 2020 and in initial operation after in-orbit tests (IOTs). GEMS provides hourly air quality information on both aerosols and gases at unprecedented spatial resolution of 7 km × 8 km. GEMS is a scanning UV–visible spectrometer measuring the hyperspectral spectrum in the ultraviolet and visible, which allows for the observation of key atmospheric constituents including O₃, NO₂, CO, SO₂, CH₂O, CHOCHO, aerosols, clouds, and UV indices. The mission opened a new era of air quality monitoring from space and will be joined by NASA’s TEMPO and ESA’s Sentinel-4 to form the GEO Air Quality Constellation in ~3 years to cover the most polluted region in the Northern Hemisphere. In this ACP–AMT special issue, the first results will be presented including instruments, IOT results, initial products, algorithm, calibration, validation from the recent GMAP field campaign, applications in modelling, etc.

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.

The Tibetan Plateau is referred to as the third pole of the world and is representative of background region of the Earth’s atmosphere. Atmospheric photochemistry, in conjunction with the unique circulation pattern of Asian Monsoon outflow and prevailing westerly wind, leads to, as of yet, unidentified implications for climate change, the oxidative capacity of the atmosphere, local air quality, and health effects on local residents and visitors.

For a better scientific understanding of the background atmospheric chemistry and its impacts on climate change, air quality, and human health, multiple international campaigns, referred to as the @Tibet field campaigns, have been conducted since 2019 under the umbrella of the second Tibetan Plateau Scientific Expedition and Research Program (STEP). As supported by the major research plan of the National Natural Science Foundation of China (NSFC) entitled fundamental researches on the formation and response mechanism of air pollution complex, the research conducted in the Tibetan Plateau and eastern China was underpinned by a number of instrument developments required to meet the specific demands of exploring air pollution and fundamental atmospheric photochemistry. This AMT–ACP joint special issue is therefore designed to intensively discuss data from @Tibet field campaigns and instrument development progress.

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.

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.

The proposed special issue has the goal of collecting original publications addressing profiles of the atmospheric boundary layer from a variety of perspectives. Aligned with the objectives of the EU COST action PROBE (http://probe-cost.eu), topics include both observational aspects (latest technological sensor developments, quality control and retrieval methods, calibration, implementation of harmonised monitoring networks, measurement campaigns, instrument synergy, combination with satellite data) and the exploitation of the profile observations (process understanding, model development and assessment, data assimilation) for a wide range of applications (including meteorology and climate, air quality, renewable energy, radio propagation).

ACTRIS statements of the purpose of the special issue on stay-at-home policies in response to the COVID-19 pandemic have resulted in an unprecedented decrease in pollutant emissions and in a well-publicized improvement of air quality in many cities in Asia, Europe and America. While the impact of lockdown on air quality was unambiguously detected in the urban areas through both in situ and space remote-sensing observations and its spatial and temporal extents, the specific role of meteorology and the cascade responses from indirect and non-linear effects are far from being fully evaluated. Throughout the very specific year 2020, ACTRIS and its partner institutions engaged in a pan-European effort to document the impact of governmental policies on atmospheric composition. ACTRIS maintained its operations at fully nominal standards, even increasing its sampling capacity in some cases. The ACTRIS data collection containing measurements of aerosol, cloud and trace gas properties across Europe measured during the year 2020 has been compiled and made available by the ACTRIS Data Centre units. Data from the year 2021 will soon follow. A community of scientists started evaluating the impact of the repeated lockdowns over the European regions. Because concentration and properties of short-lived atmospheric species are highly variable in space and time, evaluating the impact of reduced emissions is not straightforward. The special issue “Quantifying the impacts of stay-at-home policies on atmospheric composition and properties of aerosol and clouds over the European regions using ACTRIS related observations” will gather a series of scientific papers dealing with the measurable effects of lockdown measures over Europe. The special issue will particularly be dealing with • quantifying the spatial and temporal extent of stay-at-home policies on the European atmosphere, at both local and regional scales, • evaluating the impact of lockdown measures on the formation of secondary pollutants, • documenting the impact of reduced emissions (including air-traffic emissions) on cloud properties and occurrence, and • estimating the “missing” emissions using observation–model approaches. The outcome from the special issue aims to provide an in-depth analysis of the perturbation induced by the repeated lockdowns on the complex atmospheric system. The special issue is open for all submissions within its scope.

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.

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 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 proposed special issue has the goal of collecting original publications addressing profiles of the atmospheric boundary layer from a variety of perspectives. Aligned with the objectives of the EU COST action PROBE (http://probe-cost.eu), topics include both observational aspects (latest technological sensor developments, quality control and retrieval methods, calibration, implementation of harmonised monitoring networks, measurement campaigns, instrument synergy, combination with satellite data) and the exploitation of the profile observations (process understanding, model development and assessment, data assimilation) for a wide range of applications (including meteorology and climate, air quality, renewable energy, radio propagation).

The Tibetan Plateau is referred to as the third pole of the world and is representative of background region of the Earth’s atmosphere. Atmospheric photochemistry, in conjunction with the unique circulation pattern of Asian Monsoon outflow and prevailing westerly wind, leads to, as of yet, unidentified implications for climate change, the oxidative capacity of the atmosphere, local air quality, and health effects on local residents and visitors.

For a better scientific understanding of the background atmospheric chemistry and its impacts on climate change, air quality, and human health, multiple international campaigns, referred to as the @Tibet field campaigns, have been conducted since 2019 under the umbrella of the second Tibetan Plateau Scientific Expedition and Research Program (STEP). As supported by the major research plan of the National Natural Science Foundation of China (NSFC) entitled fundamental researches on the formation and response mechanism of air pollution complex, the research conducted in the Tibetan Plateau and eastern China was underpinned by a number of instrument developments required to meet the specific demands of exploring air pollution and fundamental atmospheric photochemistry. This AMT–ACP joint special issue is therefore designed to intensively discuss data from @Tibet field campaigns and instrument development progress.

Geostationary Environment Monitoring Spectrometer (GEMS) is the first instrument to observe air quality from a geostationary Earth orbit (GEO) successfully launched on 19 February 2020 and in initial operation after in-orbit tests (IOTs). GEMS provides hourly air quality information on both aerosols and gases at unprecedented spatial resolution of 7 km × 8 km. GEMS is a scanning UV–visible spectrometer measuring the hyperspectral spectrum in the ultraviolet and visible, which allows for the observation of key atmospheric constituents including O₃, NO₂, CO, SO₂, CH₂O, CHOCHO, aerosols, clouds, and UV indices. The mission opened a new era of air quality monitoring from space and will be joined by NASA’s TEMPO and ESA’s Sentinel-4 to form the GEO Air Quality Constellation in ~3 years to cover the most polluted region in the Northern Hemisphere. In this ACP–AMT special issue, the first results will be presented including instruments, IOT results, initial products, algorithm, calibration, validation from the recent GMAP field campaign, applications in modelling, etc.

Atmospheric ozone is one of the most important trace gases in the Earth's atmosphere due to its multiple roles: filtering harmful levels of ultraviolet (UV) solar radiation and shortwave absorption, being a greenhouse gas in the Earth's climate system, and being a powerful oxidant in the troposphere. Also, surface level ozone produces harmful effects on human health, ecosystems, and agricultural production. The latest findings and emerging research on ozone-related topics were presented at the 2021 Quadrennial Ozone Symposium (QOS) held online on October 3–9 2021. This special issue covers studies presented at the QOS and other relevant studies that are broadly related to the following subject areas:

1. stratospheric ozone science;
2. tropospheric ozone science;
3. ozone-depleting substances sources, sinks, and budget;
4. ozone, climate, and meteorology;
5. ozone monitoring and measurement techniques;
6. environmental and human health effects of atmospheric ozone and UV radiation.

This issue is open to all submissions within its scope.

ACTRIS statements of the purpose of the special issue on stay-at-home policies in response to the COVID-19 pandemic have resulted in an unprecedented decrease in pollutant emissions and in a well-publicized improvement of air quality in many cities in Asia, Europe and America. While the impact of lockdown on air quality was unambiguously detected in the urban areas through both in situ and space remote-sensing observations and its spatial and temporal extents, the specific role of meteorology and the cascade responses from indirect and non-linear effects are far from being fully evaluated. Throughout the very specific year 2020, ACTRIS and its partner institutions engaged in a pan-European effort to document the impact of governmental policies on atmospheric composition. ACTRIS maintained its operations at fully nominal standards, even increasing its sampling capacity in some cases. The ACTRIS data collection containing measurements of aerosol, cloud and trace gas properties across Europe measured during the year 2020 has been compiled and made available by the ACTRIS Data Centre units. Data from the year 2021 will soon follow. A community of scientists started evaluating the impact of the repeated lockdowns over the European regions. Because concentration and properties of short-lived atmospheric species are highly variable in space and time, evaluating the impact of reduced emissions is not straightforward. The special issue “Quantifying the impacts of stay-at-home policies on atmospheric composition and properties of aerosol and clouds over the European regions using ACTRIS related observations” will gather a series of scientific papers dealing with the measurable effects of lockdown measures over Europe. The special issue will particularly be dealing with • quantifying the spatial and temporal extent of stay-at-home policies on the European atmosphere, at both local and regional scales, • evaluating the impact of lockdown measures on the formation of secondary pollutants, • documenting the impact of reduced emissions (including air-traffic emissions) on cloud properties and occurrence, and • estimating the “missing” emissions using observation–model approaches. The outcome from the special issue aims to provide an in-depth analysis of the perturbation induced by the repeated lockdowns on the complex atmospheric system. The special issue is open for all submissions within its scope.

The EarthCARE satellite (Earth Cloud, Aerosol and Radiation Explorer) is a joint ESA–JAXA mission due for launch in 2023, carrying a Doppler cloud profiling radar (CPR), a high spectral-resolution atmospheric lidar (ATLID), a multi-spectral imager (MSI) and a broadband radiometer (BBR). A large number of cloud, aerosol, precipitation and radiation data products will be produced, some derived from individual EarthCARE instruments and some from the synergy of multiple instruments. This collection of papers will document the theoretical basis for the EarthCARE Level 2 algorithms and evaluate their performance. An innovative aspect that links a number of the papers together is the use of realistic 3D test scenes, 6000 km in length, with cloud, precipitation and aerosol fields from a high-resolution cloud-resolving model and an aerosol transport model, from which observations by the four Earth CARE instruments have been simulated using state-of-the-art instrument simulators. This approach has enabled these algorithms to be evaluated and compared on a common dataset. The special issue is limited to invited papers describing official ESA or JAXA retrieval algorithms and their evaluation, plus several closely related papers.

In April 2017, the Senate of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) established the Priority Programme “Polarimetric Radar Observations meet Atmospheric Modelling (PROM)” (SPP 2115). The programme is designed to run for 6 years and is now in the third funding year. The overall motivation of the SPP PROM is a better understanding and representation of cloud and precipitation processes in numerical weather prediction and climate models. Since 2015 the whole atmosphere over Germany has been monitored by 17 state-of-the-art polarimetric Doppler weather radar observations suitable to challenge the representation of cloud and precipitation processes in atmospheric models. Data assimilation merges observations and models for state estimation as a prerequisite for prediction and can be regarded as a smart interpolation between observations while exploiting the physical consistency of atmospheric models as mathematical constraints. However, considerable knowledge gaps exist both in radar polarimetry and atmospheric models, which impede the full exploitation of the triangle radar-polarimetry–atmospheric-models–data-assimilation. Objectives of SPP PROM and thus the envisioned publications within the special issue are (1) the exploitation of radar polarimetry for quantitative process detection in precipitating clouds and for model evaluation, (2) the improvement of cloud and precipitation schemes in atmospheric models based on process fingerprints detectable in polarimetric observations, (3) monitoring of the energy budget evolution due to phase changes in the cloudy, precipitating atmosphere for a better understanding of its dynamics, (4) the generation of precipitation system analyses by the assimilation of polarimetric radar observations into atmospheric models for weather forecasting, and (5) radar-based detection of the initiation of convection for the improvement of thunderstorm prediction.

The Clouds, Aerosol Monsoon Processes Philippines Experiment (CAMP2Ex) was a NASA-led study to understand the interdisciplinary relationships between aerosol lifecyle, convection and the radiation budget within the Southeast Asian boreal summer southwest monsoon system. Central to CAMP2Ex was a 25 August–5 October 2019 airborne campaign based at Clark, Philippines, including the NASA P3 and SPEC Inc. Lear 35 research aircraft outfitted with radars, microwave sounders, lidar, radiation, cloud microphysics, composition and state instrumentation, aligned with significant remote sensing data collections and informatics efforts. Operations were the pinnacle of a multi-year monitoring effort that included development of an aerosol and radiation monitoring site at the Manila Observatory, performed in partnership with the ONR-sponsored Propagation of Intraseasonal Tropical Oscillations (PISTON) R/V Sally Ride deployment and contributing to the International Years of the Maritime Continent (YMC) effort. Given its interdisciplinary nature, CAMP2Ex is generating papers on numerous classes of research. We request a Special Issue be granted to the CAMP2Ex team and its partners specifically as a venue for aerosol–cloud–radiation research.

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.

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 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 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.

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 (http://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 (http://www.iup.uni-bremen.de/emerge/home/home.html), 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.

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.

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.

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.