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).