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/).

A large number of machine learning studies are currently performed in Earth System science to extract information from observations or model data, to learn the dynamics of components of the Earth system, or to build tools to improve weather and climate predictions (for example in model post-processing). These studies require both the domain knowledge of Earth system scientists and the expertise of machine learning experts. The astounding success of machine learning in other scientific disciplines – such as computer vision, speech recognition, robotics, and autonomous driving – is not least a result of the free and open sharing of benchmark datasets and software, which allows easy reproducibility by the community as well as the quantitative comparison of the quality of machine learning solutions. Contrary to many other machine learning disciplines, Earth system datasets and validation methods require thorough documentation to achieve the goal of providing a fair and firm basis for model comparisons according to the accepted community standards. Therefore, the Earth system science community has expressed a need for dedicated benchmark datasets and machine learning tools to process Earth system data.

With this special issue, we offer a platform to develop and openly share these benchmark datasets together with specific machine learning tasks and evaluation metrics as well as innovative solutions for machine learning applications in Earth system science with the wider community of machine learners and environmental scientists.

The special issue is managed jointly by ESSD and GMD and invites manuscripts focusing either on the description of new benchmark datasets (ESSD) or new machine learning model developments (GMD). Dataset papers can reference model papers and vice versa, but this is not a requirement as long as both the datasets and model codes are made freely available and easily accessible. To be accepted as special issue contribution a manuscript must define a specific machine learning task including a loss function and evaluation metric and the authors must provide executable code which demonstrates the machine learning problem with a vanilla solution. The usual guidelines of both journals apply.

This special issue serves as a collection point for papers describing the datasets collected and developed during the "Elucidating the role of clouds–circulation coupling in climate" (EUREC4A) field campaign. This campaign, which served as an organizing umbrella for various complementary efforts such as the Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (ATOMIC), was conducted in January and February of 2020 and took place in and over the tropical Atlantic Ocean around Barbados. These efforts involved broad international participation and the deployment of a wide array of observational platforms, including several research vessels, manned and robotic research aircraft, autonomous ocean samplers, enhanced satellite observations, and land-based observational capabilities. In addition, several groups produced specialized forecasting tools in support of campaign planning, and several others are in the process of preparing forcing datasets for numerical experiments that bridge a variety of scales. This special issue is envisioned to include invited and contributed papers related to the datasets developed as part of EUREC4A, ATOMIC, and related activities.

The Arctic, the Antarctic, and the Tibetan Plateau are called the three poles. Although the regions are geographically separated, they are often regarded as a group because of the similar extreme cryosphere environments and their critical role in maintaining Earth’s climate, hydrology, and ecosystem conditions. The three poles contain the world’s highest mountains and extreme environments and store more snow, ice, and freshwater than anywhere else in the world. However, the regions are found to be particularly sensitive to climate change, and their changes could also lead to critical feedback loops of global climate changes and the sustainability of human society. Unfortunately, our understanding of the three poles, especially the connections among the regions, is still limited. The spare and scattered data collected and produced for these harsh environments are one of the main bottlenecks for promoting comprehensive studies of the regions. With this special issue, we are looking forward to inviting experts to review and present existing and new datasets for the three poles region, to recognize these valuable datasets, and to promote their use in international and interdisciplinary studies of the three poles.

The glacial–interglacial cyclicity of the climate system varied in the past, most notably during the transition from a 40 ka to a 100 ka world in the mid-Pleistocene. Gases trapped in Antarctic ice are the most direct access available to investigate the composition of the paleo-atmosphere of that age and processes related to climate variability. The International Partnerships in Ice Core Sciences (IPICS) Oldest Ice endeavour aims at obtaining an undisturbed ice-core record older than 1 Ma. In addition to ice cores, time slices of paleo-records, available, for example, in Antarctic blue-ice fields, provide further valuable information, which complements continuous time series based on ice cores. This special issue will assemble contributions dedicated to the preparatory phase of this global effort to obtain ice samples and time series older than 700 000 years. This includes a consideration of glaciogical and geophysical settings which allow the presence of old ice, results from pre-site surveys and modelling studies, aspects of ice-core and other sampling techniques and analyses, and requirements for drilling and core handling.

The climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses may give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets with respect to a variety of key diagnostics. The objectives of this project are

1. to understand the causes of differences among reanalyses;
2. to provide guidance on the appropriate usage of various reanalysis products in scientific studies;
3. to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

The project focuses predominantly on differences among reanalyses (although studies that include operational analyses are welcome and studies comparing reanalyses with observations are encouraged), with an emphasis on diagnostics in the upper troposphere, stratosphere and mesosphere. This special issue serves to collect research with relevance to S-RIP in preparation for the publication of the S-RIP report in 2018. Although participation in S-RIP is not a prerequisite for submission to this special issue, authors contributing to this collection are encouraged to consider contributing to the preparation of the S-RIP report.

The last interglacial (125 ka) represents the last warm period of the Earth’s history when climate was warmer than present. Understanding sea-level changes in the last interglacial is important for unravelling future ice melting scenarios. Although the age, elevation, and geological characteristics of last interglacial sea-level proxies have been reported in the literature, thus far few organized data collections have been compiled. With this special issue, we want to invite regional experts on last interglacial sea-level proxies to review existing (and new) last interglacial sea-level data for specific regions. Each paper will provide a description of the geological data submitted and the data itself in a standardized formatted data table (spreadsheet). Each regional data collection will be compiled using an online form connected to a mySQL database, which will be made available in an editorial, curated by the editors listed hereafter. After publication, the database and the insertion form will remain accessible online, and will be periodically updated as new data become available. This special issue is supported by PALSEA (a PAGES-INQUA working group) and by WARMCOASTS (ERC StG 802414).

Authors interested to contribute to this special issue are kindly invited to contact the Special Issue Editors (via arovere@marum.de) to define geographic areas of interest and to receive instructions on how to compile the database through an on-line interface.

A large number of machine learning studies are currently performed in Earth System science to extract information from observations or model data, to learn the dynamics of components of the Earth system, or to build tools to improve weather and climate predictions (for example in model post-processing). These studies require both the domain knowledge of Earth system scientists and the expertise of machine learning experts. The astounding success of machine learning in other scientific disciplines – such as computer vision, speech recognition, robotics, and autonomous driving – is not least a result of the free and open sharing of benchmark datasets and software, which allows easy reproducibility by the community as well as the quantitative comparison of the quality of machine learning solutions. Contrary to many other machine learning disciplines, Earth system datasets and validation methods require thorough documentation to achieve the goal of providing a fair and firm basis for model comparisons according to the accepted community standards. Therefore, the Earth system science community has expressed a need for dedicated benchmark datasets and machine learning tools to process Earth system data.

With this special issue, we offer a platform to develop and openly share these benchmark datasets together with specific machine learning tasks and evaluation metrics as well as innovative solutions for machine learning applications in Earth system science with the wider community of machine learners and environmental scientists.

The special issue is managed jointly by ESSD and GMD and invites manuscripts focusing either on the description of new benchmark datasets (ESSD) or new machine learning model developments (GMD). Dataset papers can reference model papers and vice versa, but this is not a requirement as long as both the datasets and model codes are made freely available and easily accessible. To be accepted as special issue contribution a manuscript must define a specific machine learning task including a loss function and evaluation metric and the authors must provide executable code which demonstrates the machine learning problem with a vanilla solution. The usual guidelines of both journals apply.

The Arctic, the Antarctic, and the Tibetan Plateau are called the three poles. Although the regions are geographically separated, they are often regarded as a group because of the similar extreme cryosphere environments and their critical role in maintaining Earth’s climate, hydrology, and ecosystem conditions. The three poles contain the world’s highest mountains and extreme environments and store more snow, ice, and freshwater than anywhere else in the world. However, the regions are found to be particularly sensitive to climate change, and their changes could also lead to critical feedback loops of global climate changes and the sustainability of human society. Unfortunately, our understanding of the three poles, especially the connections among the regions, is still limited. The spare and scattered data collected and produced for these harsh environments are one of the main bottlenecks for promoting comprehensive studies of the regions. With this special issue, we are looking forward to inviting experts to review and present existing and new datasets for the three poles region, to recognize these valuable datasets, and to promote their use in international and interdisciplinary studies of the three poles.

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.

This special issue serves as a collection point for papers describing the datasets collected and developed during the "Elucidating the role of clouds–circulation coupling in climate" (EUREC4A) field campaign. This campaign, which served as an organizing umbrella for various complementary efforts such as the Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (ATOMIC), was conducted in January and February of 2020 and took place in and over the tropical Atlantic Ocean around Barbados. These efforts involved broad international participation and the deployment of a wide array of observational platforms, including several research vessels, manned and robotic research aircraft, autonomous ocean samplers, enhanced satellite observations, and land-based observational capabilities. In addition, several groups produced specialized forecasting tools in support of campaign planning, and several others are in the process of preparing forcing datasets for numerical experiments that bridge a variety of scales. This special issue is envisioned to include invited and contributed papers related to the datasets developed as part of EUREC4A, ATOMIC, and related activities.

The last interglacial (125 ka) represents the last warm period of the Earth’s history when climate was warmer than present. Understanding sea-level changes in the last interglacial is important for unravelling future ice melting scenarios. Although the age, elevation, and geological characteristics of last interglacial sea-level proxies have been reported in the literature, thus far few organized data collections have been compiled. With this special issue, we want to invite regional experts on last interglacial sea-level proxies to review existing (and new) last interglacial sea-level data for specific regions. Each paper will provide a description of the geological data submitted and the data itself in a standardized formatted data table (spreadsheet). Each regional data collection will be compiled using an online form connected to a mySQL database, which will be made available in an editorial, curated by the editors listed hereafter. After publication, the database and the insertion form will remain accessible online, and will be periodically updated as new data become available. This special issue is supported by PALSEA (a PAGES-INQUA working group) and by WARMCOASTS (ERC StG 802414).

Authors interested to contribute to this special issue are kindly invited to contact the Special Issue Editors (via arovere@marum.de) to define geographic areas of interest and to receive instructions on how to compile the database through an on-line interface.

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 climate research community uses reanalyses widely to understand atmospheric processes and variability in the middle atmosphere, yet different reanalyses may give very different results for the same diagnostics. For example, the global energy budget and hydrological cycle, the Brewer–Dobson circulation, stratospheric vortex weakening and intensification events, and large-scale wave activity at the tropical tropopause are known to differ among reanalyses.

The Stratosphere–troposphere Processes And their Role in Climate (SPARC) Reanalysis Intercomparison Project (S-RIP) is a coordinated activity to compare reanalysis data sets with respect to a variety of key diagnostics. The objectives of this project are

1. to understand the causes of differences among reanalyses;
2. to provide guidance on the appropriate usage of various reanalysis products in scientific studies;
3. to contribute to future improvements in the reanalysis products by establishing collaborative links between the reanalysis centres and the SPARC community.

The project focuses predominantly on differences among reanalyses (although studies that include operational analyses are welcome and studies comparing reanalyses with observations are encouraged), with an emphasis on diagnostics in the upper troposphere, stratosphere and mesosphere. This special issue serves to collect research with relevance to S-RIP in preparation for the publication of the S-RIP report in 2018. Although participation in S-RIP is not a prerequisite for submission to this special issue, authors contributing to this collection are encouraged to consider contributing to the preparation of the S-RIP report.

The glacial–interglacial cyclicity of the climate system varied in the past, most notably during the transition from a 40 ka to a 100 ka world in the mid-Pleistocene. Gases trapped in Antarctic ice are the most direct access available to investigate the composition of the paleo-atmosphere of that age and processes related to climate variability. The International Partnerships in Ice Core Sciences (IPICS) Oldest Ice endeavour aims at obtaining an undisturbed ice-core record older than 1 Ma. In addition to ice cores, time slices of paleo-records, available, for example, in Antarctic blue-ice fields, provide further valuable information, which complements continuous time series based on ice cores. This special issue will assemble contributions dedicated to the preparatory phase of this global effort to obtain ice samples and time series older than 700 000 years. This includes a consideration of glaciogical and geophysical settings which allow the presence of old ice, results from pre-site surveys and modelling studies, aspects of ice-core and other sampling techniques and analyses, and requirements for drilling and core handling.