Through their interaction with solar and thermal infrared radiation, clouds and aerosols have been implicated as two of the greatest sources of uncertainty in climate projections. The EarthCARE (Earth Cloud, Aerosol and Radiation Explorer) satellite is a joint ESA–JAXA mission designed to provide key new information to tackle this crucial challenge. Launched on 28 May 2024, it carries a Doppler cloud profiling radar (CPR), a high-spectral-resolution atmospheric lidar (ATLID), a multi-spectral imager (MSI), and a three-view broadband radiometer (BBR). CPR is the first radar in space that can measure how fast cloud and precipitation particles are falling in the atmosphere or rising in the cores of thunderstorms, while the ability of ATLID to separately measure the backscatter of particles and air molecules enables it to retrieve the all-important profile of the cloud and aerosol extinction coefficient without having to make any assumption regarding the scattering properties of the particles. A large number of cloud, aerosol, precipitation, and radiation data products are being produced, some derived from individual EarthCARE instruments and others from the synergy of multiple instruments.

This inter-journal special issue (SI) is dedicated to presenting early scientific results using data from the satellite. It is open to the submission of papers that use EarthCARE data and fall into the remit of one of the three journals participating in the SI: Atmospheric Measurement Techniques, Atmospheric Chemistry and Physics, and Geoscientific Model Development. Topics can include, but are not limited to, the following: calibration and validation of EarthCARE data, development of new retrieval algorithms and improvement of existing algorithms, generation of new open-access datasets, improvement of the understanding of atmospheric properties and processes, evaluation of atmospheric models, assimilation of EarthCARE data into weather forecast and air quality models, and combination of EarthCARE with other datasets to estimate climate trends.

Aerosols are particles floating in the Earth's atmosphere and are linked with the largest uncertainty in estimates and interpretations of the Earth's changing energy budget. Measurement principles differ depending on the desired derived aerosol optical parameter and on the measurement platform (surface or space). Common aerosol columnar properties' retrieval techniques consist of direct measurement of a bright source of radiation (sun, star, moon, sky) with multi-wavelength photometers. Several global photometric aerosol networks exist. However, there are several instrumental, algorithm-based, and hardware-based differences in their related aerosol products, and global standardization is needed. In addition, in order to improve and optimize sun- and moon-photometric aerosol measurements, a network of aerosol scientists and operators, aerosol measurement users and software, and hardware developers is needed.

In the frame of the EU COST Action Harmonia, International network for harmonisation of atmospheric aerosol retrievals from ground based photometers (https://harmonia-cost.eu/), a network of 150 people has been established who are working towards improving sun-photometric aerosol measurements and also linking measurements with various effects and applications dealing with aerosol columnar properties. During the first 2 years of the project, various aspects were identified as scientific gaps in our current knowledge on aerosol measurements, modelling, processes, and effects, with a critical mass of scientists working towards addressing them. Such gaps include issues such as

• a need for improvement of the aerosol measurements from surface-based instruments, including use of new technologies, improved algorithms, and combined approaches;
• the harmonization of aerosol optical properties from different aerosol networks (filter radiometers, spectroradiometers);
• aerosol trend analysis and satellite validation;
• sun-photometric synergies with in situ and lidar aerosol measurements and also modelling assimilation;
• the presentation of intensive campaign results with a focus on aerosols and aerosol–cloud–radiation interactions;
• enhancements regarding aerosol effects on different communities (e.g. solar energy, aviation, health, climate, air quality, agriculture).

This special issue focuses on addressing the above-mentioned gaps through individual and combined efforts of the Harmonia community and others. A list of papers to be submitted has been created, including results related to Harmonia and also ACTRIS research infrastructure (SK2) activities, long-term measurement analysis of measured data, aerosol retrieval algorithms and techniques, and aerosol effects.

We invite original research, reviews, and case studies that focus on atmospheric greenhouse gas (GHG) monitoring, emission estimates, and mitigation strategies, particularly in the Asia–Pacific region.

The climate crisis is one of the grand challenges of the 21st century. The increase in Earth's surface temperature, commonly known as global warming or anthropogenically induced climate change, is primarily driven by the rise in greenhouse gases (GHGs) in the atmosphere. The two most significant greenhouse gases affected by human activity are carbon dioxide (CO₂) and methane (CH₄).

While human activities such as fossil fuel combustion, oil and gas exploration, waste management, and agriculture are major sources of GHGs, natural sources also play a significant role. Extensive wetlands, for example, are the largest natural source of CH₄ globally. In wetlands, methane is produced by soil microbes and plants that metabolize under anaerobic conditions and is then released into the atmosphere through diffusion, transport via plant tissues, and gas bubble emissions. These processes make global wetlands among the most important yet least understood sources and sinks in the global methane and CO₂ budget.

A major scientific challenge in this context is distinguishing between methane emissions from natural sources and those resulting from human activities. Our understanding of these processes, their relative magnitudes, and the associated feedback mechanisms – such as increased wildfire activity, permafrost thaw, or changes in inundation patterns – is still insufficient to fully meet the needs of scientists and policymakers in predicting and mitigating climate warming.

To enhance our understanding of greenhouse gas budgets, a series of airborne measurement campaigns, known as CoMet (Carbon Dioxide and Methane Mission), have been conducted using the unique capabilities of the German research aircraft HALO. The CoMet campaigns integrate active airborne remote sensing measurements with lasers, passive remote sensing with spectrometers and solar radiation, and advanced in situ greenhouse gas concentration measurements, alongside an extensive suite of meteorological parameters. These observations are further supported by extensive modelling activities that also contribute to validating existing GHG satellite data and preparing for the next generation of such missions.

The first CoMet campaign took place in 2018, and its findings were published in a special inter-journal issue of AMT/ACP/GMD. The follow-up campaign, CoMet 2.0 Arctic (https://comet2arctic.de), was successfully conducted during a 6-week intensive operation period in August and September 2022 in Canada. The research flights focused on greenhouse gas emissions from boreal wetlands, permafrost areas in the Canadian Arctic, and wildfires, as well as anthropogenic sources like oil, gas, and coal extraction sites and landfills (in Canada and, during a test flight, in Spain). This campaign provided a valuable dataset for understanding methane and carbon dioxide cycles, particularly at high northern latitudes.

CoMet 2.0 Arctic is also part of the transatlantic AMPAC (Arctic Methane and Permafrost Challenge) initiative, a collaborative effort between NASA and ESA that fosters cooperation among US, Canadian, and European research institutes in this crucial area of research.

The special issue is open to all contributions that fit the topic from participants of the CoMet 2.0 Arctic field mission, the AMPAC community, and associated research partners.

The Asian summer monsoon (ASM) plays a key role in the vertical transport of material (including anthropogenic pollutants, aerosols, aerosol precursors, and other important trace gases) across the tropopause, with significant impacts on stratospheric chemistry and dynamics regionally and globally. Additionally, the Asian region is experiencing increasingly frequent and severe weather extremes connected to the ASM that are causing unprecedented damage to public property and loss of life. Predicting localized extreme events with sufficient lead times using numerical weather prediction (NWP) models remains challenging, and recent research suggests that better representation of stratospheric processes in NWP models can help to improve the prediction of monsoon extremes. On the other hand, it is well recognized that the complexities of the underlying mechanisms of stratosphere–troposphere coupling processes are difficult to incorporate in NWP models.

Observational and modelling aspects of the stratosphere–troposphere coupling processes and extreme weather events associated with the Asian summer monsoon were addressed at an international workshop, Stratosphere-Troposphere Interactions and Prediction of Monsoon weather EXtremes (STIPMEX), in Pune, India, from 2 to 7 June 2024. The STIPMEX workshop provided a platform for discussions on dynamical, chemical, radiative, and convective processes of the atmosphere during the ASM and fostered knowledge exchange and collaboration between experts on stratosphere–troposphere interactions and extreme weather prediction. The workshop aimed to promote and improve the inclusion of stratospheric and tropospheric processes in NWP models for better predictability of monsoon extremes. Full information on the STIPMEX workshop including a detailed list of themes and topics as well as an overview of the delivered presentations can be found at https://sparc-extreme.tropmet.res.in/.

This special issue has been initiated to publish the new and original research presented during STIPMEX and make it available to the wider community, and all STIPMEX presenters are encouraged to submit full write-ups of their novel and so far unpublished scientific studies. Submissions of follow-on studies or material representing a product of conference discussions and knowledge exchange are also welcome, as are any other submissions or related and relevant work that fits the scope of the workshop and special issue.

To best accommodate the two overarching STIPMEX themes, (i) dynamical, chemical, radiative, and convective processes in general, with a particular emphasis on recent changes and trends in stratosphere–troposphere coupling and linkages between stratospheric aerosol variability (e.g. due to volcanic eruptions) and the Asian summer monsoon, and (ii) the challenges of forecasting extreme weather events during the Asian summer monsoon, the special issue is organized as an inter-journal special issue of Atmospheric Chemistry and Physics (ACP) and Weather and Climate Dynamics (WCD).