Atmospheric Measurement Techniques 

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.