The origins of ice crystals measured in mixed-phase clouds at the high-alpine site Jungfraujoch

During the winter of 2013 and 2014 measurements of cloud microphysical properties over a 5-week period at the high-alpine site Jungfraujoch, Switzerland, were carried out as part of the Cloud Aerosol Characterisation Experiments (CLACE) and the Ice Nucleation Process Investigation and Quantification project (INUPIAQ). Measurements of aerosol properties at a second, lower site, Schilthorn, Switzerland, were used as input for a primary ice nucleation scheme to predict ice nuclei concentrations at Jungfraujoch. Frequent, rapid transitions in the ice and liquid properties of the clouds at Jungfraujoch were identified that led to large fluctuations in ice mass fractions over temporal scales of seconds to hours. During the measurement period we observed high concentrations of ice particles that exceeded 1000 L-1 at temperatures around -15 degrees C, verified by multiple instruments. These concentrations could not be explained using the usual primary ice nucleation schemes, which predicted ice nucleus concentrations several orders of magnitude smaller than the peak ice crystal number concentrations. Secondary ice production through the Hallett-Mossop process as a possible explanation was ruled out, as the cloud was rarely within the active temperature range for this process. It is shown that other mechanisms of secondary ice particle production cannot explain the highest ice particle concentrations. We describe four possible mechanisms that could lead to high cloud ice concentrations generated from the snow-covered surfaces surrounding the measurement site. Of these we show that hoar frost crystals generated at the cloud enveloped snow surface could be the most important source of cloud ice concentrations. Blowing snow was also observed to make significant contributions at higher wind speeds when ice crystal concentrations were < 100 L-1

EUREC⁴A

The science guiding the EUREC⁴A campaign and its measurements is presented. EUREC⁴A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC⁴A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC⁴A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC⁴A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement

EUREC⁴A

The science guiding the EUREC⁴A campaign and its measurements is presented. EUREC⁴A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic – eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC⁴A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air–sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC⁴A explored – from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation – are presented along with an overview of EUREC⁴A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at https://doi.org/10.25326/165 (Stevens, 2021), and a film documenting the campaign is provided as a video supplement

Airborne and ground-based holographic measurement of hydrometeors in liquid-phase, mixed-phase and ice clouds

A new cloud particle spectrometer has been developed in this work with the purpose of obtaining in-situ measurements on different research aircraft. It is based on the principle of digital in-line holography and delivers six images per second of the recorded interference pattern from the laser background and particles suspended in the approx. 35 cubic centimeter large sample volume. Particle position and two-dimensional in-focus images of particles are determined via numerical reconstruction based on Fraunhofer diffraction pattern analysis. Particle properties such as diameter, area and mass are calculated from the reconstructed images. This new instrument, called &quot;HALOHolo&quot;, has been calibrated in the laboratory w.r.t. particle detectability and sizing accuracy. An instrument intercomparison with other cloud particle spectrometers confirmed the determined instrument characteristics in general. Measurements in liquid-phase, mixed-phase and ice clouds have been obtained from three airborne field campaigns where HALOHolo was operated on two different aircraft. The liquid-phase clouds measured were Arctic stratocumulus clouds, where the connection between variability of particle number concentration and mean volume diameter was investigated. The measured data were indicative of inhomogeneous mixing between the cloud and the surrounding air. A developing tropical rain shower was sampled in the process of glaciation and its microphysical evolution has been characterized. The local environment of ice crystals surrounded by supercooled liquid water droplets was studied and the spatial distribution of ice crystals and droplets was found to be a uniform random distribution for the majority of measurements in the rapid glaciation zone of the cloud. In addition, the simultaneous occurrence of small columnar ice crystals among larger rimed particles (graupels) has been documented in a variety of cases, which were most likely to be found in strong updrafts. Finally, the spatial structure of cirrus clouds has been investigated on scales between 30 m and 300 km. These clouds did sometimes exhibit bimodal distributions of particle maximum dimension, which were supposedly induced by mixing processes. In some of the cases, a connection between dynamics and microphysics could be established from the holographic measurements and the meteorological data recorded by the aircraft. It could also be shown that the size distribution of ice crystals can be described as a superposition of log-normal distributions from each individual particle habit. In this thesis, it could be shown that the new instrument is capable of delivering scientifically useful data, which extend beyond conventional instruments like the Forward Scattering Spectrometer Probe, the Cloud Imaging Probe or the Cloud Droplet Probe, and it could even provide new insights in some aspects of cloud physics in all three microphysical classes of clouds. While the majority of the data analysis is able to run automatically, some critical steps like particle classification are still semi-automatic

On the Risk of Infection by Infectious Aerosols in Large Indoor Spaces

Airborne diseases can be transmitted by infectious aerosols in the near field, i.e., in close proximity, or in the far field, i.e., by infectious aerosols that are well mixed within the indoor air. Is it possible to say which mode of disease transmission is predominant in large indoor spaces? We addressed this question by measuring the transport of aerosols equivalent to the size of human respiratory particles in two large hardware stores (V > 10000 m3). We found that aerosol concentrations in both stores decreased rapidly and almost independently of aerosol size, despite the different ventilation systems. A persistent and directional airflow on the order of a few cm/s was observed in both stores. Consequently, aerosol dynamics in such open settings can be expected to be dominated by turbulent dispersion and sweeping flows, and the accumulation of infectious aerosols in the indoor air is unlikely to contribute significantly to the risk of infection as long as the occupancy of the store is not too high. Under these conditions, well-fitting face masks are an excellent means of preventing disease transmission by human aerosols

Experimental measurement of respiratory particles dispersed by wind instruments and analysis of the associated risk of infection transmission

Activities such as singing or playing a wind instrument release respiratory particles into the air that may contain pathogens and thus pose a risk for infection transmission. Here we report measurements of the size distribution, number, and volume concentration of exhaled particles from 31 healthy musicians playing 20 types of wind instruments using aerosol size spectrometry complemented with in-line holography in a strictly controlled cleanroom environment. We find that playing wind instruments carries a lower risk of airborne disease transmission than speaking or singing. We attribute this to the fact that the resonators of wind instruments act as filters for particles 10 µm in diameter, which were found in high abundance right after a brass mouthpiece but very rarely at the instrument bell end. We have also measured the size-dependent filtering properties of different types of filters that can be used as instrument masks. Based on these measurements, we calculated the risk of airborne transmission of SARS-CoV-2 in different near- and far-field scenarios with and without masking and/or distancing. We conclude that in all cases where there is a possibility that the musician is infectious, the only safe measure to prevent airborne transmission of the disease is the use of well-fitting and well-filtering masks for the instrument and the susceptible person

La deutsche Nation et la Razão Tupiniquim - la pensée indigène face au concept étranger

During the winter of 2013 and 2014 measurements of cloud microphysical properties over a 5-week period at the high-alpine site Jungfraujoch, Switzerland, were carried out as part of the Cloud Aerosol Characterisation Experiments (CLACE) and the Ice Nucleation Process Investigation and Quantification project (INUPIAQ). Measurements of aerosol properties at a second, lower site, Schilthorn, Switzerland, were used as input for a primary ice nucleation scheme to predict ice nuclei concentrations at Jungfraujoch. Frequent, rapid transitions in the ice and liquid properties of the clouds at Jungfraujoch were identified that led to large fluctuations in ice mass fractions over temporal scales of seconds to hours. During the measurement period we observed high concentrations of ice particles that exceeded 1000 L-1 at temperatures around -15 degrees C, verified by multiple instruments. These concentrations could not be explained using the usual primary ice nucleation schemes, which predicted ice nucleus concentrations several orders of magnitude smaller than the peak ice crystal number concentrations. Secondary ice production through the Hallett-Mossop process as a possible explanation was ruled out, as the cloud was rarely within the active temperature range for this process. It is shown that other mechanisms of secondary ice particle production cannot explain the highest ice particle concentrations. We describe four possible mechanisms that could lead to high cloud ice concentrations generated from the snow-covered surfaces surrounding the measurement site. Of these we show that hoar frost crystals generated at the cloud enveloped snow surface could be the most important source of cloud ice concentrations. Blowing snow was also observed to make significant contributions at higher wind speeds when ice crystal concentrations were < 100 L-1

The origins of ice crystals measured in mixed phase clouds at High-Alpine site Jungfraujoch

Abstract. During the winter of 2013 and 2014 measurements of cloud microphysical properties over a five week period at the high Alpine site Jungfraujoch, Switzerland were carried out as part of the Cloud Aerosol Characterisation Experiments (CLACE) and the Ice Nucleation Process Investigation and Quantification project (INUPIAQ) Measurements of aerosol properties at a second, lower site, Schilthorn, Switzerland, were used as input for a primary ice nucleation scheme to predict ice nuclei concentrations at Jungfraujoch Frequent, rapid transitions in the ice and liquid properties of the clouds at Jungfraujoch were identified that led to large fluctuations in ice mass fractions over temporal scales of seconds to hours. During the measurement period we observed high concentrations of ice particles that exceeded 1000 L−1 at temperatures around −15 °C, verified by multiple instruments These concentrations could not be explained using the usual primary ice nucleation schemes, which predicted ice nucleus concentrations several orders of magnitude smaller than the peak ice crystal number concentrations. Secondary ice production through the Hallet–Mossop process as a possible explanation was ruled out, as the cloud was rarely within the active temperature range for this process It is shown that other mechanisms of secondary ice particle production cannot explain the highest ice particle concentrations. We describe 4 possible mechanisms that could lead to high cloud ice concentrations generated from the snow covered surfaces surrounding the measurement site. Of these we show that hoar frost crystals generated at the cloud enveloped snow surface could be the most important source of cloud ice concentrations Blowing snow was also observed to make significant contributions at higher wind speeds when ice crystal concentrations were &lt; 100 L−1. </jats:p

Airborne observations of the microphysical structure of two contrasting cirrus clouds

We present detailed airborne in situ measurements of cloud microphysics in two midlatitude cirrus clouds, collected as part of the Cirrus Coupled Cloud-Radiation Experiment. A new habit recognition algorithm for sorting cloud particle images using a neural network is introduced. Both flights observed clouds that were related to frontal systems, but one was actively developing while the other dissipated as it was sampled. The two clouds showed distinct differences in particle number, habit, and size. However, a number of common features were observed in the 2-D stereo data set, including a distinct bimodal size distribution within the higher-temperature regions of the clouds. This may result from a combination of local heterogeneous nucleation and large particles sedimenting from aloft. Both clouds had small ice crystals (<100 µm) present at all levels However, this small ice mode is not present in observations from a holographic probe. This raises the possibility that the small ice observed by optical array probes may at least be in part an instrument artifact due to the counting of out-of-focus large particles as small ice. The concentrations of ice crystals were a factor ~10 higher in the actively growing cloud with the stronger updrafts, with a mean concentration of 261 L−1 compared to 29 L−1 in the decaying case. Particles larger than 700 µm were largely absent from the decaying cirrus case. A comparison with ice-nucleating particle parameterizations suggests that for the developing case the ice concentrations at the lowest temperatures are best explained by homogenous nucleation