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Aviation leads to the emission of CO 2 but also exerts non-CO 2 effects on climate, such as line-shaped condensation trails (contrails) and contrail cirrus that are known to cause warming. However, little is known about the climate effect of contrails that form in already existing cirrus clouds, where conditions for contrail formation are found most often. Here, we infer the local net radiative forcing of around 40,000 embedded contrails by combining aircraft position data with height-resolved cloud observations from spaceborne lidar. Considering the period from 2015 to 2021, we find an annual mean local warming effect of 60 mW m −2 . Expanding these findings to the global scale suggests an annual global mean net radiative forcing of embedded contrails on the order of 5 mW m −2 . This corresponds to around 10% of the current estimate of the climate impact of line-shaped contrails and suggests that embedded contrails are a non-negligible contributor to aviation’s impact on climate. This study presents a quantification of the global mean net radiative forcing of contrails in cirrus clouds equal to 5 mW m −2 , obtained from matching aircraft positions with spaceborne lidar data. This suggests that such embedded contrails are a non-negligible part of aviation’s impact on climate.

This study investigates the lifetime and temporal evolution of physical properties of trade‐wind cumuli based on tracking individual clouds in observations with the Advanced Baseline Imager aboard the geostationary GOES‐16 satellite during the “ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte” (EUREC4A) campaign east of Barbados in winter 2020. A first application of our upgraded cloud‐tracking toolbox to measurements with high spatio‐temporal resolution (2 × 2 km2 and 1 min) provides probability density functions of lifetime and area of clouds that develop as a consequence of meso‐to‐synoptic scale motions. By separately considering clouds that exist during daytime and live in distinct lifetime intervals, we find that shallow marine cumuli live longer when they cover a larger surface area and show higher cloud optical thickness (COT). Besides the effect of COT, the scale of the atmospheric motions with which the clouds interact is also critical to their lifetime. Plain Language Summary We present a detailed investigation of the lifetime of Caribbean trade‐wind cumulus clouds and the temporal evolution of their physical properties based on geostationary observations with the Advanced Baseline Imager aboard the geostationary GOES‐16 satellite during the “ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte” (EUREC4A) field experiment in winter 2020. The tracking of 2.7 million individual clouds in measurements with high spatio‐temporal resolution enables the investigation of processes that control the lifetime of shallow marine cumulus (SMC) clouds. Our analysis reveals that SMC clouds live longer when they span over a surface area that exceeds an order of tens of square kilometers. While these clouds show similar median cloud droplet size and number concentration compared to shorter‐lived clouds, they contain more liquid water and, thus, show a COT that is increased by about one third. Besides the effect of COT, we find that the scale of the atmospheric motions with which the clouds interact is also critical to their lifetime. Key Points First study of the life cycle of shallow marine cumulus based on observations with the Advanced Baseline Imager aboard GOES‐16 Confirmation of the double power law in the distribution of cloud lifetime from measurements with a temporal resolution of 1 minute Cloud lifetime is related to large‐scale circulation and affects cloud optical thickness

Tracking individual clouds in geostationary satellite observations and numerical simulations allows to quantify cloud lifetime and development. These higher‐dimensional parameters are used in a novel approach to assess the fidelity of cloud‐resolving models. Clouds are tracked in 2 months of satellite observations and accompanying model simulations of trade‐wind cumulus clouds during the EUREC4A campaign. Two Icosahedral Nonhydrostatic Large Eddy Model simulations with different spatial resolution give time series of cloud cover that reasonably resemble the GOES‐16 observations, despite large differences in the number of clouds. Analysis of the tracked clouds yields similar distributions of cloud size, development, and lifetime in line with the expected values for different dynamic regimes from energy cascade theory. However, we find that the model underestimates the occurrence frequency of larger, longer‐lived clouds. This effect is enhanced for finer model resolutions and might be related to the transfer of energy from larger‐scale processes into the domain of the large‐eddy simulation.

The dielectrophoretic force is applied between two differentially heated cylinders under microgravity conditions obtained during parabolic flight. The electric field is activated at various moments of the microgravity phase in order to control the initial condition at which the dielectrophoretic force intervenes. The time evolution of the flow is measured by particle image velocimetry and that of thermal plumes is captured by shadowgraphy. The growth rate of the instability during microgravity conditions is determined from these measurements. It is found that the closer the initial condition to the purely conductive state is, the faster the instability grows.

Inertial waves exist in rotating flows and are an ubiquitous phenomena in geophysical and astrophysical flows. The present work studies inertial waves in a homogeneous weakly viscous liquid confined between two coaxial co-rotating cylinders. In a laboratory experiment waves are continuously excited and they propagate due to radial \"stratification\" of angular momentum. Inviscid theory describes the formation of wave attractors. Wave energy is reflected several times at the boundaries of the cavity and focused towards geometry dependent attractor paths. Inviscid analytical results will be confirmed by experimental visualisations. Particle image velocimetry measurements will be presented showing particular properties of inertial waves and the existence of zonal/azimuthal mean flows with different origins. Additionally, a simple theoretical model has been developed for a rotating, homogeneous, viscous fluid. The generation of a \"mean zonal motion\" due to momentum transport of vertically propagating gravity waves is well known (Plumb and McEwan, J. Atmos. Sci. 35, (1978)). Based on the mathematical analogy it will be shown that in the meridional plane propagating inertial waves can transfer their momentum in the same manner to a sheared mean flow. Even an oscillating mean flow can be driven by inertial waves. The comparison of numerical results originating from numerical computations of two simple analytical model equations and long-time measurements of velocity fields concludes the study.

We investigate the thermal convection in an annular cavity, with differentially heated inner and outer cylinders, under the influence of a dielectrophoretic (DEP) force. Applying a temperature gradient to a liquid creates buoyancy driven thermal convection. When additionally a radially acting DEP-force is applied by means of an alternating electric field, the pattern of this convective flow changes which also leads to a change in the heat transfer. Depending on the parameters, e.g. an axisymmetric structure with toroidal vortices appears. Another possible structure are columnar vortices, which extend through the annulus. To isolate the effect of the DEP-force, this experiment is not only conducted in the laboratory, but also in microgravity conditions during parabolic flights. By using DEP-induced convective flows in microgravity a comparable heat transfer as with buoyancy convection under Earth’s condition can be obtained. A better understanding of the heat transport mechanisms inside a dielectric liquid confined between two concentric cylinders can deliver solutions for the improvement of the heat transport in many technical applications.

A dielectric fluid is confined in a stationary vertical cylindrical annulus. A temperature difference is applied between the two cylinders, as well as an alternating electric potential. This configuration creates an active force called dielectrophoretic force, which acts as a thermal buoyancy force. Different axial gravity intensities are considered, so that two thermal buoyancies will affect the flow: the thermoelectric buoyancy intervenes in the radial direction and the Archimedean buoyancy acts in the axial direction. Linear stability analysis and direct numerical simulation are performed following experimental research that has been performed during parabolic flight campaigns.

Aviation affects the Earth's energy balance through CO2 and non-CO2 emissions. Contrails mark one of the latter and can occur inside the cirrus clouds where they might affect the clouds' optical and microphysical characteristics as well as their climate impact. In this study, airborne lidar observations with the German research aircraft HALO during the ML-CIRRUS and CIRRUS-HL campaigns are used together with aircraft-location data to detect the occurrence of contrails that have formed within already existing cirrus clouds. Based on manual analysis, we developed (based on ML-CIRRUS) and verified (based on CIRRUS-HL) an automated two-step method for detecting embedded contrails in lidar measurements. In the first, threshold-based step, potential embedded contrail regions are identified by particle backscatter coefficients larger than 4 Mm−1 sr−1 and particle linear depolarization ratios smaller than 30 % or 43 % depending on the impact of pollution on the background cloud. The second step assesses the area of the identified objects in a lidar curtain for finding cases that could realistically be associated with an aircraft-related perturbation. Specifically, areas smaller than 10 pixels are dismissed as noisy data, while areas larger than 50 pixels are too homogeneous to be in line with the assumptions of the manual analysis that cloud regions that are perturbed by the passage of an aircraft occur in close vicinity to unperturbed cloud areas. The resulting contrail mask enables the detection and quantification of the occurrence rate of embedded contrails in airborne lidar measurements without the need for auxiliary air-traffic information.

Studies of Arctic clouds often focus on low-level single-layer clouds (SLCs). Here, we use combined observations of soundings and cloud radar during the MOSAiC, ACSE, and AO2018 research cruises as well as from long-term observations at Ny-Ålesund, Svalbard and Utqiagvik, Alaska to investigate the occurrence of SLCs and multi-layer clouds (MLCs) in the Arctic and to assess the rate of ice-crystal seeding in cold MLCs. MOSAiC observations show cloudy conditions in between 70 % and 90 % of sounding-radar cases. SLCs show occurrence rates of 30 % to 40 % with the highest value of 45 % during October. Cold MLCs are most abundant from November to June (40 % to 55 % of cases). Seeding occurs in about half to two thirds of the identified cold MLCs during MOSAiC for which the sub-saturated layer extends between 100 and 1000 m. The seeding rate increases by as much as 20 percentage points as the assumed size of the falling ice crystals is increased from 100 to 400 µm. The observations reveal a stable rate of cloud-free conditions of around 20 % over the covered latitude range. Cloud occurrence during MOSAiC and at Ny-Ålesund in July, when the geographical distance between observations was minimal, shows reasonable agreement. Comparisons of MOSAiC and other research cruises to the central Arctic also indicate consistent occurrence rates of different cloud types despite the likely effect of year-to-year variability. The comparison of data from ship campaigns and land sites suggests that the latter are not necessarily a good indicator of cloud occurrence in the high Arctic.

The effective radiative forcing (ERF) due to aerosol–cloud interactions (ACIs) and rapid adjustments (ERFaci) still causes the largest uncertainty in the assessment of climate change. It is understood only with medium confidence and is studied primarily for warm clouds. Here, we present a novel cloud-by-cloud (C×C) approach for studying ACI in satellite observations that combines the concentration of cloud condensation nuclei (nCCN) and ice nucleating particles (nINP) from polar-orbiting lidar measurements with the development of the properties of individual clouds by tracking them in geostationary observations. We present a step-by-step description for obtaining matched aerosol–cloud cases. The application to satellite observations over central Europe and northern Africa during 2014, together with rigorous quality assurance, leads to 399 liquid-only clouds and 95 ice-containing clouds that can be matched to surrounding nCCN and nINP respectively at cloud level. We use this initial data set for assessing the impact of changes in cloud-relevant aerosol concentrations on the cloud droplet number concentration (Nd) and effective radius (reff) of liquid clouds and the phase of clouds in the regime of heterogeneous ice formation. We find a Δln⁡Nd/Δln⁡nCCN of 0.13 to 0.30, which is at the lower end of commonly inferred values of 0.3 to 0.8. The Δln⁡reff/Δln⁡nCCN between −0.09 and −0.21 suggests that reff decreases by −0.81 to −3.78 nm per increase in nCCN of 1 cm−3. We also find a tendency towards more cloud ice and more fully glaciated clouds with increasing nINP that cannot be explained by the increasingly lower cloud top temperature of supercooled-liquid, mixed-phase, and fully glaciated clouds alone. Applied to a larger number of observations, the C×C approach has the potential to enable the systematic investigation of warm and cold clouds. This marks a step change in the quantification of ERFaci from space.