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End date: 2019 × Start date: 2019 × Version: publishedVersion × Publisher: München : European Geopyhsical Union × Subject: cloud microphysics ×

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Now showing 1 - 10 of 21
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    A DNS study of aerosol and small-scale cloud turbulence interaction
    (München : European Geopyhsical Union, 2016) Babkovskaia, Natalia; Rannik, Ullar; Phillips, Vaughan; Siebert, Holger; Wehner, Birgit; Boy, Michael
    The purpose of this study is to investigate the interaction between small-scale turbulence and aerosol and cloud microphysical properties using direct numerical simulations (DNS). We consider the domain located at the height of about 2000 m from the sea level, experiencing transient high supersaturation due to atmospheric fluctuations of temperature and humidity. To study the effect of total number of particles (Ntot) on air temperature, activation and supersaturation, we vary Ntot. To investigate the effect of aerosol dynamics on small-scale turbulence and vertical air motion, we vary the intensity of turbulent fluctuations and the buoyant force. We find that even a small number of aerosol particles (55.5 cm−3), and therefore a small droplet number concentration, strongly affects the air temperature due to release of latent heat. The system comes to an equilibrium faster and the relative number of activated particles appears to be smaller for larger Ntot. We conclude that aerosol particles strongly affect the air motion. In a case of updraught coursed by buoyant force, the presence of aerosol particles results in acceleration of air motion in vertical direction and increase of turbulent fluctuations.
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    Chemical composition and mixing-state of ice residuals sampled within mixed phase clouds
    (München : European Geopyhsical Union, 2011) Ebert, M.; Worringen, A.; Benker, N.; Mertes, S.; Weingartner, E.; Weinbruch, S.
    During an intensive campaign at the high alpine research station Jungfraujoch, Switzerland, in February/March 2006 ice particle residuals within mixed-phase clouds were sampled using the Ice-counterflow virtual impactor (Ice-CVI). Size, morphology, chemical composition, mineralogy and mixing state of the ice residual and the interstitial (i.e., non-activated) aerosol particles were analyzed by scanning and transmission electron microscopy. Ice nuclei (IN) were identified from the significant enrichment of particle groups in the ice residual (IR) samples relative to the interstitial aerosol. In terms of number lead-bearing particles are enriched by a factor of approximately 25, complex internal mixtures with silicates or metal oxides as major components by a factor of 11, and mixtures of secondary aerosol and carbonaceous material (C-O-S particles) by a factor of 2. Other particle groups (sulfates, sea salt, Ca-rich particles, external silicates) observed in the ice-residual samples cannot be assigned unambiguously as IN. Between 9 and 24% of all IR are Pb-bearing particles. Pb was found as major component in around 10% of these particles (PbO, PbCl2). In the other particles, Pb was found as some 100 nm sized agglomerates consisting of 3–8 nm sized primary particles (PbS, elemental Pb). C-O-S particles are present in the IR at an abundance of 17–27%. The soot component within these particles is strongly aged. Complex internal mixtures occur in the IR at an abundance of 9–15%. Most IN identified at the Jungfraujoch station are internal mixtures containing anthropogenic components (either as main or minor constituent), and it is concluded that admixture of the anthropogenic component is responsible for the increased IN efficiency within mixed phase clouds. The mixing state appears to be a key parameter for the ice nucleation behaviour that cannot be predicted from the sole knowledge of the main component of an individual particle.
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    NLC and the background atmosphere above ALOMAR
    (München : European Geopyhsical Union, 2011) Fiedler, J.; Baumgarten, G.; Berger, U.; Hoffmann, P.; Kaifler, N.; Lübken, F.-J.
    Noctilucent clouds (NLC) have been measured by the Rayleigh/Mie/Raman-lidar at the ALOMAR research facility in Northern Norway (69° N, 16° E). From 1997 to 2010 NLC were detected during more than 1850 h on 440 different days. Colocated MF-radar measurements and calculations with the Leibniz-Institute Middle Atmosphere (LIMA-) model are used to characterize the background atmosphere. Temperatures as well as horizontal winds at 83 km altitude show distinct differences during NLC observations compared to when NLC are absent. The seasonally averaged temperature is lower and the winds are stronger westward when NLC are detected. The wind separation is a robust feature as it shows up in measurements as well as in model results and it is consistent with the current understanding that lower temperatures support the existence of ice particles. For the whole 14-year data set there is no statistically significant relation between NLC occurrence and solar Lyman-α radiation. On the other hand NLC occurrence and temperatures at 83 km show a significant anti-correlation, which suggests that the thermal state plays a major role for the existence of ice particles and dominates the pure Lyman-α influence on water vapor during certain years. We find the seasonal mean NLC altitudes to be correlated to both Lyman-α radiation and temperature. NLC above ALOMAR are strongly influenced by atmospheric tides. The cloud water content varies by a factor of 2.8 over the diurnal cycle. Diurnal and semidiurnal amplitudes and phases show some pronounced year-to-year variations. In general, amplitudes as well as phases vary in a different manner. Amplitudes change by a factor of more than 3 and phases vary by up to 7 h. Such variability could impact long-term NLC observations which do not cover the full diurnal cycle.
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    Aerosols-cloud microphysics-thermodynamics-turbulence: Evaluating supersaturation in a marine stratocumulus cloud
    (München : European Geopyhsical Union, 2012) Ditas, F.; Shaw, R.A.; Siebert, H.; Simmel, M.; Wehner, B.; Wiedensohler, A.
    This work presents a unique combination of aerosol, cloud microphysical, thermodynamic and turbulence variables to characterize supersaturation fluctuations in a turbulent marine stratocumulus (SC) layer. The analysis is based on observations with the helicopter-borne measurement platform ACTOS and a detailed cloud microphysical parcel model following three different approaches: (1) From the comparison of aerosol number size distributions inside and below the SC layer, the number of activated particles is calculated as 435±87 cm−3 and compares well with the observed median droplet number concentration of Nd = 464 cm−3. Furthermore, a 50% activation diameter of Dp50≈115 nm was derived, which was linked to a critical supersaturation Scrit of 0.16% via Köhler theory. From the shape of the fraction of activated particles, we estimated a standard deviation of supersaturation fluctuations of σS' = 0.09%. (2) These estimates are compared to more direct thermodynamic observations at cloud base. Therefore, supersaturation fluctuations (S') are calculated based on highly-resolved thermodynamic data showing a standard deviation of S' ranging within 0.1%≤σS'≤0.3 %. (3) The sensitivity of the supersaturation on observed vertical wind velocity fluctuations is investigated with the help of a detailed cloud microphysical model. These results show highest fluctuations of S' with σS'=0.1% at cloud base and a decreasing σS' with increasing liquid water content and droplet number concentration. All three approaches are independent of each other and vary only within a factor of about two.
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    Gravity wave influence on NLC: Experimental results from ALOMAR, 69° N
    (München : European Geopyhsical Union, 2013) Wilms, H.; Rapp, M.; Hoffmann, P.; Fiedler, J.; Baumgarten, G.
    The influence of gravity waves on noctilucent clouds (NLC) at ALOMAR (69° N) is analysed by relating gravity wave activity to NLC occurrence from common-volume measurements. Gravity wave kinetic energies are derived from MF-radar wind data and filtered into different period ranges by wavelet transformation. From the dataset covering the years 1999–2011, a direct correlation between gravity wave kinetic energy and NLC occurrence is not found, i.e., NLC appear independently of the simultaneously measured gravity wave kinetic energy. In addition, gravity wave activity is divided into weak and strong activity as compared to a 13 yr mean. The NLC occurrence rates during strong and weak activity are calculated separately for a given wave period and compared to each other. Again, for the full dataset no dependence of NLC occurrence on relative gravity wave activity is found. However, concentrating on 12 h of NLC detections during 2008, we do find an NLC-amplification with strong long-period gravity wave occurrence. Our analysis hence confirms previous findings that in general NLC at ALOMAR are not predominantly driven by gravity waves while exceptions to this rule are at least possible.
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    Comprehensive assessment of meteorological conditions and airflow connectivity during HCCT-2010
    (München : European Geopyhsical Union, 2014) Tilgner, A.; Schöne, L.; Bräuer, P.; van Pinxteren, D.; Hoffmann, E.; Spindler, G.; Styler, S.A.; Mertes, S.; Birmili, W.; Otto, R.; Merkel, M.; Weinhold, K.; Wiedensohler, A.; Deneke, H.; Schrödner, R.; Wolke, R.; Schneider, J.; Haunold, W.; Engel, A.; Wéber, A.; Herrmann, H.
    This study presents a comprehensive assessment of the meteorological conditions and atmospheric flow during the Lagrangian-type "Hill Cap Cloud Thuringia 2010" experiment (HCCT-2010), which was performed in September and October 2010 at Mt. Schmücke in the Thuringian Forest, Germany and which used observations at three measurement sites (upwind, in-cloud, and downwind) to study physical and chemical aerosol–cloud interactions. A Lagrangian-type hill cap cloud experiment requires not only suitable cloud conditions but also connected airflow conditions (i.e. representative air masses at the different measurement sites). The primary goal of the present study was to identify time periods during the 6-week duration of the experiment in which these conditions were fulfilled and therefore which are suitable for use in further data examinations. The following topics were studied in detail: (i) the general synoptic weather situations, including the mesoscale flow conditions, (ii) local meteorological conditions and (iii) local flow conditions. The latter were investigated by means of statistical analyses using best-available quasi-inert tracers, SF6 tracer experiments in the experiment area, and regional modelling. This study represents the first application of comprehensive analyses using statistical measures such as the coefficient of divergence (COD) and the cross-correlation in the context of a Lagrangian-type hill cap cloud experiment. This comprehensive examination of local flow connectivity yielded a total of 14 full-cloud events (FCEs), which are defined as periods during which all connected flow and cloud criteria for a suitable Lagrangian-type experiment were fulfilled, and 15 non-cloud events (NCEs), which are defined as periods with connected flow but no cloud at the summit site, and which can be used as reference cases. The overall evaluation of the identified FCEs provides the basis for subsequent investigations of the measured chemical and physical data during HCCT-2010 (see https://www.atmos-chem-phys.net/special_issue287.html).
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    Relationship between temperature and apparent shape of pristine ice crystals derived from polarimetric cloud radar observations during the ACCEPT campaign
    (München : European Geopyhsical Union, 2016) Myagkov, Alexander; Seifert, Patric; Wandinger, Ulla; Bühl, Johannes; Engelmann, Ronny
    This paper presents first quantitative estimations of apparent ice particle shape at the top of liquid-topped clouds. Analyzed ice particles were formed under mixed-phase conditions in the presence of supercooled water and in the temperature range from −20 to −3 °C. The estimation is based on polarizability ratios of ice particles measured by a Ka-band cloud radar MIRA-35 with hybrid polarimetric configuration. Polarizability ratio is a function of the geometrical axis ratio and the dielectric properties of the observed hydrometeors. For this study, 22 cases observed during the ACCEPT (Analysis of the Composition of Clouds with Extended Polarization Techniques) field campaign were used. Polarizability ratios retrieved for cloud layers with the cloud-top temperatures of  ∼ −5,  ∼ −8,  ∼ −15, and  ∼ −20 °C were 1.6, 0.9, 0.6, and 0.9, respectively. Such values correspond to prolate, quasi-isotropic, oblate, and quasi-isotropic particles, respectively. Data from a free-fall chamber were used for the comparison. A good agreement of detected apparent shapes with well-known shape–temperature dependencies observed in laboratories was found. Polarizability ratios used for the analysis were estimated for areas located close to the cloud top, where aggregation and riming processes do not strongly affect ice particles. We concluded that, in microwave scattering models, ice particles detected in these areas can be assumed to have pristine shapes. It was also found that even slight variations of ambient conditions at the cloud top with temperatures warmer than  ∼ −5 °C can lead to rapid changes of ice crystal shape.
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    High-resolution measurement of cloud microphysics and turbulence at a mountaintop station
    (München : European Geopyhsical Union, 2015) Siebert, H.; Shaw, R.A.; Ditas, J.; Schmeissner, T.; Malinowski, S.P.; Bodenschatz, E.; Xu, H.
    Mountain research stations are advantageous not only for long-term sampling of cloud properties but also for measurements that are prohibitively difficult to perform on airborne platforms due to the large true air speed or adverse factors such as weight and complexity of the equipment necessary. Some cloud–turbulence measurements, especially Lagrangian in nature, fall into this category. We report results from simultaneous, high-resolution and collocated measurements of cloud microphysical and turbulence properties during several warm cloud events at the Umweltforschungsstation Schneefernerhaus (UFS) on Zugspitze in the German Alps. The data gathered were found to be representative of observations made with similar instrumentation in free clouds. The observed turbulence shared all features known for high-Reynolds-number flows: it exhibited approximately Gaussian fluctuations for all three velocity components, a clearly defined inertial subrange following Kolmogorov scaling (power spectrum, and second- and third-order Eulerian structure functions), and highly intermittent velocity gradients, as well as approximately lognormal kinetic energy dissipation rates. The clouds were observed to have liquid water contents on the order of 1 g m−3 and size distributions typical of continental clouds, sometimes exhibiting long positive tails indicative of large drop production through turbulent mixing or coalescence growth. Dimensionless parameters relevant to cloud–turbulence interactions, the Stokes number and settling parameter are in the range typically observed in atmospheric clouds. Observed fluctuations in droplet number concentration and diameter suggest a preference for inhomogeneous mixing. Finally, enhanced variance in liquid water content fluctuations is observed at high frequencies, and the scale break occurs at a value consistent with the independently estimated phase relaxation time from microphysical measurements.
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    The Ice Selective Inlet: A novel technique for exclusive extraction of pristine ice crystals in mixed-phase clouds
    (München : European Geopyhsical Union, 2015) Kupiszewski, P.; Weingartner, E.; Vochezer, P.; Schnaiter, M.; Bigi, A.; Gysel, M.; Rosati, B.; Toprak, E.; Mertes, S.; Baltensperger, U.
    Climate predictions are affected by high uncertainties partially due to an insufficient knowledge of aerosol–cloud interactions. One of the poorly understood processes is formation of mixed-phase clouds (MPCs) via heterogeneous ice nucleation. Field measurements of the atmospheric ice phase in MPCs are challenging due to the presence of much more numerous liquid droplets. The Ice Selective Inlet (ISI), presented in this paper, is a novel inlet designed to selectively sample pristine ice crystals in mixed-phase clouds and extract the ice residual particles contained within the crystals for physical and chemical characterization. Using a modular setup composed of a cyclone impactor, droplet evaporation unit and pumped counterflow virtual impactor (PCVI), the ISI segregates particles based on their inertia and phase, exclusively extracting small ice particles between 5 and 20 μm in diameter. The setup also includes optical particle spectrometers for analysis of the number size distribution and shape of the sampled hydrometeors. The novelty of the ISI is a droplet evaporation unit, which separates liquid droplets and ice crystals in the airborne state, thus avoiding physical impaction of the hydrometeors and limiting potential artefacts. The design and validation of the droplet evaporation unit is based on modelling studies of droplet evaporation rates and computational fluid dynamics simulations of gas and particle flows through the unit. Prior to deployment in the field, an inter-comparison of the optical particle size spectrometers and a characterization of the transmission efficiency of the PCVI was conducted in the laboratory. The ISI was subsequently deployed during the Cloud and Aerosol Characterization Experiment (CLACE) 2013 and 2014 – two extensive international field campaigns encompassing comprehensive measurements of cloud microphysics, as well as bulk aerosol, ice residual and ice nuclei properties. The campaigns provided an important opportunity for a proof of concept of the inlet design. In this work we present the setup of the ISI, including the modelling and laboratory characterization of its components, as well as field measurements demonstrating the ISI performance and validating the working principle of the inlet. Finally, measurements of biological aerosol during a Saharan dust event (SDE) are presented, showing a first indication of enrichment of bio-material in sub-2 μm ice residuals.
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    Multichannel analysis of correlation length of SEVIRI images around ground-based cloud observatories to determine their representativeness
    (München : European Geopyhsical Union, 2015) Slobodda, J.; Hünerbein, A.; Lindstrot, R.; Preusker, R.; Ebell, K.; Fischer, J.
    Images of measured radiance in different channels of the geostationary Meteosat-9 SEVIRI instrument are analysed with respect to the representativeness of the observations of eight cloud observatories in Europe (e.g. measurements from cloud radars or microwave radiometers). Cloudy situations are selected to get a time series for every pixel in a 300 km × 300 km area centred around each ground station. Then a cross correlation of each time series to the pixel nearest to the corresponding ground site is calculated. In the end a correlation length is calculated to define the representativeness. It is found that measurements in the visible and near infrared channels, which respond to cloud physical properties, are correlated in an area with a 1 to 4 km radius, while the thermal channels, that correspond to cloud top temperature, are correlated to a distance of about 20 km. This also points to a higher variability of the cloud microphysical properties inside a cloud than of the cloud top temperature. The correlation length even increases for the channels at 6.2, 7.3 and 9.7 μm. They respond to radiation from the upper atmospheric layers emitted by atmospheric gases and higher level clouds, which are more homogeneous than low-level clouds. Additionally, correlations at different distances, corresponding to the grid box sizes of forecast models, were compared. The results suggest the possibility of comparisons between instantaneous cloud observations from ground sites and regional forecast models and ground-based measurements. For larger distances typical for global models the correlations decrease, especially for short-wave measurements and corresponding cloud products. By comparing daily means, the correlation length of each station is increased to about 3 to 10 times the value of instantaneous measurements and also the comparability to models grows.