1,721,522 research outputs found
The relative contribution of secondary ice processes in Alpine mixed-phase clouds
No full textIn-situ observations of mixed-phase clouds (MPCs) forming over mountain tops regularly reveal that ice crystal number concentrations (ICNCs) are orders of magnitude higher than ice-nucleating particle concentrations. This discrepancy has often been attributed to the influence of surface processes such as blowing snow and airborne hoar frost. Ιn-cloud secondary ice production (SIP) processes may also explain this discrepancy, but their contribution has received less attention. Here we explore the potential role of SIP processes on orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the mountain-top site of Jungfraujoch in the Swiss Alps using the Weather Research and Forecasting model (WRF). The Hallett-Mossop (H-M) mechanism, included in the default version of the Morrison scheme in WRF, is ruled out since the simulated clouds were outside the active temperature range for this process. This study investigates if the implementation of two additional SIP mechanisms in WRF, namely collisional break-up (BR) between ice hydrometeors and frozen droplet shattering (DS), can bridge the gap between observed and modeled ICNCs. DS is inefficient in the examined conditions due to a lack of sufficiently large raindrops to trigger this process. The BR mechanism is likely important in Alpine MPCs, but the process is activated only within seeder-feeder situations, when precipitation particles are seeding the low-level MPCs inducing their glaciation. At times when a cloud exists near the ground, blowing snow ice particles may be mixed among supercooled liquid droplets and thus contribute significantly to ice growth, but they cannot account for the observed ICNCs. Our findings indicate that outside the H-M temperature range, ice-seeding and blowing snow can initiate ice multiplication in the Alps through the BR mechanism, which is found to elevate the modeled ICNCs up to 3 orders of magnitude, providing a better agreement with in-situ measurements. This highlights the importance of considering both SIP and surface-based processes in weather-prediction and climate models.LAPILT
The impact of secondary ice processes on orographic mixed-phase clouds
No full textGround based and airborne observations of orographic mixed-phase clouds (MPCs) forming over mountain top research stations have long reported a discrepancy between the measured ice crystal number concentrations (ICNCs) and the concentration of ice nucleating particles, the former being several orders of magnitude higher (e.g., Lloyd et al., 2015). Additionally, model simulations of Alpine clouds are frequently found to underestimate the amount of ice compared with observations (Farrington et al., 2016). Although surface-based processes such as blowing snow and hoar frost have been suggested to explain this discrepancy, the potential role of secondary ice production (SIP) processes – especially mechanical breakup of cloud ice and droplet fragmentation during freezing – has been less studied. In this study we utilize the Weather Research and Forecasting model (WRF) to explore the potential contribution of SIP processes on the orographic MPCs observed during the Cloud and Aerosol Characterization Experiment (CLACE) 2014 campaign at the mountain-top site of Jungfraujoch in the Swiss Alps. The only SIP mechanism included in the default version of WRF is the Hallett–Mossop process (H-M), which is however ruled out since the recorded temperatures were generally colder than -8 ˚C. We modified the default WRF to include parameterizations of two additional SIP mechanisms, namely the collisional break-up (BR) upon collisions between ice particles and droplet shattering (DS), in order to investigate if the performance of the model is improved. Simulations suggest that the DS mechanism is not a significant source of ICNCs. The BR mechanism however is quite active, elevating the predicted ICNCs by up to 3 orders of magnitude, which is consistent with observations. The initiation of the BR mechanism is primarily associated with the occurrence of seeder-feeder situations, which are widespread phenomena over Switzerland (Proske et al., 2021). Including a source of ice crystals from the effect of blowing snow episodically affects cloud ICNCs; the numbers reaching cloud base is not large, but the concentrations are multiplied through the action of the BR mechanism. Our findings highlight the importance of considering both secondary ice and an “external” seeding mechanism – primarily falling ice from above and to a lesser degree blowing ice from the surface - in weather-prediction models in order to predict correctly the amount of liquid and ice in MPCs, which is in turn critical for the accurate representation of radiation processes and precipitation patterns. Farrington, R. J., Connolly, P. J., Lloyd, G., Bower, K. N., Flynn, M. J., Gallagher, M. W., Field, P. R., Dearden, C., and Choularton, T. W. (2016). Comparing model and measured ice crystal concentrations in orographic clouds during the INUPIAQ campaign. Atmos. Chem. Phys., 16, 4945–4966, https://doi.org/10.5194/acp-16-4945-2016 Lloyd, G., Choularton, T. W., Bower, K. N., Gallagher, M. W., Connolly, P. J., Flynn, M., Farrington, R., Crosier, J., Schlenczek, O., Fugal, J. and Henneberger, J. (2015). The origins of ice crystals measured in mixed-phase clouds at the high-alpine site Jungfraujoch. Atmos. Chem. Phys., 15, 12953–12969. https://doi.org/10.5194/acp-15-12953-2015 Proske, U., Bessenbacher, V., Dedekind, Z., Lohmann, U., and Neubauer, D. (2021). How frequent is natural cloud seeding from ice cloud layers ( < −35 °C) over Switzerland?. Atmos. Chem. Phys., 21, 5195–5216. https://doi.org/10.5194/acp-21-5195-2021LAPILT
Chemical evolution of primary and secondary biomass burning aerosols during daytime and nighttime
No full textFine particulate matter (PM) affects visibility, climate, and public health. Biomass burning (BB) in the forms of residential wood burning, wildfires, and prescribed burning is a major source of primary and secondary organic matter (OM, an important fraction fine PM), and brown and black carbon (BrC and BC). The contribution of BB to the atmospheric fine PM is only expected to increase in the foreseeable future. Recent studies have highlighted the enhancement in the biomass burning organic aerosol (bbOA) concentrations with aging and reported on the chemical composition of the secondary biomass burning organic aerosol (bbSOA) formed under different conditions. However, the chemical processing of the primary biomass burning organic aerosol (bbPOA) with aging is not well characterized. This chemical processing can potentially alter the chemical composition of bbOA drastically and render its identification and quantification in the atmosphere difficult. We used aerosol mass spectrometry (AMS) and Fourier transform infrared spectroscopy (FTIR) as two complementary methods to quantify the bbPOA aging in this study. AMS measures the bulk composition of OM with a relatively high temporal resolution but provides limited parent compound information due to the extensive fragmentation. FTIR, carried out on PTFE filter samples, provides detailed information about the functional group composition of the OM and certain bbOA makers at the expense of a relatively low temporal resolution. In a series of aging experiments at the Center for Studies of Air Qualities and Climate Change (C-STACC), primary emissions from wood and pellet stoves were injected into an environmental simulation chamber. Primary emissions were aged using hydroxyl and nitrate radicals simulating the atmospheric day-time and night-time oxidation. A high-resolution time-of-flight (HR-TOF) AMS was used to identify the composition of non-refractory PM1. PM1 was also collected on PTFE filters before and after aging for the off-line FTIR analysis. AMS and FTIR agreed well in terms of the measured bbOA mass concentrations, elemental ratios, and the evolution of biomass burning tracers. We developed a procedure to quantify the bbPOA aging using AMS and FTIR. Using AMS, we found that up to 17 % of the bbPOA mass underwent some form of transformation with aging. This transformation was more intense under day-time conditions. FTIR detected a more extensive oxidation (up to two times that of AMS), suggesting a substantial processing of bbPOA, and revealing the limitations of AMS to capture bbPOA aging due to the extensive fragmentation. Different bbOA-related ion fragments were observed to decay at different rates under different conditions (e.g., oxidants and relative humidity). These different decay rates can potentially be used to identify the extent of bbPOA aging in the atmosphere. The bbSOA formed during the daytime oxidation was dominated by acid contributions, resembling certain atmospheric biomass burning samples. The unique, acid-dominated FTIR spectrum of bbSOA can potentially be used as another indicator of the aged bbOA in the atmosphere.LAP
Using dimensionality reduction and causal inference to constrain precipitation and climate
No full textContinued greenhouse gas emissions will lead to increasing global warming. Effective adaptation and mitigation policies depend on accurately estimating climate sensitivity, the Earth's surface temperature response to increasing CO2 emissions. Global Climate models (GCMs) provide the long-term simulations essential to better understand climate and quantify this temperature change. Despite advances in numerical modelling and theory, model divergence remains significant. The Intergovernmental Panel on Climate Change (IPCC) recognized the "hot model problem" in its Sixth Assessment report (2021). One source of this model uncertainty is parametrization schemes, that encode our physical understanding of subgrid-scale processes, like clouds and convection. Convective precipitation, due to its stochastic nature and dependence to fine-scale processes, is still poorly represented in GCMs.
This thesis uses dimensionality reduction and causal inference methods on multi-model ensemble outputs to address these challenges. Model complexity has become a norm in model development, and hinders their interpretability. Major progress in these areas require a rigorous search for the processes underlying the large interdependent model outputs. The d-MAPS method offers a low-level representation of datasets, while causal inference methods question the relationships between variables. The first application focuses on Sea Surface Temperature (SST) dynamics, and investigates the potential of causal network properties to narrow down the range of plausible climate sensitivity. Chapter 1 develops the methodology for a first ensemble, with the evaluation of patterns and teleconnections in a recent period. Chapter 2 expands the methodology to climate projections. We found that SST effects weaken in warmer climates, with a significant model uncertainty in the eastern Pacific Ocean. Hot models do not exhibit more realistic teleconnection effects within a fixed causal network, but their SST patterns are more realistic with a varying structure. The second application examines the dependency of convective precipitation on its environmental conditions. Chapter 3 aims to quantify interactions among variables within a specific large-scale regime. We analyzed high-resolution model outputs from Global Storm-Resolving Models (GSRM), in which deep convection is explicitly resolved, using a multivariate causal graph. This framework helped us better understand the d-MAPS method, especially its exclusion of sharp humidity gradients. We found consistent control of large-scale variable on the convective precipitation distribution across GSRMs, and a significant uncertainty of the role of vertical velocity.
Both applications aim to better understand processes and quantify the inter-model differences. Ultimately, we use causal effects to constrain climate sensitivity, and to identify the largest control of large-scale humidity on convective precipitation. This research work, based on chain of decisions - from the inference of regions to the potential constraint of a target, through the inference of links and their effects, reveals all the complexity of the climate system, but also shows promising results in the search for robust relationships.LAP
The role of secondary ice production in mixed-phase clouds
No full textClouds are omnipresent in the Earth's atmosphere. Their phase composition significantly modulates their interaction with solar and terrestrial radiation, as well as precipitation formation. Particularly for clouds containing both phases, known as mixed-phase clouds, a thorough understanding of the processes governing the distribution of both liquid water and ice is imperative for their accurate representation in models, which is achieved through empirical parameterizations. Compared to liquid-phase processes, mechanisms related to ice formation have been notably understudied, with most weather prediction and global climate models still lacking descriptions of critical ice multiplication processes, capable of efficiently amplifying ice crystal concentrations at relatively warm subzero temperatures.
This PhD thesis aims to investigate microphysical processes that play a crucial role in determining the ice- and liquid-phase partitioning of mixed-phase clouds. We first demonstrated the ability to accurately predict cloud droplet numbers in orographic mixed-phase clouds using cloud parcel theory, along with in-situ aerosol measurements and remotely-sensed updraft velocities. Additionally, we established a relationship that could potentially be applied to decipher cloud droplet formation regimes in virtually any type of non-precipitating boundary-layer clouds.
With a primary focus on the overlooked ice-related processes, we updated the microphysics scheme of a widely-used numerical weather prediction model, to account for previously neglected ice multiplication processes. We found that the combined effect of crystal fragmentation due to collisions with seeding ice particles from above the cloud, and to a lesser extent those lofted from the snow-covered surface, significantly increased modeled ice crystal concentrations, aligning them with in-situ observations of low-level orographic clouds. These findings were further corroborated through comparisons with ground-based radar measurements in a mountainous region, underscoring the need for models to incorporate additional secondary ice processes for accurate simulations of the amount of snowfall on the ground. By coupling the model with a radar simulator, we further proposed an interpretation of complex radar signatures linked to distinct ice growth and multiplication processes.
Lastly, we introduced a novel framework to represent the impact of ice multiplication in polar stratiform mixed-phase clouds, identified as the most radiatively important cloud type. This framework, developed by applying machine learning techniques to regional climate simulations, demands fewer inputs for predicting ice multiplication, making integration into large-scale models more straightforward compared to conventional secondary ice schemes.
In summary, this thesis advances our understanding of microphysical processes in mixed-phase clouds from various perspectives, characterizing the droplet formation regime and identifying conditions favoring secondary ice production in polar and orographic clouds, inferring potential signatures of ice growth and multiplication from radar observations, and introducing a novel methodological tool to parameterize its impact in large-scale models. These outputs open new avenues for microphysical process descriptions in global climate models, with expected improvements in predicting precipitation patterns and the radiative properties of mixed-phase clouds on a global scale.LAP
Deploying online instruments to evaluate oxidative properties of combustion and atmospheric aerosols
Full text linkOutdoor air pollution is one of the greatest environmental risks to public health according to the World Health Organization. As 99% of the world’s population lives in areas which do not meet the recommended limit values, 6.7 million premature deaths per year can be attributed to the adverse health effects caused by the exposure to particulate matter (PM). This is comparable to the effects of direct tobacco use. While the link between adverse health effects and PM has been clearly established after decades of epidemiological studies the exact mechanism, sources and PM properties responsible are still poorly understood. Oxidative stress has been identified as one of the main possible toxicity pathways which is caused by an imbalance of the oxidant-to-antioxidant ratio that favours the former. PM can introduce particle-bound oxidants like reactive oxygen species (ROS) as well as compounds that can produce ROS in situ in the human body after particle inhalation, which is defined as the oxidative potential (OP). OP has been suggested as potential metric that could provide a link between PM with its specific toxicity-relevant composition and negative health effects. ROS and OP are commonly quantified using acellular assays that simulate oxidation reactions in the lungs using surrogate reductants. Conventionally, offline methods with time delays between sample collection and analysis are applied which can lead to significant underestimations. Due to long time delays between sample collection and measurement highly reactive compounds can decompose before analysis. Online adaptations avoid this by extracting PM directly into the reagents. Two recently published instruments applying different online assays have been deployed for several laboratory measurement campaigns investigating interactions of specific PM constituents as well as the effect of atmospheric processes of different anthropogenic combustion emissions (car, ship, aircraft, wood stove). Additionally, ambient measurements near a heavily congested road site and suburban background site as well as in an underground subway station in London, UK, were performed. In between campaigns, technical developments to improve the instrument’s reliability, user friendliness, and limit of detection were implemented. Significant and highly dynamic changes in ROS and OP activity of combustion PM were observed depending on combustion source and conditions (engine type, load, fuel) as well as photochemical ageing. In most cases an increase of ROS and OP with higher ages has been measured compared to primary emissions. This showed that these sources can also play a role
after atmospheric transport beyond the immediate emission. Even in a seemingly stable and isolated environment like an underground public transport system, dynamic changes of OP and ROS were observed in combination with changing physical and chemical properties of PM. These observations were made possible by applying high time resolution instruments that minimize sample degradation. Furthermore, after several deployments their operation as well as data management was streamlined and simplified
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Parameterizing the competition between homogeneous and heterogeneous freezing in ice cloud formation - Polydisperse ice nuclei
No full textThis study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from precursor aerosol and ice nuclei. The parameterization provides an analytical solution of the cloud parcel model equations and accounts for the competition effects between homogeneous and heterogeneous freezing, and, between heterogeneous freezing in different modes. The diversity of heterogeneous nuclei is described through a nucleation spectrum function which is allowed to follow any form (i.e., derived from classical nucleation theory or from observations). The parameterization reproduces the predictions of a detailed numerical parcel model over a wide range of conditions, and several expressions for the nucleation spectrum. The average error in ice crystal number concentration was ?2.0± 8.5% for conditions of pure heterogeneous freezing, and, 4.7±21% when both homogeneous and heterogeneous freezing were active. The formulation presented is fast and free from requirements of numerical integration. © 2009 Author(s).LAP
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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