Dispersion aerosol indirect effect in turbulent clouds: Laboratory measurements of effective radius

Cloud optical properties are determined not only by the number density nd and mean radius ṝ of cloud droplets but also by the shape of the droplet size distribution. The change in cloud optical depth with changing nd, due to the change in distribution shape, is known as the dispersion effect. Droplet relative dispersion is defined as d=σr / ṝ . For the first time, a commonly used effective radius parameterization is tested in a controlled laboratory environment by creating a turbulent cloud. Stochastic condensation growth suggests d independent of nd for a nonprecipitating cloud, hence nearly zero albedo susceptibility due to the dispersion effect. However, for size‐dependent removal, such as in a laboratory cloud or highly clean atmospheric conditions, stochastic condensation produces a weak dispersion effect. The albedo susceptibility due to turbulence broadening has the same sign as the Twomey effect and augments it by order 10%

Scaling of Turbulence and Microphysics in a Convection–Cloud Chamber of Varying Height

The convection–cloud chamber enables measurement of aerosol and cloud microphysics, as well as their interactions, within a turbulent environment under steady-state conditions. Increasing the size of a convection–cloud chamber, while holding the imposed temperature difference constant, leads to increased Rayleigh, Reynolds and Nusselt numbers. Large–eddy simulation coupled with a bin microphysics model allows the influence of increased velocity, time, and spatial scales on cloud microphysical properties to be explored. Simulations of a convection–cloud chamber, with fixed aspect ratio and increasing heights of H = 1, 2, 4, and (for dry conditions only) 8 m are performed. The key findings are: Velocity fluctuations scale as H1/3, consistent with the Deardorff expression for convective velocity, and implying that the turbulence correlation time scales as H2/3. Temperature and other scalar fluctuations scale as H−3/7. Droplet size distributions from chambers of different sizes can be matched by adjusting the total aerosol injection rate as the horizontal cross-sectional area (i.e., as H2 for constant aspect ratio). Injection of aerosols at a point versus distributed throughout the volume makes no difference for polluted conditions, but can lead to cloud droplet size distribution broadening in clean conditions. Cloud droplet growth by collision and coalescence leads to a broader right tail of the distribution compared to condensation growth alone, and this tail increases in magnitude and extent monotonically as the increase of chamber height. These results also have implications for scaling within turbulent, cloudy mixed-layers in the atmosphere, such as fog layers

Making Scholarly Publishing Work for You: Empowering Graduate Students to Understand the Scholarly Publishing Ecosystem Through a Graduate Academy Seminar

Understanding the landscape of scholarly publishing is an essential competency for graduate students, whether they publish during their studies or after they’ve entered their professional fields. But the scholarly publishing ecosystem can be complicated to navigate, and students cannot always rely on their advisors and colleagues to demystify the processes. To help graduate students achieve their goals when sharing their research, the ScholarWorks Center for Scholarly Publishing at the Duke University Libraries (https://scholarworks.duke.edu/) taught “Navigating Scholarly Publishing,” a five-day, interdisciplinary course introducing essential aspects of scholarly communication and empowering students to make informed, proactive decisions about sharing their work. Taught by expert instructors in the ScholarWorks Center as part of Duke’s summer Graduate Academy (https://bit.ly/47ppflT), the course involved introductory readings, short lectures, forum posts, and seminar-style discussion to explore and address student questions on each day’s topic: Day One: Big Picture (copyright, technology, economics, and ethics as lenses for understanding the scholarly publishing ecosystem) Day Two: Synthesizing Your Research (how the desired audience for one’s research can influence how it’s synthesized and shared) Day Three: Publishing Your Work (legal and ethical considerations, such as copyright, licenses, and collaborations; the economics of discoverability; evaluating publishers and publishing options; APCs and subscriptions) Day Four: Measuring and Articulating Value (impact metrics; injustices hidden by research impact; measuring what we value versus valuing what we can measure) Day Five: Creating Scholarship That Lasts (factors helping or hindering accessibility and usefulness for future scholarship) These topics not only educated students about the current state of scholarly publishing but encouraged them to (1) consider the potential audience for their research before they decide how to publish it and (2) identify their own values when it comes to sharing their research. For instance, is equitable access an essential aspect of their professional moral framework? Do they need to select a journal based on impact metrics in order to advance in their career? How can they most appropriately license their work for long-term (re)usability? We invited students to discuss what research dissemination means to them and how they can operate in the current system to their advantage—and how they can make choices that might influence the future of that system. To serve graduate students is to engage them in the wider conversation and empower them to make scholarly publishing work for them. Each of the instructors will discuss their experience teaching this course: curriculum design, learning management tools, classroom interactions, content covered, student feedback, and lessons learned from the first iteration of this course. We will also discuss how our values of student empowerment and participation infused this course, and how we see libraries as critical advocates for improving publishing (rather than simply teaching students about the status quo)

Decline in an Atlantic Puffin population : evaluation of magnitude and mechanisms

Funding: This study was funded annually by Fair Isle Bird Observatory Trust (www.fairislebirdobs.co.uk) with contributions from the Joint Nature Conservation Committee (jncc.defra.gov.uk). Funding was received from these two sources by Fair Isle Bird Observatory from 1986 to 2013. The Joint Nature Conservation Committee and Fair Isle Bird Observatory Trust supplied guidance on study design, data collection, analyses, preparation of the manuscript and the decision to publish.Peer reviewedPublisher PD

Molecular simulations reveal that heterogeneous ice nucleation occurs at higher temperatures in water under capillary tension

Homogeneous ice nucleation rates occur at higher temperatures when water is under tension, otherwise referred to as negative pressure. If also true for heterogeneous ice nucleation rates, then this phenomenon can result in higher heterogeneous freezing temperatures in water capillary bridges, pores, and other geometries where water is subjected to negative Laplace pressure. Using a molecular model of water freezing on a hydrophilic substrate, it is found that heterogeneous ice nucleation rates exhibit a similar temperature increase at negative pressures as homogeneous ice nucleation. For pressures ranging from from 1 atm to &minus;1000 atm, the simulations reveal that the temperature corresponding to the heterogeneous nucleation rate coefficient jhet (m&minus;2 s&minus;1) increases linearly as a function of negative pressure, with a slope that can be approximately predicted by the water density anomaly and the latent heat of fusion at atmospheric pressure. Simulations of water in capillary bridges confirm that negative Laplace pressure within the water corresponds to an increase in heterogeneous freezing temperature. The freezing temperature in the water capillary bridges increases linearly with inverse capillary height (1/h). Varying the height and width of the capillary bridge reveals the role of geometric factors in heterogeneous ice nucleation. When substrate surfaces are separated by less than approximately h = 20 Angstroms the nucleation rate is enhanced and when the width of the capillary bridge is less than approximately 30 Angstroms the nucleation rate is suppressed. Ice nucleation does not occur in the region within 10 Angstroms of the air-water interface and shows a preference for nucleation in the region just beyond 10 Angstroms. These results help unify multiple lines of experimental evidence for enhanced nucleation rates due to reduced pressure, either resulting from surface geometry (Laplace pressure) or mechanical agitation of water droplets. This concept is relevant to the phenomenon of contact nucleation and could potentially play a role in a number of different heterogeneous nucleation or secondary ice mechanisms.</p

Is contact nucleation caused by pressure perturbation?

The reason why ice nucleation is more efficient by contact nucleation than by immersion nucleation has been elusive for over half a century. Six proposed mechanisms are summarized in this study. Among them, the pressure perturbation hypothesis, which arose from recent experiments, can qualitatively explain nearly all existing results relevant to contact nucleation. To explore the plausibility of this hypothesis in a more quantitative fashion and to guide future investigations, this study assessed the magnitude of pressure perturbation needed to cause contact nucleation and the associated spatial scales. The pressure perturbations needed were estimated using measured contact nucleation efficiencies for illite and kaolinite, obtained from previous experiments, and immersion freezing temperatures, obtained from well-established parameterizations. Pressure perturbations were obtained by assuming a constant pressure perturbation or a Gaussian distribution of the pressure perturbation. The magnitudes of the pressure perturbations needed were found to be physically reasonable, being achievable through possible mechanisms, including bubble formation and breakup, Laplace pressure arising from the distorted contact line, and shear. The pressure perturbation hypothesis provides a physically based and experimentally constrainable foundation for parameterizing contact nucleation that may be useful in future cloud-resolving models

The Dilaton and Modified Gravity

We consider the dilaton in the strong string coupling limit and elaborate on the original idea of Damour and Polyakov whereby the dilaton coupling to matter has a minimum with a vanishing value at finite field-value. Combining this type of coupling with an exponential potential, the effective potential of the dilaton becomes matter density dependent. We study the background cosmology, showing that the dilaton can play the role of dark energy. We also analyse the constraints imposed by the absence of violation of the equivalence principle. Imposing these constraints and assuming that the dilaton plays the role of dark energy, we consider the consequences of the dilaton on large scale structures and in particular the behaviour of the slip functions and the growth index at low redshift.Comment: 14 pages, 4 figure

Effects of the Large-Scale Circulation on Temperature and Water Vapor Distributions in the Π Chamber

Microphysical processes are important for the development of clouds and thus Earth\u27s climate. For example, turbulent fluctuations in the water vapor concentration, r, and temperature, T, cause fluctuations in the saturation ratio, S. Because S is the driving factor in the condensational growth of droplets, fluctuations may broaden the cloud droplet size distribution due to individual droplets experiencing different growth rates. The small scale turbulent fluctuations in the atmosphere that are relevant to cloud droplets are difficult to quantify through field measurements. We investigate these processes in the laboratory, using Michigan Tech\u27s Π Chamber. The Π Chamber utilizes Rayleigh-Benard convection (RBC) to create the turbulent conditions inherent in clouds. In RBC it is common for a large scale circulation (LSC) to form. As a consequence of the LSC, the temperature field of the chamber is not spatially uniform. In this paper, we characterize the LSC in the Π chamber and show how it affects the shape of the distributions of r, T and S. The LSC was found to follow a single roll with an updraft and downdraft along opposing walls of the chamber. Near the updraft (downdraft), the distributions of T and r were positively (negatively) skewed. S consistently had a negatively skewed distribution, with the downdraft being the most negative

Fast and slow microphysics regimes in a minimalist model of cloudy Rayleigh-Bénard convection

A minimalist model of microphysical properties in cloudy Rayleigh-Bénard convection is developed based on mass and number balances for cloud droplets growing by vapor condensation. The model is relevant to a turbulent mixed-layer in which a steady forcing of supersaturation can be defined, e.g., a model of the cloudy boundary layer or a convection-cloud chamber. The model assumes steady injection of aerosol particles that are activated to form cloud droplets, and the removal of cloud droplets through sedimentation. Simplifying assumptions include the consideration of mean properties in steady state, neglect of coalescence growth, and no detailed representation of the droplet size distribution. Closed-form expressions for cloud droplet radius, number concentration, and liquid water content are derived. Limits of fast and slow microphysics, compared to the turbulent mixing time scale, are explored, and resulting expressions for the scaling of microphysical properties in fast and slow regimes are obtained. Scaling of microphysics with layer thickness is also explored, suggesting that liquid water content and cloud droplet number concentration increase, and mean droplet radius decreases with increasing layer thickness. Finally, the analytical model is shown to compare favorably to solutions of the fully-coupled set of governing ordinary differential equations that describe the system, and the predicted power law for liquid water mixing ratio versus droplet activation rate is observed to be consistent with measurements from the Pi convection-cloud chamber