Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G15: Focus Sessions: Fluid Dynamics of Atmospheric Clouds IGeophysical
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Chair: Lian-Ping Wang, University of Delaware Room: 601 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G15.00001: Field measurements of cloud droplet dynamics Jan Molacek, Gholamhossein Bagheri, Augustinus Bertens, Haitao Xu, Eberhard Bodenschatz We present an in-situ experiment investigating the dynamics of cloud droplets and its dependence on the turbulent flow properties. This dynamics plays a major role in the rate of growth of cloud particles by coalescence and the resulting precipitation rate. The experiment takes place at a mountain research station at an altitude of 2650m, and will make use of a movable platform that can travel with the mean wind velocity. Here we present preliminary results using a stationary setup. Simultaneous measurements of other variables such as droplet size distribution and humidity fluctuations are done in order to develop a more complete picture of the microphysical conditions within clouds. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G15.00002: LACIS-T - A moist air wind tunnel for investigating the interactions between cloud microphysics and turbulence Dennis Niedermeier, Jens Voigtl\"{a}nder, Holger Siebert, Neel Desai, Raymond Shaw, Kelken Chang, Steven Krueger, J\"{o}rg Schumacher, Frank Stratmann Turbulence - cloud droplet interaction processes have been investigated primarily through numerical simulation and field measurements over the last ten years. However, only in the laboratory we can be confident in our knowledge of initial and boundary conditions, and are able to measure for extended times under statistically stationary and repeatable conditions. Therefore, the newly built turbulent wind tunnel LACIS-T (Turbulent Leipzig Aerosol Cloud Interaction Simulator) is an ideal facility for pursuing mechanistic understanding of these processes. Within the tunnel we are able to adjust precisely controlled turbulent temperature and humidity fields so as to achieve supersaturation levels allowing for detailed investigations of the interactions between cloud microphysical processes (e.g., cloud droplet activation) and the turbulent flow, under well-defined and reproducible laboratory conditions. We will present the fundamental operating principle, first results from ongoing characterization efforts, numerical simulations as well as first droplet activation experiments. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G15.00003: Can hail and rain nucleate cloud droplets? Prasanth Prabhakaran, Stephan Weiss, Alexei Krekhov, Alain Pumir, Eberhard Bodenschatz We present results from a laboratory scale moist convection experiment composed of a mixture of pressurized sulphur hexafluoride (SF6 - liquid and vapor phase) and helium (He - gas phase) to mimic the wet (saturated water vapor) and dry components (nitrogen, oxygen etc.) of the earth's atmosphere. We operate the experiments close to critical conditions to allow for homogeneous nucleation of sulphur hexafluoride droplets. The liquid SF6 pool is heated from below and the warm SF6 vapor from the liquid-vapor interface rise and condense underneath the cold top plate. We observe the nucleation of microdroplets in the wake of cold drops falling through the SF6-He atmosphere. Using classical nucleation theory, we show that the nucleation is caused by isobaric cooling of SF6 vapor in the wake of the cold drop. Furthermore, we argue that in an atmospheric cloud, falling hail and large cold raindrops may induce heterogeneous nucleation of microdroplets in their wake. We also observe that under appropriate conditions these microdroplets form a stable horizontal layer, thus separating regions of super and sub-critical saturation. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G15.00004: Design for green, disposable, mini radiosondes to track fluctuations along isopycnic surfaces in cloud environments Tessa Chiara Basso, Michele Iovieno, Silvano Bertoldo, Giovanni Perotto, Athanassia Athanassiou, Flavio Canavero, Giovanni Perona, Daniela Tordella An introduction to innovative, bio-compatible, ultralight, disposable radiosondes that are aimed to be passively transported on isopycnic surfaces in cloud and clear air environments. Their goal is to track small-scale fluctuations of velocity, temperature, humidity, acceleration and pressure for several hours within and outside the cloud boundary. With a target weight of 15 g, the volume is chosen such that the probes float on isopycnic surfaces at constant altitudes from 1000 to 3000 m. They are filled with helium gas to obtain a buoyancy force equal to the weight of the system. Transmitters within the probes will send data to receivers on Earth to be analysed and compared with numerical simulations. To minimise their environmental impact, it is foreseen that the disposable radiosondes be made with biodegradable smart materials which keep the desired hydrophobicity and flexibility. These environmentally friendly, hydrophobic balloons will provide an insight into the unsteady life cycle of warm clouds over land, ocean and alpine environments. These explorative observations will contribute to the current understanding of microphysical processes in clouds with the purpose of improving weather prediction and climate modelling. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G15.00005: Wind-Shear Effects within the Entrainment Zone of Stratocumulus Bernhard Schulz, Juan Pedro Mellado Direct numerical simulations resolving meter and sub-meter scales of the stratocumulus cloud-top are used to investigate the interactions between a vertical shear and convective instabilities driven by evaporative and radiative cooling. Wind shear is found to thicken the entrainment interfacial layer (EIL), to enhance cloud-top cooling, and to increase the entrainment velocity substantially only if the shear velocity $\Delta u$ exceeds a critical value $(\Delta u)_\mathrm{crit}$. We provide an expression for $(\Delta u)_\mathrm{crit}$, which is based on two competing processes dominating the inversion dynamics: shear-driven turbulence, and the penetration of in-cloud turbulent convection into the inversion. For typical atmospheric conditions $(\Delta u)_\mathrm{crit}$ corresponds to a shear velocity of $1-2$ $\mathrm{m\,s}^{-1}$. However, even for $\Delta u > (\Delta u)_\mathrm{crit}$ a strong wind shear does not affect in-cloud turbulence as long as the EIL remains thin compared to the cloud layer, i.e. shear effects remain localized within the EIL. Therefore, a strong shear does not necessarily weaken in-cloud turbulence by depleting the cloud, which challenges previous conjectures. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G15.00006: Broadening of cloud droplet spectra through turbulent entrainment and eddy hopping Gustavo Abade, Wojciech Grabowski, Hanna Pawlowska This work discusses the effect of cloud turbulence and turbulent entrainment on the evolution of the cloud droplet-size spectrum. We simulate an ensemble of idealized turbulent cloud parcels that are subject to entrainment events, modeled as a random Poisson process. Entrainment events, subsequent turbulent mixing inside the parcel, supersaturation fluctuations, and the resulting stochastic droplet growth by condensation are simulated using a Monte Carlo scheme. Quantities characterizing the turbulence intensity, entrainment rate and the mean fraction of environmental air entrained in an event are specified as external parameters. Cloud microphysics is described by applying Lagrangian particles, the so-called superdroplets. They are either unactivated cloud condensation nuclei (CCN) or cloud droplets that form from activated CCN. The model accounts for the transport of environmental CCN into the cloud by the entraining eddies at the cloud edge. Turbulent mixing of the entrained dry air with cloudy air is described using a linear model. We show that turbulence plays an important role in aiding entrained CCN to activate, providing a source of small cloud droplets and thus broadening the droplet size distribution. Further simulation results will be reported at the meeting. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G15.00007: DNS of cloud-clear air mixing at a shear-free interface Paul Goetzfried, Bipin Kumar, Raymond A. Shaw, Joerg Schumacher Direct numerical simulations of a cloud-clear air interface are performed to study the response of an ensemble of cloud water droplets to the turbulent entrainment of clear air into a cloud filament. The main goal is to understand the evolution of mixing of cloudy and clear air as turbulence and thermodynamics interact through phase changes of cloud droplets. Fluid turbulence is driven solely by buoyancy, which incorporates feedbacks from the temperature, vapor and liquid water content fields. Two main parameter variations are discussed, a simulation in a larger domain and a variation of the turbulence level of the clear air environment. It is found that due to droplet evaporation buoyancy dominates the subsequent evolution of the mixing layer. Its feedback leads initially to downdrafts at the cloudy-clear air interface and to updrafts in the bulk. The strength of the turbulence is domain size dependent, showing that the range of scales is an important parameter. In contrast, the level of turbulence in the clear air is found to have little effect on the evolution of the mixing process. The distributions of cloud water droplet size, supersaturation at the droplet positions and vertical velocity are more sensitive to large scale as opposed to small-scale properties of the flow. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G15.00008: Entrainment in stratocumulus-topped boundary layers: a DNS study Mona Karimi, Juan Pedro Mellado Entrainment is an important multi-physics and multi-scale process in stratocumulus-topped boundary layers. Integral analysis provides analytical expressions of the mean entrainment velocity that exhibit the contributions from different cloud-top processes such as: turbulent mixing, radiative and evaporative cooling, and deformation across the region where the turbulent/non-turbulent interface lies. Previous analyses have investigated these contributions by simulating the cloud-top region alone without retaining the large-scale turbulent motions characteristic of the boundary layer, the vertical variation of in-cloud thermodynamic properties, and the effect of the surface fluxes. We extend this previous work by performing, for the first time, DNS of complete stratocumulus-topped boundary layers. By comparing DNS of the cloud-top region with DNS of the complete boundary layer, we aim to assess the role of large-scale structures on small-scale aspects of entrainment. Initial results indicate that the contribution of deformation, which cannot be neglected in the local analysis, is small in the stratocumulus-topped boundary layers, simplifying further analysis. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G15.00009: Hydrodynamic clustering of droplets in turbulence Rudie Kunnen, Altug Yavuz, GertJan van Heijst, Herman Clercx Small, inertial particles are known to cluster in turbulent flows: particles are centrifuged out of eddies and gather in the strain-dominated regions. This so-called preferential concentration is reflected in the radial distribution function (RDF; a quantitative measure of clustering). We study clustering of water droplets in a loudspeaker-driven turbulence chamber. We track the motion of droplets in 3D and calculate the RDF. At moderate scales (a few Kolmogorov lengths) we find the typical power-law scaling of preferential concentration in the RDF. However, at even smaller scales (a few droplet diameters), we encounter a hitherto unobserved additional clustering. We postulate that the additional clustering is due to hydrodynamic interactions, an effect which is typically disregarded in modeling. Using a perturbative expansion of inertial effects in a Stokes-flow description of two interacting spheres, we obtain an expression for the RDF which indeed includes the additional clustering. The additional clustering enhances the collision probability of droplets, which enhances their growth rate due to coalescence. The additional clustering is thus an essential effect in precipitation modeling. [Preview Abstract] |
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