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 F32: Geophysical Fluid Dynamics: AtmosphericGeophysical

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Chair: Chenning Tong, Clemson University Room: 104 
Monday, November 20, 2017 8:00AM  8:13AM 
F32.00001: Diurnal Ekman layer cycles at White Sands, New Mexico observed with Doppler lidar Andrew Gunn, Douglas Jerolmack Atmospheric boundary layer turbulence is produced by shear and buoyancy, which are forced by largerscale geostrophic and solar controls. Their absolute and relative inputs result in a nonlinear response of boundarynormal flow deflection within the Ekman layer. Classical analytical solutions for the deflection were found by assuming extrema of the shearbuoyancy phase space. At the chosen field site, a lowroughness longfetch salt flat upwind of White Sands dune field, we employ a groundbased upwardfacing Doppler lidar to observe the layerâ€™s dynamics throughout a continuous 70day time series of wind vector measurements between 10 and 300 metres aloft. Within this domain we identify transition path bifurcation in the shearbuoyancy phase space, where deflection transience and the associated transformation of the vertical extent of the Ekman layer, are dependent on the daily stability cycle. Our analysis probes the natural system that is often modelled with idealised theory, revealing nonequilibrium dynamics that have implications within atmospheric science and geomorphology. [Preview Abstract] 
Monday, November 20, 2017 8:13AM  8:26AM 
F32.00002: Numerical model error in largeeddy simulations of a stable atmospheric boundary layer Georgios Matheou Aspects of a largeeddy simulation model performance are investigated in simulations of a moderately stable boundary layer. The model utilizes the constantcoefficient SmagorinskyLilly subgridscale closure. Three model parameters are considered: the grid spacing, the value of the SGS model constant and the order of accuracy (resolving power) of the advection discretization. Depending on the value of the model constant, two main errorproducing mechanisms are identified. For high values of the model constant, spurious turbulence collapse is observed. For low values of the model constant, numerical discretization errors dominate, leading to accumulation of energy at small scales and overprediction of the magnitude of the surface heat flux. Surface fluxes vary for different model constants, even for finely resolved runs with highorder advection discretizations. Although the impact of the surface fluxes on the boundarylayer dynamics is expected, the observed differences are relatively large given that the flow configuration is identical. The present runs show the importance of the feedback between boundarylayer turbulence and surface flux. To eliminate the feedback between turbulent flow and surface heat flux, a series of runs with prescribed surface heat flux was carried out. [Preview Abstract] 
Monday, November 20, 2017 8:26AM  8:39AM 
F32.00003: Airborne Measurement of Insolation Impact on the Atmospheric Surface Boundary Layer Jamey Jacob, Phil Chilson, Adam Houston, Carrick Detweiler, Sean Bailey Atmospheric surface boundary layer measurements of wind and thermodynamic parameters are conducted during variable insolation conditions, including the 2017 eclipse, using an unmanned aircraft system. It is well known that the air temperatures can drop significantly during a total solar eclipse as has been previously observed. In past eclipses, these observations have primarily been made on the ground. We present results from airborne measurements of the near surface boundary layer using a small unmanned aircraft with high temporal resolution wind and thermodynamic observations. Questions that motivate the study include: How does the temperature within the lower atmospheric boundary vary during an eclipse? What impact does the immediate removal of radiative heating on the ground have on the lower ABL? Do local wind patterns change during an eclipse event and if so why? Will there be a manifestation of the nocturnal boundary layer wind maximum? Comparisons are made with the DOE ARM SGP site that experiences a lower but still significant insolation. Supported by the National Science Foundation under award number 1539070. [Preview Abstract] 
Monday, November 20, 2017 8:39AM  8:52AM 
F32.00004: Numerical simulations with a FSIcalibrated actuator disk model of wind turbines operating in stratified ABLs S. M. Iman Gohari, Sutanu Sarkar, Artem Korobenko, Yuri Bazilevs Numerical simulations of wind turbines operating under different regimes of stability are performed using LES. A reduced model, based on the generalized actuator disk model (ADM), is implemented to represent the wind turbines within the ABL. Data from the fluidsolid interaction (FSI) simulations of wind turbines have been used to calibrate and validate the reduced model. The computational cost of this method to include wind turbines is affordable and incurs an overhead as low as 1.45\%. Using this reduced model, we study the coupling of unsteady turbulent flow with the wind turbine under different ABL conditions: (i) A neutral ABL with zero heatflux and inversion layer at 350m, in which the incoming wind has the maximum mean shear between the heights of uppertip and lowertip; (2) A shallow ABL with surface cooling rate of 1 K/hr wherein the low level jet occurs at the wind turbine hub height. We will discuss how the differences in the unsteady flow between the two ABL regimes impact the wind turbine performance. [Preview Abstract] 
Monday, November 20, 2017 8:52AM  9:05AM 
F32.00005: Measurement of atmospheric surface layer turbulence using unmanned aerial vehicles Sean Bailey, Caleb Canter We describe measurements of the turbulence within the atmospheric surface layer using highly instrumented and autonomous unmanned aerial vehicles (UAVs). Results from the CLOUDMAP measurement campaign in Stillwater Oklahoma are presented including turbulence statistics measured during the transition from stably stratified to convective conditions. The measurements were made using prefabricated fixedwing remotecontrol aircraft adapted to fly autonomously and carry multihole pressure probes, pressure, temperature and humidity sensors. Two aircraft were flown simultaneously, with one flying a flight path intended to profile the boundary layer up to 100 m and the other flying at a constant fixed altitude of 50 m. The evolution of various turbulent statistics was determined from these flights, including Reynolds stresses, correlations, spectra and structure functions. These results were compared to those measured by a sonic anemometer located on a 7.5 m tower. [Preview Abstract] 
Monday, November 20, 2017 9:05AM  9:18AM 
F32.00006: Local free convection in the atmospheric boundary layer using matched asymptotic expensions Mengjie Ding, Chenning Tong The local free convection (LFC) scaling in the convective atmospheric boundary layer (ABL) was previously obtained using dimensional analysis, assuming the limits $z/L\rightarrow\infty$ and $z/z_i\rightarrow 0$, where $z$, $z_i$, and $L$ are the distance from the ground, the boundary layer height, and the MoninObukhov length respectively. However, when the conditions are not satisfied, there is a departure from the LFC scaling, which cannot be obtained using dimensional analysis. In this study we derive the LFC limit from the equations for the velocity variances using matched asymptotic expansions. In the outer layer ($z/L > 1$) buoyancy dominates whereas in the inner layer ($z/L<1$) shear production is important, resulting in a singular perturbation problem. Matching between the inner and outer solutions results in the LFC scaling. We also obtain the corrections to the LFC scaling of the vertical velocity variance near the two ends of the LFC region ($z/L \rightarrow 1$ and $z/z_i \rightarrow 1$). The analysis reveals the influence of $z_i/L$ on the correction for reduced $z/L$ values, which is absent from previous empirical formulae. The composite expansion provides a unified expression for the vertical velocity variance in the convective layer of the convective ABL. [Preview Abstract] 
Monday, November 20, 2017 9:18AM  9:31AM 
F32.00007: Numerical Study of motion of Falling Conical Graupel ChihChe Chueh, Pao K. Wang, Tempei Hashino Each year, large hailstones falling from a thunderstorm cause massive loss of crops and properties, pose a serious threat to aviation, and, on occasion, some deaths in the world. Graupel particles often serve as hailstone embryos, and are frequently observed forms of convective precipitation almost everywhere. And it is sufficiently evident that the major factor that determines collision efficiency for ice accretion is the flow field. In the present study, the attitudes of freelyfalling conical graupel particles with a realistic range of densities are investigated numerically by solving the transient NavierStokes equations and the body dynamics equations representing the 6degreesoffreedom motion, allowing us to determine the position and orientation of the graupel in response to the coupling of the hydrodynamic force and torque of the flow fields, gravitational force, as well as Magnus force due to selfrotation. The results show significant horizontal movements (on the order of 1 km in one hour) and also show that when Reynolds number is small, a typical damped oscillation occurs, whereas when Reynolds number is high, amplifying oscillation may occur which leads to more complicated and unpredictable flying attitudes such as tumbling. [Preview Abstract] 
Monday, November 20, 2017 9:31AM  9:44AM 
F32.00008: Atmospheric propagation of infrasound across mountain ranges Florentin Damiens, Christophe Millet, Francois Lott Linear theory of acoustic propagation is used to analyze trapping of infrasound within the lower tropospheric waveguide during propagation above a mountain range. Atmospheric flow produced by the mountains is predicted by a nonlinear mounatin wave model. For the infrasound component, we solve the wave equation under the effective sound speed approximation using both a spectral collocation method and a WKB approach. It is shown that in realistic configurations, the mountain waves can deeply perturb the low level waveguide, which leads to significant acoustic dispersion. To interpret these results each acoustic mode is tracked separately as the horizontal distance increases. It is shown that during statically stable situations, roughly representative of winter or night situations, the mountain waves induce a Foehn effect downstream which shrinks significantly the waveguide. This yields a new form of infrasound absorption, that can largely outweigh the direct effect the moutain induces on the lowlevel waveguide. For the opposite case, when the low level flow is less statically stable (summer or day situations), mountain wave dynamics do not produce dramatic responses downstream. Instead, it favors the passage of infrasound, which somehow mitigates the direct effect of the obstacle. [Preview Abstract] 
Monday, November 20, 2017 9:44AM  9:57AM 
F32.00009: On the use of infrasound for constraining global climate models Christophe Millet, Bruno Ribstein, Francois Lott, David Cugnet Numerical prediction of infrasound is a complex issue due to constantly changing atmospheric conditions and to the random nature of smallscale flows. Although part of the upward propagating wave is refracted at stratospheric levels, where gravity waves significantly affect the temperature and the wind, yet the process by which the gravity wave field changes the infrasound arrivals remains poorly understood. In the present work, we use a stochastic parameterization to represent the subgrid scale gravity wave field from the atmospheric specifications provided by the European Centre for MediumRange Weather Forecasts. It is shown that regardless of whether the gravity wave field possesses relatively small or large features, the sensitivity of acoustic waveforms to atmospheric disturbances can be extremely different. Using infrasound signals recorded during campaigns of ammunition destruction explosions, a new set of tunable parameters is proposed which more accurately predicts the smallscale content of gravity wave fields in the middle atmosphere. Climate simulations are performed using the updated parameterization. Numerical results demonstrate that a network of groundbased infrasound stations is a promising technology for dynamically tuning the gravity wave parameterization. [Preview Abstract] 
Monday, November 20, 2017 9:57AM  10:10AM 
F32.00010: Modeling and optimal design of CO$_{\mathrm{2}}$ Direct Air Capture systems in large arrays Samaneh Sadri Irani, Paolo LuzzattoFegiz As noted by the 2014 IPCC report, while the rise in atmospheric CO$_{\mathrm{2}}$ would be slowed by emissions reductions, removing atmospheric CO$_{\mathrm{2}}$ is an important part of possible paths to climate stabilization. Direct Air Capture of CO$_{\mathrm{2}}$ with chemicals (DAC) is one of several proposed carbon capture technologies. There is an ongoing debate on whether DAC is an economically viable approach to alleviate climate change. In addition, like all air capture strategies, DAC is strongly constrained by the netcarbon problem, namely the need to control CO$_{\mathrm{2}}$ emissions associated with the capture process (for example, if DAC not powered by renewables). Research to date has focused on the chemistry and economics of individual DAC devices. However, the fluid mechanics of their largescale deployment has not been examined in the literature, to the best of our knowledge. In this presentation, we develop a model for flow through an array of DAC devices, varying their lateral extent and their separation. We build on a recent theory of canopy flows, introducing terms for CO$_{\mathrm{2}}$ entrainment into the array boundary layer, and transport into the farm. In addition, we examine the possibility of driving flow passively by wind, thereby reducing energy consumption. The optimal operational design is established considering the total cost, drag force, energy consumption and total CO$_{\mathrm{2}}$ capture. [Preview Abstract] 
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