Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session D1: Geophysical Flows: Atmospheric II |
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Chair: Joe Fernando, University of Notre Dame Room: 301 |
Sunday, November 20, 2011 2:10PM - 2:23PM |
D1.00001: Thermally driven upslope flow in mountainous terrain Dan Liberzon, Christopher Hocut, Harindra J. Fernando Buoyancy driven up-slope flow and its separation from mountain apex are two important processes that determine meso and regional flows in mountainous areas. Such flow configurations have applications from mountain meteorology to large scale monsoonal circulation. A combined experimental and theoretical study toward improving our understanding of the mechanisms governing upslope flow processes, in particular, generation of upstream circulating cells and plume rise at the apex is presented. The experiments were performed in a 1.25x.35x.3 m water tank, using an inclined (10 to 30 degrees from the horizontal) electrical foil as the heated slope. Under certain condition the flow configuration produced stable circulation cells and rising limited plumes of finite height. Particle Tracking Velocimetry and flow visualization techniques were used for the diagnostics of velocity field and plume rise height, and relevant salient dimensionless quantities were evaluated in terms of governing parameters. Theoretical arguments are presented to explain the results. Parameter ranges for the appearance of characteristic flow patterns are also delineated. [Preview Abstract] |
Sunday, November 20, 2011 2:23PM - 2:36PM |
D1.00002: Thermal stability effects on the turbulent boundary-layer flow over a steep 2-D hill: Near-wake structure Wei Zhang, Fernando Porte-Agel Thermally stratified boundary-layer flows over complex topography are of great interest to the atmospheric science and wind engineering communities. Neutral boundary-layer flows over topography have been extensively studied by wind-tunnel experiments and numerical simulation techniques such as Large-Eddy Simulation (LES). Thermal stability effect, however, is seldom considered due to difficulty of physical simulation in wind tunnels. Experimental studies of thermal stability (neutral, stable and convective) effects on the flow over a steep 2-D hill were conducted in an atmospheric boundary-layer wind tunnel. The 2-D model hill had a steepest slope of 0.73 and its shape followed a cosine shape with height of 7 cm and length of 14.5 cm. High-resolution PIV was employed to characterize the onset of separation, the recirculation zone and flow reattachment location. Results show that thermal stability significantly affects the turbulent flow downwind of the hill, with an elongated recirculation zone and delay of the reattachment in the stable case, and a shorter recirculation zone and early attachment in the convective case. The present study leads to better understanding of thermal stability effects on the boundary-layer flow over a steep 2-D hill, and provides a reliable database for validation and improvement of LES models. [Preview Abstract] |
Sunday, November 20, 2011 2:36PM - 2:49PM |
D1.00003: Atmospheric stability analysis over statically and dynamically rough surfaces Emina Maric, Meredith Metzger, Arindam Singha, Reza Sadr The ratio of buoyancy flux to turbulent kinetic energy production in the atmospheric surface layer is investigated experimentally for air flow over two types of surfaces characterized by static and dynamic roughness. In this study, ``static'' refers to the time-invariant nature of naturally-occurring roughness over a mud/salt playa; while, ``dynamic'' refers to the behavior of water waves along an air-water interface. In both cases, time-resolved measurements of the momentum and heat fluxes were acquired from synchronized 3D sonic anemometers mounted on a vertical tower. Field campaigns were conducted at two sites, representing the ``statically'' and ``dynamically'' rough surfaces, respectively: (1) the SLTEST facility in Utah's western desert, and (2) the new Doha airport in Qatar under construction along the coast of the Persian Gulf. Note, at site 2, anemometers were located directly above the water by extension from a tower secured to the end of a 1 km-long pier. Comparisons of the Monin-Obukhov length, flux Richardson number, and gradient Richardson number are presented, and discussed in the context of the observed evolution of the turbulent spectra in response to diurnal variations of atmospheric stability. [Preview Abstract] |
Sunday, November 20, 2011 2:49PM - 3:02PM |
D1.00004: LES of atmospheric boundary layer flow over fluvial-like anisotropic topography with a dynamic surface drag model William Anderson, Charles Meneveau A dynamic surface drag model (A. \& M. 2011, JFM 679, 288 - 314) is applied in LES of atmospheric boundary layer (ABL) flow over fractal-like topography where the height field exhibits power-law energy spectrum. Initially, the dynamic drag model was applied in LES of ABL flow over isotropic synthetic fractal-like roughness. Here we consider fluvial-like anisotropic landscapes. Two main cases are considered. The first is a fluvial-like topography built through numerical solution of the Kardar-Parisi-Zhang equation.\footnote{Thanks also to Profs. P. Passalacqua and F. Porte-Agel for providing KPZ solution fields.} The second is a rescaled topography (Texas) map from the U.S. National Elevation Dataset. These landscapes are dominated by anisotropic modes that have emerged through geomorphological erosion processes. The dynamic model yields stable solutions even in these highly anisotropic cases: performance is strongest for cases where the LES grid- and test-filter width are within the landscape ``self-similar'' range. Weaknesses are reported for cases where spectral exponent changes with wavenumber, motivating the development of a scale-dependent version of the dynamic approach using two test-filters. [Preview Abstract] |
Sunday, November 20, 2011 3:02PM - 3:15PM |
D1.00005: Lagrangian coherent structures in hurricanes Doug Lipinski, Kamran Mohseni We present the results of a ``surface tracking'' algorithm for efficiently computing Lagrangian coherent structure (LCS) surfaces in three dimensions. The algorithm is applied to data from a Weather Research and Forecasting simulation of hurricane Rita. The highly complicated LCS surfaces reveal complex dynamics and transport in the hurricane, particularly in the lower atmosphere boundary layer and the upper level outflow. The lower level transport in the hurricane is of particular importance for accurate intensity prediction in hurricane forecasts due to the uncertainty in the ocean-atmosphere interaction. Understanding the lower level transport and mixing behavior in hurricanes could lead to significant advances in hurricane intensity prediction. [Preview Abstract] |
Sunday, November 20, 2011 3:15PM - 3:28PM |
D1.00006: Description of coherent features in the atmospheric boundary layer by low-dimensional analysis of surface pressures Gregory Lyons, Nathan Murray Short-term localized forecasting of the near-earth atmospheric boundary layer could enhance empirical models and provide input for wind energy control systems. It is expected that a successful forecasting method could be developed through characterization of the coherent turbulent structures in the near-earth flow field. The coherence of these features suggests description by a low-dimensional method, such as the dynamic mode decomposition. To this end, a circular array of pressure transducers approximately 60 meters in diameter was deployed on the earth's surface, and time series were recorded at each sensor location. As scalar observables, these pressure measurements are functions of the flow field system. As such, by decomposing these data into dynamic modes, the spatial and temporal coherence of local features in the near-earth atmospheric boundary layer is described. [Preview Abstract] |
Sunday, November 20, 2011 3:28PM - 3:41PM |
D1.00007: A comparison of scaling in satellite and in situ water vapor measurements Kyle Pressel, William Collins, Ankur Desai Improvements in the characterization of cloud scale variability of water vapor are a direct means of improving stochastic parametrization of clouds in global climate models (GCMs). The Atmospheric Infrared Sounder (AIRS) provides nearly global physical retrievals of the vertical profile of water vapor mass mixing ratio, at resolutions far greater than cloud scale. The 447m WLEF tower located near Park Falls, WI is instrumented to provide high frequency (10Hz) observations of water vapor at 396, 122, and 30m above ground level. Using Taylor's hypothesis, the high frequency tower observations can be used to characterize the variability of water vapor at or near cloud scales, but only at a single location. In this work, we will compare the climatology of water vapor scaling (scale-invariance) observed by AIRS and from the WLEF tower, in an effort to examine the potential use of the scale-invariant properties of the water vapor field to gain global information about cloud scale water vapor variability from AIRS. [Preview Abstract] |
Sunday, November 20, 2011 3:41PM - 3:54PM |
D1.00008: Investigating multiscale modeling with the Kuramoto-Sivashinsky equation Lauren Padilla, Clarence Rowley, Geoffrey Vallis The multiscale modeling framework (MMF), also known as superparameterization, is a new approach to solving the sub-gridscale closure problem in weather and climate simulations that are too expensive for direct numerical simulation. In the MMF, the true equations of motion for the sub-gridscale processes replace typical parameterizations which provide closure based on large-scale variables alone where no local dynamics are allowed to develop. The extent to which MMF represents improvement over conventional models is an open question that we investigate systematically in a simple fluid-like system, the Kuramoto-Sivashinsky (K-S) equation. This one-dimensional, non-linear, partial differential equation is an ideal test case for the MMF approach. The K-S equation exhibits chaotic behavior and includes the effects of production, dissipation, and advection yet is simple enough to sample many initial conditions over long time periods. We present results comparing the energy spectra, error, and turbulent kinetic energy of solutions achieved through direct numerical simulation, MMF with different coupling schemes, and conventional large-scale closure. [Preview Abstract] |
Sunday, November 20, 2011 3:54PM - 4:07PM |
D1.00009: A leaky rigid lid Lyubov Chumakova, Ruben Rosales, Esteban Tabak Various models of the atmosphere use the rigid lid approximation. In reality the atmosphere does not have a definite top. Furthermore, mathematically the two problems are drastically different. In the study of linear problem in hydrostatic balance the rigid lid approximation leads to a problem with discrete spectrum, while a semi-infinite atmosphere problem has a continuous one. The energy that is projected onto the continuous part of the spectrum is disregarded in the rigid lid approach. We propose to fix this ``leak'' problem via changing the boundary conditions at the top of the rigid lid. [Preview Abstract] |
Sunday, November 20, 2011 4:07PM - 4:20PM |
D1.00010: Parameterization of turbulence characteristics of Atmospheric surface layer in Qatar Arindam Singha, Reza Sadr Turbulent characteristics of atmospheric boundary layer are of utmost importance in modeling the large-scale meteorological processes, diffusion of atmospheric contaminants, heat transfer and evaporation from the earth surface. Available data are for some areas on the globe and are really sparse in tropical regions, except a few recent studies in Asia. There had been some recent studies in tropical weather in southwestern Asia but no study is carried out in Persian Gulf region. An atmospheric measurement station has been designed and installed in a site in the coastal region of Doha, Qatar, to characterize the nature of atmosphere surface layer (ASL) and ocean wave characteristics in this field. The aim of the present study is to report the micrometeorological data collected from this site. The normalized variation of the turbulent velocity components and temperature were studied using Monin-Obukhov similarity theory (MOST). This study also attempts to verify the validity of MOST in the context of the data collected for a marine ASL in Qatar, and compares the modeling parameters with other investigations around the world. This is the first ever study of ASL in this area, and is expected to be a foundation of further atmospheric research endeavors in Qatar. [Preview Abstract] |
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