Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C20: Geophysical Fluid Dynamics Atmosphere |
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Chair: Leslie Smith, University of Wisconsin, Madison Room: 602 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C20.00001: Velocity-defect laws, log law and logarithmic friction law in the convective atmospheric boundary layer Mengjie Ding, Chenning Tong The mean velocity profile (MVP) in the convective atmospheric boundary layer (CBL) is derived analytically employing the shear-stress budget equations and the mean momentum equations. The multi-point Monin--Obukhov similarity (MMO) recently proposed provides the scaling properties of the one-point statistics in these equations. Our previous studies have shown that the CBL is mathematically a singular perturbation problem. Thus, we obtain the MVP using the method of matched asymptotic expansions. Three scaling layers are identified: the outer layer, the inner-outer layer and the inner-inner layer. Two new velocity--defect laws are discovered: the mixed-layer velocity--defect law and the surface-layer velocity--defect law. The local-free-convection MVP is obtained by asymptotically matching the expansions in the first two layers. The log law is obtained by matching in the last two layers. The von Karman constant is obtained using velocity and length scales, and therefore has a physical interpretation. A new friction law, the convective logarithmic friction law, is obtained. The present work provides an analytical derivation of the MVP hypothesized in the Monin--Obukhov similarity theory, and is part of a comprehensive derivation of the MMO scaling from first principles. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C20.00002: Annual variability of atmospheric surface layer characteristics and wind/temperature patterns in Qatar Reza Sadr, Yuan Li Surface wind patterns influence the anemochory, pollutants dissipation. The research of the characteristics of surface layer and turbulence exchange processes can contribute to the local economic construction and understanding of regional plant ecological environment. This work reports on the weather variation in the coastal region of northern Qatar peninsula in the Persian Gulf. Wind velocity, direction, humidity and temperature data for the coastal site of Qatar are recorded from August 2015 to September 2016. Sonic anemometer and weather station data was collected at 9 m height tower. Seasonal wind patterns are analyzed. Shamal wind from Northwest is prevailing for all the four seasons and the annual wind speed is 4.67 m/s. Temperature in June, July, August and September are the highest, with the lowest air pressure and the most occurrence of the summer Shamal. Two other public data are used for comparison with the present data. The normalized variance of wind components and temperature are studied within the framework of Monin-Obukhov similarity theory. Heat and momentum fluxes are calculated and compared with other reported values world-wide. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C20.00003: Turbulent characteristics analysis of atmospheric surface layer in coastal region of Qatar Yuan Li, Reza Sadr The turbulence characteristics in coastal region of Qatar are analyzed. The micrometeorological data is collected in the coastal site at (26.08N, 51.36E) by three sonic anemometers located on a 9 m height tower from September 2015 to August 2016. The friction velocity, Obukhov length and the normalized variance of wind components and temperature are studied within the framework of Monin-Obukhov similarity theory. During the 1-year measurement period, 33{\%} of data are characterized as stable. The normalized variances are in agreement to empirical fits from other reported values under both unstable and stable atmospheric stratification. However, the normalized variance of stream wise and transverse velocity at near-neutral condition is slightly higher than other observations while the normalized variance of vertical velocity is slightly lower. The measured wind components and temperature in the surface layer shows the sea breeze circulation. The diurnal pattern characteristics in the coastal region is also analyzed for the onshore and offshore breeze. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C20.00004: Internal Boundary Layer Induced by Dune-Field Roughness Andrew Gunn, Ryan Ewing, Matthew Wanker, Douglas Edmonds, Phillip Schmutz, Douglas Jerolmack Within atmospheric boundary layers(BL), internal BL (IBL) develop across sharp flow-normal topographic changes in planetary surfaces. At White Sands, New Mexico, topographic change is brought about by the BL itself in the form of wind-blown sand dunes. There is an abrupt roughness change from a smooth playa to a dune field, inducing an IBL. We have measured wind profiles of the unidirectional BL across the roughness transition with 3 meteorological towers simultaneously on an along-flow axis, and 2 Doppler lidar deployments. Our results show profiles downwind from the roughness transition have decreased surface stress and northward deflected flow w.r.t. those upwind, consistent with an equilibrium expectation from Rossby similarity theory. An IBL, however, is a non-equilibrium phenomena and is yet to be observed to include secondary flow. Using a Lagrangian 1D BL model, we show this large change in roughness can, with sufficient fetch, introduce geostrophic secondary flow and match our observations. We close this land-atmosphere feedback loop via repeat high-resolution aerial topographic surveys, finding that this IBL in-turn steers and slows the dune migration along-wind. We believe our results can generalize to other cases such as canopies and coasts. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C20.00005: Characterizing Intermittency in the Stable Arctic Atmospheric Boundary Layer Mohammad Allouche, Elie Bou-Zeid, Jose Fuentes, Marcelo Chamecki, Otavio Acevedo, Sham Thanekar, Cedrick Ansorge To elucidate the physics of surface-atmosphere exchange processes in Polar Regions, our understanding of the stable atmospheric boundary layer (SABL) where buoyancy damps turbulent kinetic energy needs to advance significantly. We seek to understand the intermittent turbulence regime observed in the strongly stable case. The inertial sublayer, referred to as the atmospheric surface layer (ASL), under such regime is characterized by abrupt transitions between turbulent and laminar states. In this study, we analyze field experimental data from Barrow, Alaska to detect intermittent periods based on non-dimensional statistical metrics. We reveal three clusters of turbulence regimes, two of which correspond to the weakly turbulent periods that feature intermittent behavior (cluster 1: intermittent, cluster 2: transitional) and the third cluster is a fully turbulent regime (cluster 3) only mildly damped by stability. We then investigate the origins of the intermittent bursts based on analyses of the Turbulent Kinetic Energy (TKE) budget equation over these bursts in the TKE time series, and assess the combination of velocity and length scales needed in the eddy diffusion theory under intermittent conditions. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C20.00006: Diabatic vortices: a simple framework for tropical cyclones and the martian polar vortex Richard Scott The formation and subsequent evolution of annular distributions of potential vorticity, forced by the combination of diabatic heating and the angular momentum transport of secondary, transverse circulations, is investigated in the framework of the forced shallow water equations at various levels of complexity. Annular potential vorticity is found to develop under a range of forcing conditions depending on the radial location of diabatic heating and the structure of secondary circulation; the simplest, axisymmetric formulation of the model allows the effect of the two forcings to be examined in isolation. Further eddy permitting calculations allow the nonlinear transient evolution to be studied, where the competition of forcing and shear instability can give rise to quasi-regular vacillation cycles. The framework is applied to two very distinct geophysical systems, the terrestrial tropical cyclone and the martian winter polar vortex, where diabatic heating is due to the condensation of water vapor or carbon dioxide, respectively, and where eddy transience gives rise to such effects as eye-wall replacement cycles and controls the polar transport of dust and ice aerosols. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C20.00007: Balanced and Unbalanced Components of Atmospheric Flows with Phase Changes of Water Leslie Smith Atmospheric variables (temperature, velocity, etc.) are often decomposed into balanced and unbalanced components that represent low-frequency and high-frequency waves, respectively. Such decompositions can be defined, for instance, in terms of eigenmodes of a linear operator. Traditionally these decompositions do not account for phase changes of water since the latter create a piecewise-linear operator that differs in different phases (cloudy versus non-cloudy). Here we demonstrate how a balanced--unbalanced decomposition can be performed in the presence of phase changes, by including a slow thermodynamic variable involving total water in addition to a slow potential vorticity variable, which are both associated with the nullspace of the linear operator with phase changes. Evolution of the fast and slow components of water is illustrated in simulations of moist Boussinesq dynamics, as well an in a turbulent steady-state of the idealized Weather Research Forecasting (WRF) model. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C20.00008: Morphological Properties of Large-Scale Motions Remain Self-Similar Across Thermal Regimes Scott Salesky, William Anderson Studies of high-Re wall turbulence have revealed the existence of large scale motions (LSMs) that populate the logarithmic layer and modulate the amplitude of small-scale turbulent fluctuations near the wall. In flows with unstable thermal stratification, conventional wisdom states that these structures become shorter in the streamwise direction and steepen with increasing buoyancy forcing. Using a suite of large eddy simulations of wall-bounded turbulent shear flows with increasingly unstable thermal stratification, we demonstrate from instantaneous flow visualizations and conditional averages that morphological properties of these structures remain self-similar with increasing unstable thermal stratification. As thermal stratification increases, the downstream edge of an LSM begins to detach from the wall, leaving a “wedge” of cool, high-momentum fluid beneath. A simple model is developed for the inclination angle of LSMs with increasing thermal instability; atmospheric surface layer observations from the Advection Horizontal Array Turbulence Study (AHATS) are found to be in good agreement with the predicted inclination angles. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C20.00009: Efficient Dynamical Downscaling of General Circulation Models Using Continuous Data Assimilation Omar Knio, Srinivas Desamsetti, Hari Dasari, Sabique Langodan, Ibrahim Hoteit, Edriss Titi Continuous data assimilation (CDA) is implemented for efficient dynamical downscaling of a global atmospheric reanalysis. A comparison of the performance of CDA with grid and spectral nudging techniques is conducted, using the Weather Research and Forecast model. The model is configured at $0.25^\circ \times 0.25^\circ$ horizontal resolution and is driven by $2.5^\circ \times 2.5^\circ$ initial and boundary conditions from NCEP/NCAR reanalysis fields. Downscaling experiments are performed over a one-month period. Results are compared for the outputs of the WRF model with different downscaling techniques, NCEP/NCAR reanalysis, and NCEP Final Analysis. Both spectral nudging and CDA describe better the small-scale features compared to grid nudging. CDA maintains the balance of large- and small-scale features similar to that of the best simulation achieved using spectral nudging, without the need of a spectral decomposition. The results indicate that different atmospheric variables downscaled with CDA are most consistent with observations, and that CDA consequently provides an attractive approach for dynamical downscaling. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C20.00010: Unmanned Aircraft for Mapping Atmospheric Boundary Layer Induced Geomorphological Changes Victoria Natalie, Jamey Jacob The process of acquiring lower atmospheric measurements onboard unmanned aircraft systems (UAS) is becoming a widely available solution. This study ties atmospheric observations using UAS with photogrammetrically mapped rapid geomorphology to observe and relate the terrain induced effects on lower atmospheric phenomena. Lower atmospheric boundary conditions are characterized through wind velocity and turbulence measurements. In order to achieve this data fusion, empirical data was obtained by UAS mounted anemometers and photogrammetric terrain modeling was acquired utilizing airborne imagery. The results of both are analyzed and related to compare the coupled effects. Wind induced terrain variations are also compared through multiple terrain models, showing the temporal evolution of the landscape. This study was performed at Little Sahara is a State park in Oklahoma, a naturally occurring collection of barchan structures and sand dunes that are heavily influenced by the prevailing southern wind. The datasets are months apart, and the changes in the topography demonstrate how closely linked the atmospheric movement and the changes in topography are to each other. Wind observations were made with multiple direct and indirect methods and are presented. [Preview Abstract] |
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