Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session M13: Geophysical Fluid Dynamics: Atmospheric Dynamics |
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Sponsoring Units: DFD GPC Chair: Harindra Fernando, University of Notre Dame Room: C124 |
Tuesday, November 22, 2016 8:00AM - 8:13AM |
M13.00001: Turbulence in the Stable Atmospheric Boundary Layer Harindra Fernando, Eliezer Kit, Patrick Conry, Christopher Hocut, Dan Liberzon During the field campaigns of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program, fine-scale measurements of turbulence in the atmospheric boundary layer (ABL) were made using a novel sonic and hot-film anemometer dyad (a combo probe). A swath of scales, from large down to Kolmogorov scales, was covered. The hot-film was located on a gimbal within the sonic probe volume, and was automated to rotate in the horizontal plane to align with the mean flow measured by sonic. This procedure not only helped satisfy the requirement of hot-film alignment with the mean flow, but also allowed in-situ calibration of hot-films. This paper analyzes a period of nocturnal flow that was similar to an idealized stratified parallel shear flow. Some new phenomena were identified, which included the occurrence of strong bursts in the velocity records indicative of turbulence generation at finer scales that are not captured by conventional sonic anemometers. The spectra showed bottleneck effect, but its manifestation did not fit into the framework of previous bottleneck-effect theories and was unequivocally related to bursts of turbulence. The measurements were also used to evaluate the energetics of stratified shear flows typical of the environment. [Preview Abstract] |
Tuesday, November 22, 2016 8:13AM - 8:26AM |
M13.00002: Katabatic flow: a closed-form solution with spatially-varying eddy diffusivities Marco G. Giometto, Riccardo Grandi, Jiannong Fang, Peter A. Monkewitz, Marc B. Parlange The Nieuwstadt closed-form solution for the stationary Ekman layer is generalized for katabatic flows within the conceptual framework of the Prandtl model. The proposed solution is valid for spatially-varying eddy diffusivities (O'Brien type) and constant Prandtl number $(Pr)$. Variations in the velocity and buoyancy profiles will be discussed as a function of the dimensionless model parameters $z_0 \equiv \hat{z}_0 \hat{N}^2 Pr \sin{(\alpha)} |\hat{b}_s |^{-1}$ and $\lambda \equiv \hat{u}_{\rm{ref}} \hat{N} \sqrt{Pr} |\hat{b}_s |^{-1}$, where $\hat{z}_0$ is the hydrodynamic roughness length, $\hat{N}$ is the buoyancy frequency, $\alpha$ is the surface sloping angle, $\hat{b}_s$ is the imposed surface buoyancy, and $\hat{u}_{\rm{ref}}$ is a reference velocity scale used to define eddy diffusivities. Profiles show significant variations in both phase and amplitude of extrema with respect to the classic constant $K$ model and with respect to a recent approximate analytic solution based on the Wentzel-Kramers-Brillouin theory, hence shedding new light on the problem. [Preview Abstract] |
Tuesday, November 22, 2016 8:26AM - 8:39AM |
M13.00003: Deviations from Equilibrium in Daytime Atmospheric Boundary Layer Turbulence arising from Nonstationary Mesoscale Forcing Balaji Jayaraman, James Brasseur, Sue Haupt, Jared Lee LES of the ``canonical'' daytime atmospheric boundary layer (ABL) over flat topography is developed as an equilibrium ABL with steady surface heat flux, $Q_{0}$ and steady unidirectional ``geostrophic'' wind vector $V_{g}$ above a capping inversion. A strong inversion layer in daytime ABL acts as a ``lid'' that sharply separates 3D ``microscale'' ABL turbulence at the O(10) m scale from the quasi-2D ``mesoscale'' turbulent weather eddies (O(100) km scale). While ``canonical'' ABL is equilibrium, quasi-stationary and characterized statistically by the ratio of boundary layer depth ($z_{i})$ to Obukhov length scale ($-L)$, the real mesoscale influences ($U_{g}$ and $Q_{0})$ that force a true daytime ABL are nonstationary at both diurnal and sub-diurnal time scales. We study the consequences of this non-stationarity on ABL dynamics by forcing ABL LES with realistic WRF simulations over flat Kansas terrain. Considering horizontal homogeneity, we relate the mesoscale and geostrophic winds, $U_{g}$ and $V_{g}$, and systematically study the ABL turbulence response to non-steady variations in $Q_{0}$ and $U_{g}$. We observe significant deviations from equilibrium, that manifest in many ways, such as the formation of ``roll'' eddies purely from changes in mesoscale wind direction that are normally associated with increased surface heat flux. [Preview Abstract] |
Tuesday, November 22, 2016 8:39AM - 8:52AM |
M13.00004: Turbulence dynamics in unsteady atmospheric flows Mostafa Momen, Elie Bou-Zeid Unsteady pressure-gradient forcing in geophysical flows challenges the quasi-steady state assumption, and can strongly impact the mean wind and higher-order turbulence statistics. Under such conditions, it is essential to understand when turbulence is in quasi-equilibrium, and what are the implications of unsteadiness on flow characteristics. The present study focuses on the unsteady atmospheric boundary layer (ABL) where pressure gradient, Coriolis, buoyancy, and friction forces interact. We perform a suite of LES with variable pressure-gradient. The results indicate that the dynamics are mainly controlled by the relative magnitudes of three time scales: $T_{inertial}$, $T_{turbulence}$, and $T_{forcing}$. It is shown that when $T_{f}\approx T_{t}$, the turbulence is no longer in a quasi-equilibrium state due to highly complex mean-turbulence interactions; consequently, the log-law and turbulence closures are no longer valid in these conditions. However, for longer and, surprisingly, for shorter forcing times, quasi-equilibrium is maintained. Varying the pressure gradient in the presence of surface buoyancy fluxes primarily influences the buoyant destruction in the stable ABLs, while under unstable conditions it mainly influences the transport terms. [Preview Abstract] |
Tuesday, November 22, 2016 8:52AM - 9:05AM |
M13.00005: Influence of cross-flow on the entrainment of bending plumes Graham Freedland, Larry Mastin, Solovitz Steven, Raul Cal Volcanic eruption columns inject high concentrations of ash into the atmosphere. Some of this ash is carried downwind forming ash clouds in the atmosphere that are hazardous for private and commercial aviation. Current models rely on inputs such as plume height, duration, eruption rate, and meteorological wind fields. Eruption rate is estimated from plume height using relations that depend on the rate of air entrainment into the plume, which is not well quantified. A wind tunnel experiment has been designed to investigate these models by injecting a vertical air jet into a cross-flow. The ratio of the cross-flow and jet velocities is varied to simulate a weak plume, and flow response is measured using particle image velocimetry. The plumes are characterized and profile data is examined to measure the growth of weak plumes and the entrainment velocity along its trajectory. This allows for the study of the flow field, mean, and second order moments, and obtain information to improve models of volcanic ash concentrations in the atmosphere. [Preview Abstract] |
Tuesday, November 22, 2016 9:05AM - 9:18AM |
M13.00006: Dynamic roughness model for LES of turbulent flow over multiscale urban-like topography Xiaowei Zhu, William Anderson Urban-like topographies are composed of a wide spectrum of topographic elements, which results in multiscale, fractal-like distributions. This has important implications for microscale numerical weather prediction in urban environments, or urban meteorology: the range of scales inhibits the use of numerical schemes where the topography is fully resolved, but the self-similar nature of the topography inspires development of closures that leverage such self-similarity to parameterize unresolved information. That is, a natural urban landscape can be low-pass filtered at the large-eddy simulation grid scale, thereby removing details between the grid scale and the smallest scale of the landscape, but the effects of these truncated topographic modes can be modeled based on details of the large scale. LES has been used to investigate the effects of subgrid-scale (SGS) topography on the roughness length of multiscale urban-like topographies. First, high-resolution multiscale urban-like topographies were generated with random distribution function. Then, the high-resolution multiscale topography was filtered and separated into large- and small-scale topographies with the Reynolds decomposition. Thus, the topography was decomposed into resolved (scale larger than the grid scale) and SGS part (scale smaller than the filter scale). The resolved part was resolved in LES, while the SGS terrain must be parameterized. New models for urban roughness will be used to parameterize SGS topography. [Preview Abstract] |
Tuesday, November 22, 2016 9:18AM - 9:31AM |
M13.00007: Modulation of energetic coherent motions by large-scale topography. Wing Lai, Ali M. Hamed, Dan Troolin, Leonardo P. Chamorro The distinctive characteristics and dynamics of the large-scale coherent motions induced over 2D and 3D large-scale wavy walls were explored experimentally with time-resolved volumetric PIV, and selected wall-normal high-resolution stereo PIV in a refractive-index-matching channel. The 2D wall consists of a sinusoidal wave in the streamwise direction with amplitude to wavelength ratio a/$\lambda $x $=$ 0.05, while the 3D wall has an additional wave in the spanwise direction with a/$\lambda $y $=$ 0.1. The ?ow was characterized at Re \textasciitilde 8000, based on the bulk velocity and the channel half height. The walls are such that the amplitude to boundary layer thickness ratio is a/$\delta $99 $\approx $ 0.1, which resemble geophysical-like topography. Insight on the dynamics of the coherent motions, Reynolds stress and spatial interaction of sweep and ejection events will be discussed in terms of the wall topography modulation. [Preview Abstract] |
Tuesday, November 22, 2016 9:31AM - 9:44AM |
M13.00008: A nonlinear self-similar solution to barotropic flow over rapidly varying topography Ruy Ibanez, Joseph Kuehl Beginning from the Shallow Water Equations (SWE), a nonlinear self-similar analytic solution is derived for barotropic flow over rapidly varying topography. We study conditions relevant to the ocean slope where the flow is dominated by Earth's rotation and topography. Attention is paid to the northern Gulf of Mexico slope with application to pollutant dispersion and the Norwegian Coastal Current which sheds eddies into the Lofoten Basin that are believe to influence deep water formation. The solution is found to extend the topographic $\beta$-plume solution (Kuehl 2014, GRL) in two ways: 1) The solution is valid for intensifying jets. 2) The influence of nonlinear advection is included. The SWE are scaled to the case of a topographically controlled jet, then solved by introducing a similarity variable $\eta = Cxy$. The nonlinear solution, valid for topographies $h = h_{0} - \alpha x y^{3}$, takes the form of the Lambert W Function for velocity. The linear solution, valid for topographies $h = h_{0} - \alpha x y^{\gamma}$, takes the form of the Error Function for transport. Kuehl's results considered the case $-1 \le \gamma < 1$ which admits expanding jets, while the new result consider the case $\gamma < -1$ which admits intensifying jets. [Preview Abstract] |
Tuesday, November 22, 2016 9:44AM - 9:57AM |
M13.00009: Interactions between intermittent gravity waves and infrasounds Bruno Ribstein, Christophe Millet, Francois Lott, Alvaro de la Camara Even though the accuracy of atmospheric specifications is constantly improving, it is well known that the main part of gravity waves is still yet not resolved in the available data. In most infrasound modeling studies, the unresolved gravity wave field is often represented as a deterministic field that is superimposed on a given average background state. Direct observations in the lower stratosphere show, however, that the gravity wave field is very intermittent, and is often dominated by rather well defined wave packets. In this study we sample the gravity wave spectrum by launching few monochromatic waves and choose their properties stochastically to mimic the intermittency. The statistics of acoustic signals are computed by decomposing the original signal into a sum of modal pulses. Owing to the disparity of the gravity and acoustic lengthscales, the interaction can be described using a multiplescale analysis and the appropriate amplitude evolution equation involves certain random terms that are related to the gravity wave sources. More specifically, it is shown how the unpredictable low level small-scale dynamics triggers multiple random stratospheric waveguides in which high frequency infrasound components can propagate efficiently. [Preview Abstract] |
Tuesday, November 22, 2016 9:57AM - 10:10AM |
M13.00010: Modeling Radiation Fog Sreenivas K R, Rafiuddin Mohammad Predicting the fog-onset, its growth and dissipation helps in managing airports and other modes of transport. After sunset, occurrence of fog requires moist air, low wind and clear-sky conditions. Under these circumstances radiative heat transfer plays a vital role in the NBL. Locally, initiation of fog happens when the air temperature falls below the dew-point. Thus, to predict the onset of fog at a given location, one has to compute evolution of vertical temperature profile. Earlier,our group has shown that the presence of aerosols and vertical variation in their number density determines the radiative-cooling and hence development of vertical temperature profile$^1$. Aerosols, through radiation in the window-band, provides an efficient path for air layers to lose heat to the cold, upper atmosphere. This process creates cooler air layer between warmer ground and upper air layers and resulting temperature profile facilitate the initiation of fog. Our results clearly indicates that accounting for the presence of aerosols and their radiative-transfer is important in modeling micro-meteorological process of fog formation and its evolution. . [1] Field and laboratory experiments on aerosolāinduced cooling in the nocturnal boundary layer. Q.J.R.Meteorol.Soc. 140.678 2014, 151-169. [Preview Abstract] |
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