Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session GL: Geophysical: Atmospheric II |
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Chair: Marko Princevac, University of California, Riverside Room: Salt Palace Convention Center 250 F |
Monday, November 19, 2007 10:30AM - 10:43AM |
GL.00001: Efficient and effective mixed turbulence models for LES of the atmospheric boundary layer Fotini Chow Dynamic mixed turbulence models have recently been applied to large-eddy simulation of idealized atmospheric flows, with great improvement in comparisons to similarity theory. Mixed models combine a scale-similarity (or velocity reconstruction) component with an eddy-viscosity component. A dynamic eddy-viscosity model can be used for the latter. Difficulty in implementation of dynamic models over rough surfaces has limited their use to simplified geometries. Here it is shown that combining a scale-similarity model with standard non-dynamic eddy viscosity formulations leads to a simple mixed model which still provides significant needed improvements to atmospheric boundary layer simulations and can easily be implemented for general flows over complex terrain. [Preview Abstract] |
Monday, November 19, 2007 10:43AM - 10:56AM |
GL.00002: Improving Simulations of Atmospheric Flow with an Immersed Boundary Method to Represent Complex Terrain Katherine Lundquist, Fotini Chow, Julie Lundquist, Jeff Mirocha A framework is being developed for building-resolving urban flow simulations using the Weather Research and Forecasting (WRF) model. WRF is formulated as a meso-scale model which solves the compressible Navier-Stokes equations. Additionally, WRF allows for lateral boundary forcing based on synoptic data and performs two-way grid nesting at subsequent finer nested grid levels. To accommodate the complex geometries of urban scale domains, we have implemented an immersed boundary method (IBM) along with a rough surface parameterization. This IBM alleviates difficulties associated with terrain following coordinates in WRF, and allows seamless nesting of domains ranging from the synoptic to urban scale. Illustrative examples will be used to demonstrate that both Neumann and Dirichlet type boundary conditions have been implemented with the IBM. Simulations with Neumann boundary conditions include nonlinear topographic mountain waves over idealized terrain. Dirichlet boundary conditions are validated in a variety of laminar and turbulent flows. Examples include the simulation of external flows over bluff bodies and channel flow with a sinusoidal lower boundary. [Preview Abstract] |
Monday, November 19, 2007 10:56AM - 11:09AM |
GL.00003: Effects of soil moisture initialization on simulations of atmospheric boundary layer flow over complex terrain Megan Daniels, Fotini Chow Soil moisture affects flow in the atmospheric boundary layer (ABL) by changing the surface heat fluxes. Standard surface boundary condition initialization procedures often rely on coarse grid data sets that are unable to capture variation over topography resolved by finer grids. High resolution simulations of ABL flow over Owens Valley, CA are carried out first using standard surface boundary condition initialization procedures, then using field observations of soil moisture and temperature as well as adjusted snow cover. A quiescent and a strongly forced case are considered. Simulation results are compared to observations gathered during the Terrain-Induced Rotor Experiment in March and April, 2006. Preliminary results indicate that more accurate soil moisture characterization changes flow evolution under quiescent conditions. The effect of soil moisture initialization under strongly forced conditions is also examined. [Preview Abstract] |
Monday, November 19, 2007 11:09AM - 11:22AM |
GL.00004: Large Eddy Simulation of Pollen Transport in the Atmospheric Boundary Layer Marcelo Chamecki, Charles Meneveau, Marc B. Parlange The development of genetically modified crops and questions about cross-pollination and contamination of natural plant populations enhanced the importance of understanding wind dispersion of airborne pollen. The main objective of this work is to simulate the dispersal of pollen grains in the atmospheric surface layer using large eddy simulation. Pollen concentrations are simulated by an advection-diffusion equation including gravitational settling. Of great importance is the specification of the bottom boundary conditions characterizing the pollen source over the canopy and the deposition process everywhere else. The velocity field is discretized using a pseudospectral approach. However the application of the same discretization scheme to the pollen equation generates unphysical solutions (i.e. negative concentrations). The finite-volume bounded scheme SMART is used for the pollen equation. A conservative interpolation scheme to determine the velocity field on the finite volume surfaces was developed. The implementation is validated against field experiments of point source and area field releases of pollen. [Preview Abstract] |
Monday, November 19, 2007 11:22AM - 11:35AM |
GL.00005: Transition in energy spectrum for forced stratified turbulence Yoshi Kimura, Jackson Herring Energy spectrum for forced stably stratified turbulence is investigated numerically. The 3D momentum equation under the Boussinesq approximation is solved pseudo- spectrally with stochastic forcing applied to the largest velocity scales. Following Lesieur \& Rogallo (1989) and Carnevale {\it et.~al.}(2001), spectral eddy viscosity, $ \nu_t(k)=(a_1+a_2 \exp(- a_3k_c/k))\sqrt{E(k_c)/k_c}$, is used for small scale dissipation. Using toroidal-poloidal decomposition (Craya-Herring decomposition), the velocity field is divided into the vortex mode ($\phi_1$) and the wave mode ($ \phi_2 $). With the initial kinetic energy being zero, the $\phi_1$ spectra as a function of horizontal wave numbers, $k_{\perp}$, first develops a $k_{\perp}^{-3}$ spectra for the whole $k_{\perp}$ range, and then $k_{\perp}^{-5/3}$ part appears with rather a sharp transition wave number. Meanwhile the $\phi_2$ spectra shows $k_{\perp}^{-2}$ first, and then $k_{\perp}^{-5/3}$ part appears with the same transition wave number. According to Carnevale {\it et.~al.}, the transition wave number is understood as the Ozmidov scale with a correction by the coefficients of the buoyancy spectrum, $E(k) =\alpha N^2k^{-3}$, and the Kolmogorov spectrum, $E(k)=C_K\epsilon^{2/3} k^{-5/3}$. By equating these spectra, we obtain $k_b \sim (\alpha/C_K)^{3/4}\sqrt {N^3/ \epsilon}$. This assessment will be discussed. \par\medskip\noindent Carnevale,G.F. {\it et.~al}: 2001 J.~Fluid Mech. {\bf 427} 205--239.\par\noindent Lesieur, M. \& Rogallo, R. 1989 Phys. Fluids A{\bf 1} 718--722.\par [Preview Abstract] |
Monday, November 19, 2007 11:35AM - 11:48AM |
GL.00006: Extracting Lagrangian Coherent Structures and locating Clear Air Turbulence Wenbo Tang, Manikandan Mathur, George Haller Clear Air Turbulence (CAT) is a small-scale meteorological hazard that poses significant threat to aviation safety. We apply dynamical systems methods on three-dimensional atmospheric data to detect Lagrangian Coherent Structures that play a crucial role in CAT. We also discuss the development of an automated algorithm that provides real-time CAT alert based on on-board sensor data. Our analysis is also of use in weather forecasting. [Preview Abstract] |
Monday, November 19, 2007 11:48AM - 12:01PM |
GL.00007: Rotors in inviscid flow John McHugh, Robert Sharman Rotors are large coherent vortex structures that form in the lee of a mountain ridge. They are known to be strongly turbulent, and a severe hazard to aircraft. Rotors are also believed to be responsible for stirring up large quantities of dust and other pollutants, transporting it to higher altitudes. The onset of rotor flow is poorly understood. Queney (1955) suggested that a rotor will appear behind a mountain as a result of steep mountain waves, which should be predictable by inviscid theory. The wave amplitude that results in a vertical stream line is the critical amplitude, and larger wave amplitudes will have rotors, according to Queney (1955). However, recent inviscid simulations using mesoscale models with very large amplitude waves fail to show the expected rotor, as reported by Doyle and Durran (2002). Doyle and Durran successfully initiate rotor flow, but only with viscosity and the no-slip condition at the bottom boundary. Recent simulations have revisited the inviscid case by considering initial conditions that are not uniform, including flows with discrete vortices. Simulations with an idealized mountain show rotor-like behavior in the lee of the mountain for some of these initial conditions. The primary conclusion is that a rotor may form in inviscid flow given the proper choice of initial conditions. The strength of the stratification is also important. [Preview Abstract] |
Monday, November 19, 2007 12:01PM - 12:14PM |
GL.00008: A Forecast Model for Atmospheric Internal Waves Produced by a Mountain James Rottman, Dave Broutman The breaking of mountain-generated internal waves in the atmosphere is an important mechanism for mixing in the upper parts of the troposphere and the stratosphere, and the associated turbulence also is a hazard for aircraft. We describe a new forecast model for mountain waves in a height-dependent atmosphere. The model is based on a nonlinear numerical simulation of the near-field flow around the topography combined with a ray-tracing of the far-field propagation of the internal waves generated by the near-field flow. A mesoscale model is used to simulate the near-field flow. This flow can be substantially nonlinear, with vortex shedding, flow separation, and wavebreaking. The more linear internal wavefield that emerges from this low-level flow is modeled by a Fourier ray method, which is initialized with the mesoscale model results. The Fourier ray method is used to propagate the waves to higher altitudes. The ray model allows for the refraction and reflection of the waves by height-dependent winds and stratification, including the possibility of two turning points, and with much higher resolution than is practical with a mesoscale model. [Preview Abstract] |
Monday, November 19, 2007 12:14PM - 12:27PM |
GL.00009: Jupiter's New Red Spot - An Indication of Climate Change? Philip Marcus, Chung-Hsiang Jiang, Xylar Asay-Davis Jupiter now has two red spots. The new Red Oval, is similar to the Great Red Spot and is Jupiter's second largest storm. The Oval was originally white, but turned red in 2005. This color change was the first sign of the current jovian ``upheaval'' at the latitudes north of the Oval. Using a new method, Data Assimilated Correlation Image Velocimetry, we derive the Oval's velocities with unprecedented accuracy from spacecraft images. These data show that the velocities of the Oval did not between 2000 and 2005 and therefore cannot account for its color change. Although the dynamics of upheavals and other jovian climate cycles are not well understood, in 2001 we predicted that a large-scale warming at, and north of, the Oval, was about to begin, with effects becoming visible in 2006. Currently, no instrument can directly confirm whether the average jovian temperatures changed, and therefore we do not know whether a warming was responsible for the current upheaval or for the Oval's red color. However, we show that the Oval's color change is consistent with the location, magnitude, and timescale of the predicted temperature change. We show also that other proposed explanations for the color change are not plausible. [Preview Abstract] |
Monday, November 19, 2007 12:27PM - 12:40PM |
GL.00010: Laboratory model of non-local herricane generation Albert Sharifulin, Anatoly Poludnitsin, Alexander Kravchuk The new physical mechanism of the non-stationary large-scale intensive cyclonic vortex origin is described on the basis of laboratory modeling. Unlike previous attempt to explain the phenomena our one does not demand presence of rotation and a local source of heat. Laboratory modeling of large-scale vortical structures formation in the atmosphere has been undertaken. The experimental results obtained are compared with numerical and analytical study of low mode models. For modeling and research of this mechanism the laboratory model, - a cubic inclined cavity fueled by air is used. Inclination is used for modeling of anomalous advective motion. Thus the origin of a cyclonic vortex is consequence of fast transition from anomalous advective motion to normal one. In process of transition from anomalous advective motion to normal one the weight of air turns as a solid body around of a vertical axis. The offered mechanism due to the symmetry conditions lead to heterogeneity of heating. In turn it may start the known mechanisms of a cyclonic vortex local excitation and self-maintenance effect increasing its lifetime. [Preview Abstract] |
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