Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session KG: Geophysical Fluid Dynamics VI: Ocean and Atmospheric Dynamics |
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Chair: Daniel Jacob, Harvard University Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 5 |
Monday, November 20, 2006 5:15PM - 5:28PM |
KG.00001: Observations of the surface temperature field at an air-water interface for both stable and unstable conditions Geoffrey Smith, Robert Handler, Nicholas Scott High-resolution infrared (IR) imagery of the air-water interface was obtained during experiments performed at the ASIST facility at the University of Miami. During the experiments wind speeds ranged from approximately 2 ms$^{-1}$ to 10 ms$^{-1}$, and flux based Richardson numbers ranged from about 10$^{-2}$ to 10$^{-5}$. Results from two regimes will be presented: the cool-skin case, where the water surface temperature is less than the bulk fluid, and the warm-skin case, where the water surface is warmer than the bulk. In the cool skin case the low wind speed data show a cellular structure in which the lateral length scale of the cells varies as the inverse of the friction velocity. The imagery clearly shows a progression from no waves, through non-breaking gravity waves, to a system of seemingly omnidirectional breaking events. In the warm-skin case the dominant cellular structure is still present, strongly suggesting that these small scale features are due to shear instabilities in the surface layer. However, the natural stability of the system appears to surpress the smallest scales of the surface turbulence. [Preview Abstract] |
Monday, November 20, 2006 5:28PM - 5:41PM |
KG.00002: Scattering of small-scale internal waves by near-inertial wavepackets in the ocean Julie Vanderhoff, James Rottman, Keiko Nomura The breaking of oceanic internal waves is an essential part of the deep-ocean mixing processes that contribute to the general circulation of the ocean, the exchange of heat and gases with the atmosphere, the distribution of nutrients and the dispersal of pollutants. The aim of this work is to further our understanding of how a spectrum of a small-scale internal gravity waves is modified as the waves propagate through a realistic ocean environment. In particular we use ray theory and fully nonlinear numerical simulations to compute the evolution of an initial spectrum of small-scale internal waves as they propagate through a collection of near-inertial waves. Previous work on this topic is restricted to ensembles of small scale waves with a single horizontal wavelength. We extend this analysis to include a realistic ensemble of horizontal wavelengths. [Preview Abstract] |
Monday, November 20, 2006 5:41PM - 5:54PM |
KG.00003: Coherent Structures in a Convective Urban Boundary Layer: An Adjoint Lidar-Data Assimilation Study. Quanxin Xia, Ching-Long Lin, Ronald Calhoun The accuracy of the four-dimensional variational data assimilation (4DVAR) method is first evaluated using the dual lidar data measured during the Joint Urban 2003 atmospheric dispersion field experiment held in Oklahoma City. By comparing with the second lidar observational data, the single lidar 4DVAR is found to retrieval radial velocity fields with an accuracy of 80-90{\%} in the cross-beam direction despite of the missing cross-beam information. This suggests that the current single lidar 4DVAR is able to retrieve reasonably accurate 3D wind fields. The retrieved complete wind and temperature fields are then used to identify coherent flow structures in a convective urban boundary layer, such as convective rolls. The correlation between the retrieved flow structures and the building data, such as the airpark, the central business district, and restaurants, is examined. The multi-scale nature of these structures is further analyzed by using the proper orthogonal decomposition (POD) technique. The interplay between different spatial and temporal POD eigenfunctions is discussed. [Preview Abstract] |
Monday, November 20, 2006 5:54PM - 6:07PM |
KG.00004: Spatial Reduction Method for Global Atmospheric Modeling Yevgenii Rastigejev, Michael Brenner, Daniel Jacob Numerical modeling of global atmospheric chemical dynamics presents an enormous challenge. The main problem is associated with a broad range of time scales varying from milliseconds to several years. This introduces significant stiffness into the governing equations and causes formation of fine spatial structures. The described difficulties are exacerbated by the fact that a typical chemical mechanism includes hundreds of species and thousands of chemical reactions. We offer a dynamically adaptive spatial reduction method for numerical modeling of atmospheric chemical evolution equations. The algorithm diagnoses the chemical dynamics on-line rather than a priori, locally and separately for every species according to its characteristic reaction time and splits the region into different domains where, depending on chemical complexity, a properly reduced chemical model is used. Unlike conventional time-scale separation methods, the spatial reduction algorithm speeds up not only a ``chemical solver'' but also advection-diffusion integration. The algorithm reduces computational cost by at least an order of magnitude for typical atmospheric chemical kinetic mechanism. [Preview Abstract] |
Monday, November 20, 2006 6:07PM - 6:20PM |
KG.00005: LES of the atmospheric boundary layer diurnal cycle with the Lagrangian scale-dependent subgrid-scale model Vijayant Kumar, Jan Kleissl, Charles Meneveau, Marc Parlange Accurately simulating the turbulent structure of the diurnal cycle of the atmospheric boundary layer (ABL) is a critical test for large-eddy simulation (LES). Extreme disparities in the nature of turbulence between unstable, day-time ABL and stably stratified, night-time ABL present a challenge for subgrid-scale models. The Lagrangian scale-dependent (LASD) subgrid-scale model has been recently shown to produce accurate results in flows over heterogeneous surfaces in the neutral boundary layer. In order to evaluate the performance of the LASD model in simulations of ABL flows spanning a wide range of stabilities, a diurnal cycle of the ABL is simulated using the time series of surface heat flux from the HATS experiment (Horst et al. 2004, Kleissl et al. 2004) as the surface boundary condition. Good results are obtained over the entire daily cycle, highlighting the adaptability of the LASD model to disparate flow conditions. Profiles of several flow variables plotted as a function of the surface-layer stability parameter show ``hysteretic'' behavior, whereas when plotted as a function of the local Obukhov length, they show no ``hysteresis.'' This confirms the validity of Nieuwstadt's local scaling hypothesis for the entire stability regime of the diurnal cycle. Research funded by the National Science Foundation (WCR-0233464). [Preview Abstract] |
Monday, November 20, 2006 6:20PM - 6:33PM |
KG.00006: SNOHATS: Subgrid-scale fluxes in stably stratified atmospheric flow over snow surfaces Elie Bou-Zeid, Marc B. Parlange, Hendrik Huwald, Marcelo Chamecki, Charles Meneveau Stably stratified turbulence presents particular challenges both from an experimental and a modeling perspective. Many of the characteristics of stable flows complicate the formulation of effective models for unresolved, subgrid scale (SGS), turbulence in Large Eddy Simulation (LES). To address these concerns, a field study (SNOHATS) was held at the extensive ``Plaine-Morte'' glacier in the Swiss Alps (3000 m) from February to April 2006. Two horizontal arrays of vertically separated 3D sonic anemometers were deployed; this setup was specifically designed to measure subgrid scale fluxes (upwind uninterrupted fetch of 2 km) and then to assess the success of various models in reproducing these fluxes. We first study the influence of stratification on the spectra and co-spectra of velocity and temperature. Subsequently, the eddy-viscosity subgid scale model is assessed for LES of stably stratified wall-bounded flows. Specifically, we measure the Smagorinsky coefficient and the SGS turbulent Prandtl number by matching measured and modeled dissipation rates. Finally we present the dependence of these coefficients on stability, height above the ground, filter size, and strain rates. [Preview Abstract] |
Monday, November 20, 2006 6:33PM - 6:46PM |
KG.00007: Progressive internal waves John McHugh Progressive internal waves of permanent form in a finite-depth layer of stratified fluid are considered. Such waves were previously considered by Thorpe (1968)and Yih (1974) using a Stokes expansion and including non-Boussinesq effects. The non-Boussinesq effects are important for obtaining the correct wave-generated mean flow. The previous results give the now well-known exponential growth of wave amplitude with altitude. Each successive nonlinear harmonic also has an exponential growth with altitude, with each successive harmonic growing faster than the previous one. This result indicates a problem with uniform validity, which becomes critically important in the unbounded layer. Uniformly valid results are obtained here for a finite depth layer using a different expansion, but still assuming small wave amplitude. Long's equation is used for the governing equation.The upstream velocity is assumed constant and equal to the wave speed, making the problem steady. The upstream density profile is adjusted to match the downstream average density profile, as in Yih (1974). This adjustment provides the mostimportant nonlinear effects. The results show a saturated wave profile. The second-order correction to wavespeed for some parameters is negative, meaning larger amplitude waves travel slower, in agreement with Yih (1974). However, other parameter values have a positive value, indicating that larger amplitude waves travel faster. [Preview Abstract] |
Monday, November 20, 2006 6:46PM - 6:59PM |
KG.00008: Numerical investigations on some lightning phenomena Jean-Francois Ripoll, Christoffer Jeffery, Patrick Colestock, John Zinn We currently use and develop a cylindrical coupled radiation-hydrodynamic code which contains over 600 chemical reaction and ionization equations that are essential for the detailed multi-spectral predication of air opacity. These hydrodynamic methods and detailed chemistry allow us simulating the lightning return stroke, which is the radiating shock wave of hot ionized air produced by the electrical discharge. Our longterm goals are (i) the understanding the complex chemical reactions (e.g. formation of NOx, destruction of ozone, etc.) that occur during a lightning discharge and their cumulative effect on atmospheric chemistry and composition, and (ii) determining the multi-spectral radiation signature of a lightning discharge for different altitudes. We will discuss some of the lightning signatures we simulate. In addition, we have coupled to our code a simplified Maxwell system which allows resolving the discharge growth and the air plasma formation in the return stroke. We will show how we simulate the creation of the lightning channel followed by its expansion. [Preview Abstract] |
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