Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session M23: Geophysical Fluid Dynamics: Atmospheric and Oceanic Applications |
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Chair: Alberto Scotti, University of North Carolina at Chapel Hill Room: 2001 |
Tuesday, November 25, 2014 8:00AM - 8:13AM |
M23.00001: Tornado-like flows driven by magnetic body forces Gunter Gerbeth, Ilmars Grants, Tobias Vogt, Sven Eckert Alternating magnetic fields produce well-defined flow-independent body forces in electrically conducting media. This property is used to construct a laboratory analogue of the Fiedler chamber with a room-temperature liquid metal as working fluid. A continuously applied rotating magnetic field (RMF) provides the source of the angular momentum. A pulse of a much stronger travelling magnetic field drives a converging flow at the metal surface, which focuses this angular momentum towards the axis of the container. The resulting vortex is studied experimentally and numerically. In a certain range of the ratio of both driving actions the axial velocity changes its direction in the vortex core, resembling the subsidence in an eye of a tropical cyclone or a large tornado. During the initial deterministic spin-up stage (T. Vogt et al., JFM 736, 2013, pp. 641) the vortex is well described by axisymmetric direct numerical simulation. Being strong enough the flow develops a funnel-shaped surface depression that enables visual observation of the vortex structure. As the RMF strength is increased the eyewall diameter grows until it breaks down to multiple vortices. A number of further observed similarities to tornado-like vortices will be discussed. [Preview Abstract] |
Tuesday, November 25, 2014 8:13AM - 8:26AM |
M23.00002: A Minimal Model for Precipitating Turbulent Convection Leslie Smith, Gerardo Hernandez-Duenas, Sam Stechmann To construct a minimal model for precipitating turbulent convection, we consider simplified bulk cloud physics assuming infinitely fast condensation, evaporation and auto-conversion from cloud to rain water. The model sacrifices all microphysics but retains important conservations principles. It is demonstrated numerically that the model is able to capture convective organization, such as squall lines. Linear analysis of a saturated base state identifies the stable, unstable and conditionally stable regions of parameter space. The two delineating parameters are established numerically in the general case of finite rainfall speed. Each parameter is also derived in an appropriate limiting scenario: the condition sufficient for instability (stability) is analytically found for the limit of zero (infinite) rainfall speed. Energy considerations further support the numerical and limiting analytical calculations. [Preview Abstract] |
Tuesday, November 25, 2014 8:26AM - 8:39AM |
M23.00003: Turbulent growth of cloud droplets without collisions Alberto de Lozar, Lukas Muessle, Juan Pedro Mellado It is believed that the increase of droplet collisions due to turbulence is key for the initiation of rain in warm clouds. In particular, the turbulence enhancement of collisions might explain how some lucky droplets grow from 20 to 50 micrometers, a regime in which neither condensation nor collisions due to settling are effective. Stratocumulus clouds, however, do not fit in this picture because typical turbulence dissipation rates are too low to enhance collisions appreciably, but at the same time these clouds produce significant drizzle. We explore the possibility that long-wave radiation causes a significant part of the droplet growth in stratocumulus. In our simulations the bulk properties of the cloud are calculated in the Eulerian field, while at the same time some droplets are tracked using a Lagrangian scheme. The advantage of our formulation is that condensation-evaporation processes are assumed to be infinitely fast, and do not need to be resolved explicitly. This allows us to investigate domains hundreds of meters wide for several minutes, thus resolving the relevant scales for radiative cooling. In this talk we will show results of how the droplet size distribution evolves due to radiation and turbulence, using one billion Lagrangian droplets. [Preview Abstract] |
Tuesday, November 25, 2014 8:39AM - 8:52AM |
M23.00004: The Walker circulation, diabatic heating, and outgoing longwave radiation Reed Ogrosky, Samuel Stechmann The Matsuno-Gill model, derived from the forced shallow-water equations in the tropics, has been widely used to describe the large-scale overturning circulation in the tropical atmosphere. This model contains damping terms in the form of Rayleigh friction and Newtonian cooling. Here, using new data analysis techniques, evidence suggests that damping is actually negligible. Specifically, near the equator, the east--west overturning circulation is in agreement with the undamped wave response to atmospheric heating. To estimate the heating, satellite observations of outgoing longwave radiation (OLR) are used. Frequently OLR is used as a heuristic indicator of cloudiness. Here, the results further suggest that OLR variations are actually proportional to total diabatic heating variations, with a proportionality constant of 18 W m$^{-2}$(K day$^{-1})^{-1}$. While the agreement holds best over long time averages of years or decades, it also holds over shorter periods of one season or one month. Consequently, it is suggested that the strength of the Walker circulation---and its evolution in time---could be estimated using satellite data. [Preview Abstract] |
Tuesday, November 25, 2014 8:52AM - 9:05AM |
M23.00005: Coordinated in-situ observation of developing hurricanes using atmospheric balloons -- a Model Predictive Control approach Gianluca Meneghello, Thomas Bewley Current operational methods used to monitor the development of hurricanes and typhoons include radar and satellite imagery as well as dropsondes parachuted from repeated aircraft flights above the hurricane itself. The accurate in-situ measurements provided by dropsondes are especially valuable for generating an accurate forecast of a hurricane's evolution and landfall. Unfortunately, the data from dropsondes is expensive to obtain (requiring many hazardous high-altitude flights) and limited both spatially (to the vertical profile of its path) and temporally (to the ten or twenty minutes it takes to fall). We show in the present work how receding-horizon MPC can be used to coordinate a formation of sensor-laden atmospheric balloons, distributing them quasi-uniformly across a realistic developing hurricane flowfield for days at a time. Several atmospheric balloons can be released from a high-altitude aircraft, or launched from a ship at sea level, and distributed over the hurricane thereafter. Certain target orbits of interest in the hurricane can be continuously sampled by some balloons, while other balloons make continuous sweeps between the eye and the spiral rain bands. Various solution methods for the optimal control problem arising within the MPC framework are considered. [Preview Abstract] |
Tuesday, November 25, 2014 9:05AM - 9:18AM |
M23.00006: Enabling high-resolution simulations of atmospheric flow over complex terrain in the WRF model Katherine Lundquist, Jeff Mirocha, David Wiersema, Jingyi Bao, Megan Daniels, Fotini Chow As model grid resolution increases, atmospheric models are able to represent fine scale terrain, which can result in steep terrain slopes. The standard terrain-following coordinates used by models such as WRF (Weather and Research Forecasting) are unable to handle very steep terrain because of the grid distortion and related numerical errors. This has prompted the development of an alternative gridding technique in the WRF model, known as the immersed boundary method (IBM), which eliminates terrain-following grids and the associated errors (Lundquist et al. 2010,2012). This implementation, WRF-IBM, has been validated for idealized cases and real urban cases with excellent results; however, to date WRF-IBM has been applied with idealized lateral boundary conditions, and uses a no-slip boundary condition. In this work, we detail a multi-year effort to develop WRF-IBM for real, multi-scale simulations, including full atmospheric physics. Results from three aspects of this project are presented: initializing IBM domains using real meteorological and surface data, developing a nest interface between domains using terrain-following and IBM coordinates, and modifying the IBM boundary condition to include a wall model. [Preview Abstract] |
Tuesday, November 25, 2014 9:18AM - 9:31AM |
M23.00007: Urban Flow and Pollutant Dispersion Simulation with Multi-scale coupling of Meteorological Model with Computational Fluid Dynamic Analysis Yushi Liu, Hee Joo Poh The Computational Fluid Dynamics analysis has become increasingly important in modern urban planning in order to create highly livable city. This paper presents a multi-scale modeling methodology which couples Weather Research and Forecasting (WRF) Model with open source CFD simulation tool, OpenFOAM. This coupling enables the simulation of the wind flow and pollutant dispersion in urban built-up area with high resolution mesh. In this methodology meso-scale model WRF provides the boundary condition for the micro-scale CFD model OpenFOAM. The advantage is that the realistic weather condition is taken into account in the CFD simulation and complexity of building layout can be handled with ease by meshing utility of OpenFOAM. The result is validated against the Joint Urban 2003 Tracer Field Tests in Oklahoma City and there is reasonably good agreement between the CFD simulation and field observation. The coupling of WRF- OpenFOAM provide urban planners with reliable environmental modeling tool in actual urban built-up area; and it can be further extended with consideration of future weather conditions for the scenario studies on climate change impact. [Preview Abstract] |
Tuesday, November 25, 2014 9:31AM - 9:44AM |
M23.00008: An experimental study of the flow pattern and heat transport behavior in horizontal convection with large Rayleigh number and small aspect ratio Ke-Qing Xia, Shi-Di Huang Horizontal convection is a simple conceptual model to understand the role of buoyancy in the Meridional Overturning Circulation (MOC). Here we report an experimental study of the flow pattern and heat transport behavior in horizontal convection with Rayleigh number Ra up to $2 \times 10^{12}$ and aspect ratio of 0.1 using a long apparatus. Flow visualization studies reveal that it is not necessary for the returning flow to penetrate the strong stratification in the thermal BLs, suggesting that much less energy may be required to maintain a global circulation than is generally believed. Moreover, both the heat transport efficiency and thermal BL thicknesses are found to follow a 0.3 power law, which indicates a stronger heat transport in horizontal convection with large Ra number than is suggested in the literature. These findings on horizontal convection may be relevant to the driving mechanism of the MOC. [Preview Abstract] |
Tuesday, November 25, 2014 9:44AM - 9:57AM |
M23.00009: DNS of Horizontal Convection Brian White, Alberto Scotti We perform three-dimensional DNS of Horizontal Convection in a rectangular tank with idealized boundary conditions. The flow is driven by imposing the profile for the buoyancy $b$ at the surface, where it ranges from $b_0$ to $b_0+\Delta b$ and the transition region is confined to a very small area. The Rayleigh based on the domain depth ranges from $10^5$ to $10^{12}$. The scaling observed for the Nusselt number and the strength of the circulation is consistent with Rossby's scaling across the range of Rayleigh numbers considered, indicating that the dynamics in the boundary layer under the ``warming'' side throttles the flow. Energetically, we find that Available Potential Energy (APE) is generated along the surface, and converted to Kinetic Energy (KE). Along the descending plume energy goes from APE to KE up to ${\rm Ra}\sim 10^{11}$. For higher Rayleigh numbers the plume becomes a net sink of APE. When the switch occurs, a stagnant layer develops near the bottom, and the overall circulation becomes characterized by a narrow plume which retroflects rapidly towards the surface, with a shallow recirculation to close the flow. This may indicate the beginning of a Sandstr\"om regime characterized by a stagnant abyssal region and a shallow circulation. [Preview Abstract] |
Tuesday, November 25, 2014 9:57AM - 10:10AM |
M23.00010: An experimental study of bottom heating effects in horizontal convection Fei Wang, Shi-Di Huang, Sheng-Qi Zhou, Ke-Qing Xia We report an experimental study of bottom heating effects in horizontal convection. The horizontal convection is driven by a surface heat flux Q$_{\mathrm{s}}$ and a small amount of heat flux Q$_{\mathrm{b}}$ is applied at the bottom boundary as a perturbation. It is found that while the bottom heating has negligible effect on the thermal properties at the top surface, its influences on the interior temperature and the strength of the downwelling flow are remarkable and such influences are more significant for stronger bottom heating and larger Rayleigh number Ra. Most importantly, direct velocity measurements at Ra around 5 $\times 10^{9}$ reveal that the overturning rate, characterized by the maximum stream function, is increased by up to 111{\%} and 256{\%} for $\eta =$ Q$_{\mathrm{b}}$/ Q$_{\mathrm{s}}=$ 2{\%} and 6.8{\%} cases, respectively, which is consistent with previous numerical studies. These results might be helpful to understand how geothermal heating will affect the oceanic circulation. [Preview Abstract] |
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