Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session E31: Geophysical Fluid Dynamics: Atmospheres |
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Sponsoring Units: DFD GPC Chair: Brad Marston, Brown University Room: 312 |
Sunday, November 22, 2015 4:50PM - 5:03PM |
E31.00001: Direct Statistical Simulation of Geophysical Flows Brad Marston, Greg Chini, Steve Tobias Statistics of models of geophysical and astrophysical fluids may be directly accessed by solving the equations of motion for the statistics themselves as proposed by Lorenz nearly 50 years ago. Motivated by the desire to capture seamlessly multiscale physics we introduce a new approach to such Direct Statistical Simulation (DSS) based upon separating eddies by length scale. Discarding triads that involve only small-scale waves, the equations of motion generalize the quasi-linear approximation (GQL) and are able to accurately reproduce the low-order statistics of a stochastically-driven barotropic jet. Furthermore the two-point statistics of high wavenumber modes close and thus generalize second-order cumulant expansions (CE2) that employ zonal averaging. This GCE2 approach is tested on two-layer primitive equations. Comparison to statistics accumulated from numerical simulation finds GCE2 to be quantitatively accurate. DSS thus leads to new insight into important processes in geophysical and astrophysical flows. [Preview Abstract] |
Sunday, November 22, 2015 5:03PM - 5:16PM |
E31.00002: Gravity wave emission in an atmosphere-like configuration of the differentially heated rotating annulus experiment Ulrich Achatz, Sebastian Borchert, Mark Fruman, Steffen Hien, Joran Rolland A finite-volume model of the classic differentially heated rotating annulus experiment is used to study the spontaneous emission of gravity waves (GWs) from jet stream imbalances, which may be an important source of these waves in the atmosphere and for which no satisfactory parameterisation exists. Experiments were performed using a classic laboratory configuration as well as using a much wider and shallower annulus with a much larger temperature difference between the inner and outer cylinder walls. The latter configuration is more atmosphere-like, in particular since the Brunt--V\"{a}is\"{a}l\"{a} frequency is larger than the inertial frequency, resulting in more realistic GW dispersion properties. In both experiments, the model is initialised with a baroclinically unstable axisymmetric state established using a two-dimensional version of the code, and a low-azimuthal-mode baroclinic wave featuring a meandering jet is allowed to develop. Possible regions of GW activity are identified by the horizontal velocity divergence and a modal decomposition of the small-scale structures of the flow. Results indicate GW activity in both annulus configurations close to the inner cylinder wall and within the baroclinic wave. The former is attributable to boundary layer instabilities, while the latter seems to originate in part from spontaneous GW emission from the baroclinic wave. [Preview Abstract] |
Sunday, November 22, 2015 5:16PM - 5:29PM |
E31.00003: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 5:29PM - 5:42PM |
E31.00004: Mean flow and anisotropic cascades in decaying 2D turbulence Chien-chia Liu, Rory Cerbus, Gustavo Gioia, Pinaki Chakraborty Many large-scale atmospheric and oceanic flows are decaying 2D turbulent flows embedded in a non-uniform mean flow. Despite its importance for large-scale weather systems, the affect of non-uniform mean flows on decaying 2D turbulence remains unknown. In the absence of mean flow it is well known that decaying 2D turbulent flows exhibit the enstrophy cascade. More generally, for any 2D turbulent flow, all computational, experimental and field data amassed to date indicate that the spectrum of longitudinal and transverse velocity fluctuations correspond to the same cascade, signifying isotropy of cascades. Here we report experiments on decaying 2D turbulence in soap films with a non-uniform mean flow. We find that the flow transitions from the usual isotropic enstrophy cascade to a series of unusual and, to our knowledge, never before observed or predicted, anisotropic cascades where the longitudinal and transverse spectra are mutually independent. We discuss implications of our results for decaying geophysical turbulence. [Preview Abstract] |
Sunday, November 22, 2015 5:42PM - 5:55PM |
E31.00005: Convectively and Orographically Forced Mesoscale Flows in a Stably Stratified Atmosphere Jaemyeong Mango Seo, Jong-Jin Baik Mesoscale flows forced by convective-orographic forcing in a stably stratified atmosphere are theoretically examined. We consider a two-dimensional, steady-state, hydrostatic, nonrotating linear system satisfying the Boussinesq approximation with convective elevated thermal forcing and mountain mechanical forcing. Solutions for perturbation horizontal and vertical velocities are obtained by analytically solving the equation system. Flows forced by thermal forcing linearly affect flows forced by orographic forcing, and vice versa. Convectively forced flows are affected by the maximum height and half-width of a mountain and its relative location to the thermal forcing. Orographically forced updrafts (downdrafts) strengthen (weaken) the convective system. In a stably stratified atmosphere, both thermal forcing and orographic forcing generate vertically propagating internal gravity waves. The vertical flux of the horizontal momentum is obtained analytically. In the total momentum flux, nonlinear interaction terms between convectively and orographically forced components are contained. The effects of the nonlinear interaction terms on the total momentum flux are examined with different values of the parameters related to orographic forcing. [Preview Abstract] |
Sunday, November 22, 2015 5:55PM - 6:08PM |
E31.00006: Advection-condensation of water vapor with coherent stirring: a stochastic approach Yue-Kin Tsang, Jacques Vanneste, Geoffrey Vallis The dynamics of atmospheric water is an essential ingredient of weather and climate. Water vapor, in particular, is an important greenhouse gas whose distribution has a strong impact on climate. To gain insight into the factors controlling the distribution of atmospheric moisture, we study an advection-condensation model in which water vapor is passively advected by a prescribed velocity and condensation acts as a sink that maintains the specific humidity below a prescribed, spatially dependent saturation value. The velocity consists of two parts: a single vortex representing large-scale coherent flow (e.g. the Hadley cell) and a white noise component mimicking small-scale turbulence. Steady-state is achieved in the presence of a moisture source at a boundary. We formulate this model as a set of stochastic differential equations. In the fast advection limit, analytical expression for the water vapor distribution is obtained by matched asymptotics. This allows us to make various predictions including the dependence of total precipitation on the vortex strength. These analytical results are verified by Monte Carlo simulations. [Preview Abstract] |
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