145 research outputs found

    Developing and testing a hydrostatic atmospheric dynamical core on triangular grids

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    Design of a nonhydrostatic atmospheric model based on a generalized vertical coordinate

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    Department Head: Dick Johnson.2008 Summer.Includes bibliographical references.The isentropic system of equations has particular advantages in the numerical modeling of weather and climate. These include the elimination of the vertical velocity in adiabatic flow, which simplifies the motion to a two-dimensional problem and greatly reduces the numerical errors associated with vertical advection. Vertical resolution is enhanced in regions of high static stability which leads to better resolving of features such as the tropopause boundary. Also, sharp horizontal gradients of atmospheric properties found along frontal boundaries in traditional Eulerian coordinate systems are nonexistent in the isentropic coordinate framework. The extreme isentropic overturning that can occur in fine-scale atmospheric motion presents a challenge to nonhydrostatic modeling with the isentropic vertical coordinate. This dissertation presents a new nonhydrostatic atmospheric model based on a generalized vertical coordinate. The coordinate is specified in a similar manner as Konor and Arakawa, but elements of arbitrary Eulerian-Lagrangian methods are added to provide the flexibility to maintain coordinate monotonicity in regions of negative static stability and return the coordinate levels to their isentropic targets in statically stable regions. The model is mass-conserving and implements a vertical differencing scheme that satisfies two additional integral constraints for the limiting case of z-coordinates. The hybrid vertical coordinate model is tested with mountain wave experiments which include a downslope windstorm with breaking gravity waves. The results show that the advantages of the isentropic coordinate are realized in the model with regards to vertical tracer and momentum transport. Also, the isentropic overturning associated with the wave breaking is successfully handled by the coordinate formulation

    Large-scale circulation with small diapycnal diffusion: The two-thermocline limit

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    The structure and dynamics of the large-scale circulation of a single-hemisphere, closed-basin ocean with small diapycnal diffusion are studied by numerical and analytical methods. The investigation is motivated in part by recent differing theoretical descriptions of the dynamics that control the stratification of the upper ocean, and in part by recent observational evidence that diapycnal diffusivities due to small-scale turbulence in the ocean thermocline are small (≈0.1 cm2 s−1). Numerical solutions of a computationally efficient, three-dimensional, planetary geostrophic ocean circulation model are obtained in a square basin on a mid-latitude β-plane. The forcing consists of a zonal wind stress (imposed meridional Ekman flow) and a surface heat flux proportional to the difference between surface temperature and an imposed air temperature. For small diapycnal diffusivities (vertical: κv ≈0.1 – 0.5 cm2 s−1, horizontal: κh ≈105 – 5 × 106 cm2 s−1), two distinct thermocline regimes occur. On isopycnals that outcrop in the subtropical gyre, in the region of Ekman downwelling, a ventilated thermocline forms. In this regime, advection dominates diapycnal diffusion, and the heat balance is closed by surface cooling and convection in the northwest part of the subtropical gyre. An ‘advective’ vertical scale describes the depth to which the wind-driven motion penetrates, that is, the thickness of the ventilated thermocline. At the base of the wind-driven fluid layer, a second thermocline forms beneath a layer of vertically homogeneous fluid (‘mode water’). This ‘internal’ thermocline is intrinsically diffusive. An ‘internal boundary layer’ vertical scale (proportional to κv1/2) describes the thickness of this internal thermocline. Two varieties of subtropical mode waters are distinguished. The temperature difference across the ventilated thermocline is determined to first order by the meridional air temperature difference across the subtropical gyre. The temperature difference across the internal thermocline is determined to first order by the temperature difference across the subpolar gyre. The diffusively-driven meridional overturning cell is effectively confined below the ventilated thermocline, and driven to first order by the temperature difference across the internal thermocline, not the basin-wide meridional air temperature difference. Consequently, for small diapycnal diffusion, the abyssal circulation depends to first order only on the wind-forcing and the subpolar gyre air temperatures. The numerical solutions have a qualitative resemblance to the observed structure of the North Atlantic in and above the main thermocline (that is, to a depth of roughly 1500 m). Below the main thermocline, the predicted stratification is much weaker than observed

    NEMO ocean engine

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    100 Years of Earth System Model Development

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    This is the final version. Available from American Meteorological Society via the DOI in this recordToday’s global Earth System Models began as simple regional models of tropospheric weather systems. Over the past century, the physical realism of the models has steadily increased, while the scope of the models has broadened to include the global troposphere and stratosphere, the ocean, the vegetated land surface, and terrestrial ice sheets. This chapter gives an approximately chronological account of the many and profound conceptual and technological advances that made today’s models possible. For brevity, we omit any discussion of the roles of chemistry and biogeochemistry, and terrestrial ice sheets

    Tracer and Timescale Methods for Passive and Reactive Transport in Fluid Flows

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    Geophysical, environmental, and urban fluid flows (i.e., flows developing in oceans, seas, estuaries, rivers, aquifers, reservoirs, etc.) exhibit a wide range of reactive and transport processes. Therefore, identifying key phenomena, understanding their relative importance, and establishing causal relationships between them is no trivial task. Analysis of primitive variables (e.g., velocity components, pressure, temperature, concentration) is not always conducive to the most fruitful interpretations. Examining auxiliary variables introduced for diagnostic purposes is an option worth considering. In this respect, tracer and timescale methods are proving to be very effective. Such methods can help address questions such as, "where does a fluid-born dissolved or particulate substance come from and where will it go?" or, "how fast are the transport and reaction phenomena controlling the appearance and disappearance such substances?" These issues have been dealt with since the 19th century, essentially by means of ad hoc approaches. However, over the past three decades, methods resting on solid theoretical foundations have been developed, which permit the evaluation of tracer concentrations and diagnostic timescales (age, residence/exposure time, etc.) across space and time and using numerical models and field data. This book comprises research and review articles, introducing state-of-the-art diagnostic theories and their applications to domains ranging from shallow human-made reservoirs to lakes, river networks, marine domains, and subsurface flow

    The Role of Horizontal Processes in Upper-Ocean Prediction: A Forecast Simulation in the Sea of Japan.

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    Present-day, operational, upper-ocean, thermal-structure forecast models consist of mixed-layer models with local wind-generated horizontal and vertical advection. To extend their applicability into dynamically active regions, e.g. western boundary current regions, the next generation models are envisioned to include mesoscale advection provided by high horizontal resolution circulation nowcast and, eventually forecast models. In this study, I consider the impact of this additional component of advection in a representative dynamic ocean region. I perform four experiments using a modified version of an operational, upper-ocean, thermal prediction model. Each of the four experiments consists of a series of daily, 72-hr-duration, upper-ocean hindcasts. They were each conducted for four weeks during the warming season in the Sea of Japan. The first experiment uses an Nx1 dimensional mixed layer model with no horizontal processes included. The second experiment uses the same model with the addition of horizontal diffusion and local wind-generated horizontal and vertical advection. This model is comparable to present-day operational models. The third experiment repeats the second with the addition of a fixed geostrophic component to the horizontal advection. The fourth experiment allows daily variation of the geostrophic component through each three day forecast. A suite of statistical measures applied to the results indicates a small but statistically significant increase in forecast skill due to the addition of the nowcast mesoscale advection. The additional analysis of a representative individual forecast strengthens this result. The statistical plus individual analyses together lead to three conclusions. First, the addition of geostrophic flow can have a statistically significant impact, especially in frontal regions. Second, global statistical measures alone are not sufficient model comparison criteria since they can mask specific regions or times of significant change. Third, the use of forecast mesoscale circulation in future upper-ocean thermal forecast models will require care due to the potential for artificial cross-frontal advection

    Lagrangian Coherent Structures: Application to Unsteady Oceanic and Laboratory Flows

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    In this thesis we study several spatio-temporal transport problems in two-dimensional time-dependent flows that generate chaotic advection. We aim to clarify the role of advective transport for passive and also for reactive tracers. Therefore, we characterize transport in these flows in the very detail with Lagrangian coherent structures (LCS) that reveal the geometry of irregular fluid motion, and especially emphasize lines of separation and merging of fluid patches. In particular, (1) we study the spreading of a phytoplankton plankton patch in a numerical NPZ model, (2) we examine the role of advection for the Madagascar plankton bloom , (3) we extract flow patterns in the surface flow of the Ria de Vigo, an estuary in NW Spain, and (4) we analyze transport patterns in a turbulent laboratory flow induced by Faraday waves and relate the resulting Lagrangian coherent structures with the filamentous wave front of a chemical reaction. The results show that LCS are a very useful method to visualize the fluid transport in different chaotic flows
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