Fall 2001

The presentation will start with the efforts of modelling the ocean current off Cape Farewell, following by the ideas for simulating the Labrador Sea and Baffin Bay.

Second part of the presentation will be on the drift of sea ice in the Cape Farewell area. During the study a sea ice model was developed and is in the process of being applied for operational forecasts. The forecasting system will be described. The model is a relatively simple drift model. Results from the validation highlight the most important limitations of the model, and provides directions for further development.

Abstract: We used a spectral element shallow water model for investigating the "inertial runaway problem" in irregular domains for the single-gyre Munk problem. Ideally, one would like the statistical equilibrium observed at large Reynolds number to be insensitive to model choices that are not well founded, e.g., the precise value of the viscous coefficient, and choice of dynamic boundary condition. Simple models of geophysical flows are indeed very sensitive to these choices. For example, flows typically converge to unrealisticly strong circulations, particularly under free-slip boundary conditions, even at rather modest Reynolds numbers. This is referred to as the "inertial runaway problem". We showed that the addition of irregular coastlines to the canonical problem helps to slow considerably the circulation, but does not prevent runway.

Abstract: In the summer of 2000 I experimented with the use of a digital camera to observe the sea-surface signature of high-frequency internal gravity waves. Internal waves could be clearly identified from areal photographs as propagating bands of slightly different sea-surface roughness. After refinements this method could provide an interesting mean to evaluate the direction of propagation, the phase speed, and wavelength of internal waves.

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Abstract: The surface layer is the lowest portion of the atmospheric boundary layer in direct contact with the underlying surface and whose properties are determined by exchanges at that surface. Simple schemes are sometimes sufficient to parameterize such transfer processes (using roughness lengths, transfer coefficients etc.) but it is often necessary, and always scientifically interesting, to examine how the atmosphere interacts with a vertically distributed array of sources and sinks.

We choose as our example, a forest canopy and examine the distinct changes in aerodynamic processes that occur at the interface with the overlying atmosphere, and within the air spaces of the stand. Most obviously, the branches, leaves and trunks of trees act as a blockage to the flow, diminishing internal wind speeds. More interesting is the presence of elevated wind shear at treetop height and a mixing layer that results in much greater organization to canopy turbulence than is found over smoother surfaces. The array of canopy elements, which have dimensions smaller than the energy containing eddies, acts to accelerate the dissipation process and diminish turbulence levels. Finally, pressure perturbations created in the shear zone and imposed on flow deep inside a forest canopy dominate the structure of turbulence at that level.

This presentation examines these topics within the framework of a large-eddy simulation of atmospheric surface layer flow that includes air layers within a canopy such as a forest.

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Abstract: High latitudes have received attention recently because of significant changes in the atmosphere - sea ice - ocean system. The surface air temperature in the Arctic has increased by 1.1^oC over the last 50 years, i.e. more than three times as fast as the global air temperature, which increased by 0.6^oC over 100 years. The Arctic sea ice extent in spring and summer decreased by 10% since the 1950s, and it was reported that the sea ice thickness during late summer declined by about 40% since 1958. A warming is also observed in the Arctic Ocean at intermediate depths. These amplified trends are in agreement with warming scenarios performed with coupled climate models, which indicate an amplified response in high latitudes to increased greenhouse gas concentrations. But details of the complex interaction between atmosphere, sea ice and ocean are still only marginally known.

In this presentation the optimisation of sea ice models and an application to 50 years of NCEP/NCAR atmospheric forcing will be discussed, with special emphasis on the long-term variability of sea ice extent, thickness and export to the North Atlantic.

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