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Geophysical Turbulence Program Workshop

Turbulence and Scalar Transport in Roughness Sublayers

26-28 September 2006; Boulder, Colorado, USA

Venue:

Organizing Committee:

The National Center for Atmospheric Research will host a workshop in Boulder, Colorado, September 26-28, 2006 sponsored by the Geophysical Turbulence Program (GTP). The workshop will bring together researchers interested in the roughness sublayer; the layer of flow immediately above (or below) a boundary surface (air-land or air-water interface) containing stationary and moving roughness elements, such as stones, vegetation (trees), buildings, wind turbines, or water waves. The approaches considered are numerical and laboratory modeling, observations, and theory.

Scientific Background and Rationale:

The roughness sublayer normally extends from the interface up to about two to several times the height of the roughness elements. Within the roughness sublayer the flow is three-dimensional and site-dependent since it is dynamically influenced by the length scales and density/porosity of the individual roughness elements. Turbulence properties and scalar transport (including deposition) may vary with the surface type and texture, e.g., bluff obstacles in cities versus the fibrous "fine-grained" elements of vegetation.

In general, Monin-Obukhov (M-O) similarity theory successfully predicts mean and turbulence statistics in the surface layer above the roughness sublayer, but is not applicable within the roughness sublayer as turbulence and local circulations are strongly affected by individual roughness elements. The transport of passive scalars (e.g., CO₂, water vapor, other trace gas species, and aerosols) can be quite different from dynamical quantities such as temperature and momentum. For example, pressure drag dominates momentum transport within the roughness elements while passive species are not so constrained. Turbulent eddies in roughness sublayers are particularly anisotropic and highly intermittent in space and time, especially under stable situations. The complicated nature of turbulence within the roughness sublayer also dramatically influences chemical reaction rates.

Roughness sublayers develop in geophysical flows over a myriad of surfaces, including those containing forests, hills, wind farms, cities, and even bare soil. Oceanic waves present a special problem in that the underlying "roughness" (the wave field) propagates at directions not necessarily related to the local wind and because of wave breaking which leads to multiphase flows near the air-water interface. There is an increasing need to understand turbulence in these complicated regimes; e.g., monitoring the global carbon budget, parameterizing boundary-layer flow, deploying observational networks, simulating urban pollutant dispersion, forecasting hurricanes, and predicting extreme events. Since there are currently no viable parameterization schemes to adequately describe the structure of flow in roughness sublayers, numerical models typically rely on M-O similarity to bridge the gap between the outer regions of the flow and surfaces, despite the known shortcomings. Indeed, the roughness sublayer is likely one of the weakest links among the entire suite of boundary layer parameterizations in global and mesoscale models.

Workshop Format:

The workshop will feature a series of invited talks, interspersed with discussion sessions, and short contributed oral presentations. The invited talks will focus on recent observational, numerical modeling, and laboratory simulation results to better understand turbulence structure and transport in roughness sublayers. The discussions will focus on fundamental issues related to the roughness sublayer and how we deal with the roughness sublayer in a variety of applications. A summary paper is planned to be submitted to a scientific publication.

Voluntary abstract submission:

Anyone who wishes to give a brief oral presentation is required to provide a one-page (maximum) abstract by 1 June 2006. Submission of an abstract does not guarantee that you will be speaking, since there may not be enough time to include all who wish to speak. Please email your abstract (in pdf format) to Liz Rothney rothney "at" ucar .dot. edu

Extended abstract for invited speakers:

All invited speakers should email a 2-page extended abstract (in pdf) to Liz Rothney rothney "at" ucar .dot. edu by 15 August 2006.