P.I.M.S. Summer School in Fluid Dynamics

Abstracts of Invited Talks

Turbulence and Mixing in Stably Stratified Fluid Layers
H. J. S. Fernando, Arizona State University

Stable stratification and turbulence are ubiquitous in nature, and their interaction plays an important role in determining circulation patterns of environmental flows. In predicting the nature of such flows, governing equations of fluid motions should be solved with certain space and time resolutions, but the practicable resolutions of current models cannot resolve a host of phenomena that play crucial roles in momentum and mass transfer in geophysical flows. One such phenomena is turbulent mixing ac ross stably stratified layers, which needs to be parameterized in environmental forecasting models as a sub-grid scale process. Recent attempts to develop descriptions of turbulent mixing in density stratified flows, based on fundamental understanding ac quired through theoretical studies and laboratory and field experiments will be presented. Particular attention will be given to mixing across sheared and shear-free interfaces and turbulent convective flows bounded by inversion layers. The sensitivity of numerical model results to sub-grid scale parameterizations will also be illustrated.

noon, Wednesday June 6 in room 251 Civil Engineering Building

Geophysical Convection
H. J. S. Fernando, Arizona State University

Aspects of turbulent convection in geophysical fluids will be discussed. For non-rotating fluids, scales of convection have been well established and have been tested using laboratory and field measurements. Much progress has been made recently with regard to rotating convection and these developments will be reviewed. An intriguing variety of vortex structures arise during buoyant convection, especially in the presence of background stratification and rotation. These vortices play an important role in environmental fluid motions, bearing upon small-scale turbulence to planetary-scale circulation. A review of vortex motions associated with buoyant convection will also be presented, emphasizing the sources of vorticity, evolution of vortex structures and their role in oceanic and atmospheric dynamics. The genesis of a variety of vortices, for example, mushroom vortices, geostrophic and ageostrophic vortices, dipolar structures and hetons in buoyant convection as well as parameterizations that have been developed to represent their properties will be presented.

10:30am, Thursday June 7 in room 251 Civil Engineering Building

Fluid dynamics of the middle atmosphere
T. G. Shepherd, University of Toronto

The middle atmosphere (stratosphere and mesosphere) has some special characteristics that make it interesting from a fluid dynamical point of view. It is a far simpler environment than the troposphere: there is no planetary boundary layer, essentially no latent heat release, and no convection. Thus the fluid dynamics is much closer to "ideal" than in the troposphere. While the troposphere is thermally forced and mechanically damped (a heat engine), the middle atmosphere is mechanically forced and thermally damped (a refrigerator). The mechanical forcing comes from non-local angular momentum transfer by waves, and leads to fascinating fluid dynamical phenomena such as the quasi-biennial oscillation. Because of the stable stratification, short-lived greenhouse gases such as ozone and methane exhibit strong spatial inhomogeneity, and the question of transport of trace species becomes a significant question.

Yet the middle atmosphere is also a region of the atmosphere where some basic fluid dynamical questions remain unanswered. Part of this is due to the relative scarcity of measurements, as compared with the troposphere, so there is plenty of scope for speculation and debate (as with planetary atmospheres). For example, solar radiative forcing should lead to an inertial adjustment process at the equator which is akin to convective adjustment, only sideways --- but this has never been clearly observed. While the stratosphere is dominated by planetary-scale Rossby waves, the mesosphere is thought to be dominated by internal gravity waves forced in the troposphere, which reach large amplitude and break down into turbulence as they propagate upwards into the less dense upper atmosphere --- but a reliable quantification of their effects remains elusive. These and other current research issues will be discussed, with an emphasis on where further fluid dynamical insight is required.

10:30am, Wednesday June 6 and noon, Thursday June 7 in room 251 Civil Engineering Building

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Bruce R. Sutherland, May 2001.