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Atmospheric Boundary Layers: Impacts of heterogeneity

The American Meteorological Society is celebrating 100 years of annual meetings in Jan 2020 and the journal, Boundary-Layer Meteorology is preparing to celebrate 50 years of operation. There are a number of retrospective reviews in preparation.

Before the 1960s most atmospheric boundary-layer studies dealt with steady flow over flat homogeneous surfaces, land or water. Heterogeneity was acknowledged but the main concern was to have enough upwind fetch over a uniform surface so that it could be considered locally homogeneous. Monin-Obukhov Similarity Theory (MOST) was beginning to be widely appreciated (i.e. beyond the USSR) and logarithmic profiles were replacing power laws. Ted Munn's (ECCC and UoT) book, Descriptive Micrometeorology (1966) included MOST and moved things forward in terms of standard texts. It also discussed Internal Boundary Layers.

Initial work on Internal Boundary Layers involved flow over changes in the underlying surface roughness but they can also arise as a result of changes in thermal conditions at the surface, especially in the classic Thermal IBL situation at coastlines with cold stably stratified air advected over warm surfaces. Another situation arises when warm moist air is advected over cold surfaces and fog can form, as over Sable Island or the Grand Banks. Also material, such as snow particles can be lifted from the surface and the density profile of the air/particle mix can modify the turbulence – a variation of MOST.
Topography is the other component of heterogeneity. To quote John Finnigan, “There was a burst of interest in boundary layer flow over hills in the late 1970s into the 1980s, sparked partly by the advent of wind farms as a viable energy source and partly by a general desire in the community to move the discipline beyond ‘flat earth micrometeorology’. This sparked development of theory, modelling and field and wind tunnel experiments.” Much of this involved Canadian research, including the Askervein field study (1982, 83) which remains a significant test case for model comparisons of flow over topography.
So the talk will be mostly retrospective, but we are still working on several aspects and making some progress.