Pan Gnammas (Weathering Pits) across Australia: Morphology in Response to Formative Processes

Pan gnammas occur across Australia, mainly in granites and sandstones. Their morphology and origins on Eyre Peninsula, South Australia are known, but this Twidale model is not applicable everywhere. I investigated their morphology and theorized on their origins at eight sites across the continent, noting particularly their edge profiles, floors and depths in relation to their geological placement. The steep sides, flat floors, depths up to about 25 cm in laminated granites apply only in the hot climates of Western Australia and South Australia. Laminations are rare in Victorian granites, so their pan gnammas have sloped edges and are not as deep (ca. 10 cm) and in the granitic uplands of southeast Queensland, corrosion at the edge is concentrated to a narrow zone and most gnammas are only ca. 5 cm deep. Gnammas in horizontal sandstones in New South Wales and Queensland have steep or sloping edges and reach ca 10 cm deep but in dipping sandstones of the Grampians, Victoria horizontality of corrosion has produced visors at pan edges. The vertically bedded Uluru sandstones have enabled deeper corrosion and pans of different morphology. Regional depth variation has biological consequences.

Numerical study of the effects of Planetary Boundary Layer structure on the pollutant dispersion within built-up areas

The effects of different Planetary Boundary Layer（PBL） structures on pollutant dispersion processes within two idealized street canyon configurations and a realistic urban area were numerically examined by a Computational Fluid Dynamics（CFD） model. The boundary conditions of different PBL structures/conditions were provided by simulations of the Weather Researching and Forecasting model. The simulated results of the idealized 2D and 3D street canyon experiments showed that the increment of PBL instability favored the downward transport of momentum from the upper flow above the roof to the pedestrian level within the street canyon. As a result, the flow and turbulent fields within the street canyon under the more unstable PBL condition are stronger. Therefore, more pollutants within the street canyon would be removed by the stronger advection and turbulent diffusion processes under the unstable PBL condition. On the contrary, more pollutants would be concentrated in the street canyon under the stable PBL condition. In addition, the simulations of the realistic building cluster experiments showed that the density of buildings was a crucial factor determining the dynamic effects of the PBL structure on the flow patterns. The momentum field within a denser building configuration was mostly transported from the upper flow, and was more sensitive to the PBL structures than that of the sparser building configuration. Finally, it was recommended to use the Mellor-Yamada-Nakanishi-Niino（MYNN） PBL scheme, which can explicitly output the needed turbulent variables, to provide the boundary conditions to the CFD simulation.