The impact of basin topography on surface layer turbulence - an observational study

The influence of topography of a small, close basin on turbulence properties are examined by comparing turbulence data from 4-level 3D sonic anemometers on a flux tower inside the Arizona's Meteor Crater with simultaneous turbulence observations at a nearby site on the plain outside the crater. The comparisons indicate that the bowl-shaped crater significantly amplifies the diurnal oscillations of temperature and heat flux, with the amplification most pronounced under quiescent synoptic conditions. The crater's rim shelters the crater atmosphere from mean background flows with wind speeds inside the crater usually less than half the speeds on the outside plain. However, the flows in the crater are much more turbulent, with turbulence generated by the conversion of mean flow into turbulent motion. On days with near-surface winds outside the crater greater than 10 m s-1, turbulent kinetic energy can reach extremely large values (~15 m2 s-2) inside the crater. Spectral analyses indicate that spectral peaks occur at lower frequencies inside the crater compared to outside, especially for the cross-stream wind component. The results also show that a constant flux layer, if exists, is very shallow (less than 2 m) on the crater floor, suggesting that the similarity theory-based empirical formulas may not be proper for describing properties of the surface-layer within the crater.